Convective Outlook Valid for Today and Tomorrow

Regional Impacts

Labrador: some isolated thundershowers expected today associated with a southeastward moving trough of low pressure. Gusts to 70 km/h are possible.

Central/eastern/northeastern Newfoundland: a slight chance of late afternoon and evening thundershowers.

Maritimes: a chance of thundershowers today and tonight in association with a fairly stationary upper level low.

 

Convective Discussion

Labrador:

There is a fairly strong 700-500 mb jet descending from Ungava Bay area (YVP has about 55 knots of shear based on the 12Z tephi). A surface frontal zone could trigger some isolated cells this afternoon and into this evening as daytime highs hit the upper 20’s to low 30’s. PWAT at YVP is close to 40, but the air mass is moving into a drier zone, so heavy downpours shouldn’t be an issue at this time. Given the dryness below 700 mb at YYR, some wind gusts in cells is a possibility. Right now gusts to 70 seem reasonable.

 

Central/eastern/northeastern Newfoundland:

There is a weak area of instability laying across the aforementioned areas on the island. Given the latest sat pix skies appear to be clearing so there is a risk of isolated cells this afternoon and into this evening. 12Z YJT tephi is not particularly interesting, but with enough solar insolation CAPE could reach 300. Given about 15 knots of 0-6 km shear, weak pulse cells are expected.

 

Maritimes:

Lastly, the stubborn upper area of low pressure west of the MRTMS. Yesterday the guidance was suggesting potential for some heavy downpours given PWAT was above 40mm and in an area of modest divergence aloft, but there was next to nothing. Lots of activity over the marine areas, but nothing at all over land. So given the history of the upper low only a slight risk of a TSRA is expected, and this would mainly apply to Nova Scotia tonight. Not included in the risk zone is northwestern New Brunswick. If given enough surface heating an isolated cell is possible. Right now I am expected mostly TCUs.

 

Thunderstorm Outlook for Day 1

 

Convective Discussion

The greyed area is a combination of the leftover upper low that will be weakening somewhat and the approaching frontal trough over Labrador. With the increasing shear associated with the trough some gusts to 70 km/h are possible over southern Labrador (of course where only the caribou roam).

 

Thunderstorm Outlook for Day 2

 

Jeremy

Convective Outlook Valid for Today and Tomorrow

Regional Impacts

Nova Scotia, PEI and southern New Brunswick: likely some elevated convection this afternoon and through the night associated with an upper level area of low pressure over New England. Heavy downpours are the only threat.

Newfoundland: a weak trough of low pressure may give an isolated thundershower to southeastern regions through the course of the day.

Labrador: nil.

 

Convective Discussion

The aforementioned upper area of low pressure has produced quite a bit of elevated convection to the east/southeast where the area of divergence is maximized. The sun has been warming the tops some this morning which has lead to a weakening in the lightning but things will continue gradually progress northeastward through the course of the day. There could be some locally heavy downpours later today and tonight. The PWAT is close to 40 mm. There is a slight risk of an embedded thundershower across portions of southeastern Newfoundland in association with a weak trough of low pressure. OTWZ nothing to talk about for the rest of the region.

 

Thunderstorm Outlook for Day 1

 

Convective Discussion

There will remain a risk of thundershowers over portions of SE NL and the MRTMS associated with the upper area of low pressure. Also a frontal trough will be moving southeastward across Labrador that may trigger a couple thunderstorms.

 

Thunderstorm Outlook for Day 2

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

New Brunswick: Some thunderstorms this afternoon, with heavy downpours, gusty winds, an a low chance for hail. In the northwest the winds and rain may reach warning criteria.

Newfoundland: Some thunderstorms this afternoon, with possible heavy downpours, some gusty winds, and a chance for hail. In the north, roughly from Grand Lake  east, there is a slight chance for severe weather, with rainfall followed by gusty winds being the greatest risk.

Nova Scotia: some thundershowers possibly this afternoon into this evening. Non-severe elevated convection, mostly.

Prince Edward Island: Possible non-severe thundershowers.

Western Labrador: Thundershowers this afternoon with some gusty winds.

 

 

Convective Discussion

The broad upper low has move southeast to northwest of the St. Lawrence River, with an associated broad surface low just north of Vermont. A trough stretching east over northern New Brunswick and continuing east over Newfoundland will be part of the trigger for this afternoon. Similarly,  an associated low/trough southwest of Yarmouth will give embedded elevated convection to the eastern part of Nova Scotia as it moves east today, and will start  impacting southern Newfoundland overnight tonight, and mat give a little more elevated convection to the southern Island. Southwestern Labrador will see some more afternoon into this evening.

 

Northwest New Brunswick and north-central Newfoundland have measurable risks for severe convective impacts, mainly for strong gusts and heavy downpours. For New Brunswick, there is a little dynamic forcing with some weak upper PVA near the trough, and PWAT is higher than yesterday at around 30-35 mm. Adjusting the Caribou and Gray  tephis for this afternoon’s max temperature gave CAPEs between 700-1500 J/kg. assuming the clouds break up a bit. The shear is low, and we’re not expecting supercells, but some heavy downpours and strong downdrafts are possible. A watch will be issued shortly. For Newfoundland the ingredients are weaker, but there is still a possibility forstrong downdrafts and some local heavy showers.

 

 

 

 

Thunderstorm Outlook for Day 2

 

Northern Labrador may have some weak thundershowers in the afternoon. Newfoundland may see some embedded convection associated with the trough from Nova Scotia moving into the southern island. For Nova Scotia and souther New Brunswick  a second disturbance may give some embedded convection.

 

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

Nova Scotia, New Brunswick and Prince Edward Island, and Newfoundland:  a few thundershowers with a slight risk of small hail, some gusty winds, and brief downpours.

Labrador: similar to the Maritimes, but a bit weaker.

 

Convective Discussion

Currently there is a broad occluded low west of Labrador with a deep cold upper trough extending south to southeast across the eastern Gulf of St. Lawrence. In addition there is a weak broad east-west trough across most of Atlantic Canada.  Currently there are strikes along the deep upper trough from Gulf - Port au Port to the vicinity of Wabush. The ingredients are fairly moderate for most regions, with PWAT generally between 20-30 mm, with most of it below 700 mb, freezing levels not far from 10,000 ft, giving a possibility of small hail, and dry air above 700 mb, giving some gusty winds. Shear is a bit more variable. Over Labrador it varies from 15-25 kts, which probably will inhibit hail. For Newfoundland it increase from 15 near Stephenville to near 45 kts near St. John’s, and with some partially clear skies over the island may give some small hail and good but non-severe wind gusts as you move east.  Surface dewpoints range from 8-12 for Labrador and 10-15 for Newfoundland. The CAPEs for Newfoundland should stay below 500 L’kg, and for Labrador below 250.

 

For the Maritimes there is little in the way of upper support except for the weak broad trough near New Brunswick.  Clear skies over most of the Maritimes will give good insolation, except for New Brunswick where some cloud is moving in.  For the Maritimes PWAT is similar, with dewpoints ranging from  13-16 C  for New Brunswick and 15-18 for the rest of the Maritimes.  The shear is between 25-35 kts, and mean layer CAPEs may exceed 500 J/kg, especially for eastern Nova Scotia. Again, freezing levels near 10,000 ft may give small hail, as seen yesterday in New Brunswick. For tonight a trough will approach from the southwest and may give some nocturnal convection.

 

 

 

Thunderstorm Outlook for Day 2

 

Roughly similar areas, with somewhat different dynamics. There will be a  broad low over Maine and extending north, with a trough extending east to Newfoundland, part of which will push north twards Labrador, initiating some thundershowers.  A second trough, mentioned above, will continue to  move across Nova Scotia tomorrow.  The freezing levels will be a bit higher tomorrow, reducing the chance for hail.

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

 

New Brunswick: strong wind gusts likely and near severe possible, possibly small hail, and some brief heavy downpours.

Nova Scotia and Prince Edward Island: chance for weak thundershowers, with some gusty winds and brief downpours.

Labrador: strong wind gusts likely and near severe possible, possibly small hail, and some brief heavy downpours.

 

Convective Discussion

The situation is roughly similar to yesterday, with an occluded low southeast of James Bay and a trough extending from it east to the Great Northern Peninsula of Newfoundland. A weak low has developed northeast of the Island, with yesterday’s cold front extending south. The low is moving away and will no longer be of concern.  For the Maritimes there is a deep cold north-south trough near western Maine which will slowly drift east, destabilising mid to upper levels.  The associated upper jet over New Brunswick is not aligned to help with lift, however.  There is a moderate but no generous amount of moisture, with PWAT in the 20-30 mm range, and mainly between 700-900mb, with surface dewpoints near 15 C being advected in from the southwest. So the main lift mechanism is daytime heating, a bit of PVA from the trough near west and northwest New Brunswick, and enough moisture to make things interesting. With the dry mid to upper layers, strong downdrafts are possible. With the freezing level near 10,00 ft, there is a chance for small hail.  For Nova Scotia there is less dynamic lift, so marginal thundershowers are possible. For Labrador, condifitons are similar to New Brunswick, but with the freezing level higher and some weak PVA associated with the east-west trough.

 

 

 

 

Thunderstorm Outlook for Day 2

 

For the Maritimes basically a repeat from yesterday, with the area of highest threat moved southeast into parts of Nova Scotia. The low freezing levels from yesterday persist, so there is a chance of small hail, and gusty winds are also likely.

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts:

Labrador: generally non-severe convection, with some heavy showers and gusty winds.

Eastern Nova Scotia: A chance for topographic enhancement to trigger a thundershower or two.

Western Newfoundland: A chance for topographic enhancement to trigger a thundershower or two.

Southern waters: weak embedded convection from Sable south. It may get close to Cape Breton Island.

 

Convective Discussion

There’s a deep occluded low southeast of James Bay, and with a trowel  extending to a warm front-cold front system over western Labrador. The warn front extends east across Labrador, while the cold front extends south across the western Gulf of St. Lawrence and Nova Scotia through to Georges Bank. The frontal system is moving almost directly east, with the cold front also moving east across the Gulf.  The convection over Labrador is currently along the warm front-trowal line (the main trigger) and has some moderate moisture, shear in the 40-60 kt range, and marginal CAPE.  The most likely impacts are some heavy showers and possible gusty winds.  Southwest of the low and trowel there will be some cooling and drying aloft behind a deep thermal ridge. There will also be some PVA associated with the right exit of the upper jet, and the clouds . The only significant moisture source will be below 850 mb, with dewpoints maxing out near 12 C. This probably implies fairly gusty winds as the largest threat.

 

Ahead of the cold front over the Maritimes the moisture is great, but the profiles are closer to moist adiabatic with little cape. The font will be the primary lift mechanism, with heavier downpours. There’s a chance this afternoon for Cape Breton, and mor into the evening for western Newfoundland to have a couple of thundershowers enhance by topography.

 

 

 

Thunderstorm Outlook for Day 2

 

On Friday there will be atrough extending itself over southern Labrador, possibly giving some thundershowers mainly south of Goose Bay and west to Wabush in the afternoon. PWAT will be near 30 mm, surface based CAPEs near 500 J/kg, and with shear in the 20-30 kt range in the west and 10-15 kts stronger in the east, where there’s a slight chance for a severe thunderstorm. For Newfoundland, a chance for a final thundershower as things quiet down. For New Brunswick a similar setup as for southern Labrador, but a bit warmer. The Maritimes might have the highest risk for severe convection.

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

 

New Brunswick: Near-severe thunderstorms possible late this afternoon and evening mainly over northwestern regions giving localised heavy rainfall (20-30 mm/hr), moderate to strong wind gusts (80 km/hr) and small hail (<2 cm).

 

Labrador: Isolated non-severe thunderstorms over southern parts of the region early this afternoon and evening.

 

Convective Discussion

 

An upper level low over western Quebec will slowly move northeastward today. A weak cold front currently over southern Quebec will move over Maine and into western New Brunswick this evening. Meanwhile a broad warm front extends into southern Labrador. Convection has been ongoing along both features this morning. Embedded convection may continue into the early afternoon over southern and eastern Labrador.

 

Ahead and along the cold front extending into southern Quebec and western Maine, organized convection is expected which may continue into western New Brunswick in the early evening and overnight period. The main inhibitors for now will depend on the current cloud deck over Maine and western New Brunswick which is taking a while to burn off. The lapse rates over the area are also not ideal as the 500mb cold trough remains back over Ontario and there is not much further destabilization expected in the upper levels. However, there is ample moisture available with dew points reaching 20C. The 12Z Caribou sounding shows quite a warm nose around 900 mb and that cap is expected to be broken once the cloud clears. It also has a decent dry layer from 500-700mb, indicating a risk for strong downdraft gusts. The  0-6 km shear has increased to 23 KT with Should things align, there is the risk for convection to organize into lines which could make their way into northwestern New Brunswick this evening. Frequent lightning, locally heavy downpours and strong wind gusts are the main risks.

 

 

 

 

 

Thunderstorm Outlook for Day 2

 

The upper low somewhat stall over central Quebec tomorrow. The threat for any severe convection diminishes. Only daytime heating pop up TCUs in showers with a slight risk of a CB seems likely over New Brunswick and Newfoundland with some embedded convection over western Labrador where the warm frontal feature is stalled.

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional impacts:

 

New Brunswick: Isolated to scattered thundershowers this afternoon giving localised heavy downpours (20 mm), moderate wind gusts, and small hail (1-2cm).

 

Nova Scotia: Isolated showers with possible non-severe thundershowers mainly over the Annapolis Valley and parts of northern Nova Scotia giving localised downpours.

 

Newfoundland: Isolated showers with possible non-severe thundershowers over parts of the interior and northeast giving localised downpours.

 

Labrador: Isolated thundershowers over southern Labrador with possible localized downpour, moderate wind gusts, and small hail.

 

Convective discussion:

 

A trough over Ontario will be slowly pushing eastward today, which will continue to bring in more humid air further east into Maine and eastern Quebec and the western Maritimes. Pulse type storms are expected for New Brunswick and possibly reaching southwestern Labrador by the evening.  The 12Z Caribou sounding shows that there is a lot of buoyant potential given the forecasted high temperatures today and increasing dew points with possible CAPE values in excess of 1500 J/kg. The area however remains in a weak area of shear with no real upper dynamics in the area to keep cells prolonged for too long. There is enough energy however that small hail may still be possible but heavy downpours are likely the biggest concern.

 

Sea-breeze convergence over mainland Nova Scotia could also kick up some TCUs with showers with a risk of CBs this afternoon. With shear remaining low and little flow aloft combined with precipitable water of 39 mm, heavy downpours will be the main concern but downpours should be relatively short lived. Daytime heating with a light southwest flow over Newfoundland may also give showers and possible CBs over the interior and northeastern regions this afternoon and evening.  

 

 

Thunderstorm Outlook for Day 2

 

A low pressure system moves into central Quebec today from Ontario with increasing moisture in the low levels. Ahead of a trough extending into southern Quebec and western Maine, organized convection is expected which may continue into western New Brunswick in the early evening and overnight. Given the timing, the highest risk of severe thunderstorms should remain over Maine and southern Quebec. A warm front will extends from central Quebec east toward the Lower North Shore. Along this front embedded convection is possible which may move into parts of southern Labrador early in the afternoon.

 

Convective Outlook Valid for Today and Tomorrow

Regional Impacts

 

Southern Labrador: Scattered thunderstorms possible for southern Labrador this afternoon and evening giving localised heavy downpours, strong wind gusts and small hail.

New Brunswick: Isolated thunderstorms are possible also over northern New Brunswick giving localised heavy rainfall and strong wind gusts early this afternoon and evening.

Southern Maritime marine district: Isolated  Embedded thunderstorms today.

 

Convective Discussion

 

There are two features to consider today. An upper low resides over central Quebec with a warm front extending into central Labrador and southeast towards Newfoundland with some embedded convection this morning. A broad area of lower pressure extends from western Labrador and southwest into northwestern New Brunswick and Maine. There is also a low pressure system south of Nova Scotia It is within this area convection will likely pop up this afternoon and evening.

 

Over Labrador, the upper dynamics are favourable for convective initiation this afternoon. The area falls within the right entrance to a 250 mb jet and there is some destabilizing at 500 mb with a cold trough approaching western Labrador. That along with 0-6km shear of 47 KT and relatively high precipitable water levels, and low freezing levels will help prolong convection later into the evening and cells may form into organized lines of convection or possibly supercells. Heavy downpours, strong winds gusts (60-70 km/h) and small hail (1-2cm) are possible.

 

There is a capping inversion to break this morning over New Brunswick but with forecasted high temperatures are expected to be near the convective temperature needed to initiate lift. Additional lift will be aided by the higher terrain over northern New Brunswick. Some areas along the Bay of Chaleur that see a sea breeze this afternoon may also help to provide lift.  There is the potential for CAPE values to be in excess of 1500 J/kg but the area is lacking in upper dynamics. Moisture has been slowly increasing over the area so heavy downpours are possible as the steering flow is weak. 0-6km shear remains low as with Caribou sounding at 12Z so pulse or multi-cell type storms seems the most probable. Freezing levels are somewhat high so the risk of hail seems minimal. The threat of training heavy rainfall over some areas is have the highest impact.

 

Thunderstorm Outlook for Day 1

 

 

 

 

Thunderstorm Outlook for Day 2

 

Tuesday will be similar to today over New Brunswick with some afternoon convection possible over mainly northern areas. A trough over Ontario will be slowly pushing eastward, which will continue to bring in more humid air further east into Maine and western Quebec. Pulse type storms again expected for New Brunswick and possibly western Labrador. Sea-breeze convergence could also kick up some TCUs in showers over mainland Nova Scotia and Newfoundland.

 

 

Convective Outlook Valid for Today and Tomorrow

Regional Impacts

Risk of isolated thunderstorms producing downpours and wind gusts over northwestern NB, and northern Gulf of St. Lawrence regions.

Convective Discussion

This morning, the upper ridge at 500mb remains positioned from Hudson Bay to Maine. At the surface, a warm front extends from a low pressure area over northern Ontario to James Bay then southeastward through central Quebec, Maine and western Nova Scotia. Based on this mornings upper air soundings, the most likely areas for convective activity today is northwestern New Brunswick. Surface moisture is increasing over northern Maine and some of this moisture will have to advect over NB in order for convection to initiate.

Modifying the Caribou profile with a surface temperature of 32 and dewpoint of 18 results show:

·         Mixed layer cape values in excess of 1000 J per kg

·         A fair amount of this instability would be situated between 700-450 mb which would suggest a chance of hail (possibly around 1cm)

·         Slow moving storms could produce downpours with precipitable water in order of 30mm

·         And finally wind gusts 40kt or more.

 

On Monday, a non-tropical low pressure system east of the Carolinas will reach Georges Bank and bring isolated showers and thundershowers to the Atlantic coast of Nova Scotia. Further north, afternoon thunderstorms are possible.

 

Thunderstorm Outlook for Day 1

 

 

Thunderstorm Outlook for Day 2

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

 

Nova Scotia and New Brunswick: A risk of thundershowers this afternoon and early evening.

Newfoundland and Labrador and Prince Edward Island: None

 

Convective Discussion

 

Cape values of 500-1000 J/kg and shear of less than 20 kts will yield the potential for pulse storms along a weak trough over western NB and NS this afternoon and evening. If the convection does occur then the slow moving cells could give local downpours of 25 mm or higher.  The precipital water indicates 25-30 mm. Dry mid-levels will give the potential for some gusty winds if the convection does occur.  

 

Thunderstorm Outlook for Day 2

 

Pulse storms possible for northwestern NB on Sunday giving similar impacts as this afternoon.

 

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

 

Low risk of high based thunderstorms over the higher terrain in northern New Brunswick and western Newfoundland late this afternoon and evening. Convective downdraft gusts are the main concern should anything develop.

 

Convective Discussion

 

A very hot and slightly humid air mass continues to reside over the Maritime provinces today. A trough over southern Labrador into northern Newfoundland will slowly sink southward today and reach northeastern New Brunswick and the Gulf of St. Lawrence tonight.  Temperatures profiles over NB this morning suggest a very dry air mass but with a lot of buoyant potential.  An isolated thunderstorm cannot be ruled out, especially with the help of forcing from hills and where forest and other vegetation may provide the extra moisture and energy needed or with the help of a sea breeze convergence off the Bay of Chaleur. The forecast highs are such that the convective temperature of 37 could even be reached on its own. Given such a dry and warm profile at all levels, there may not be much in the way of precipitation but convective wind gusts could be a concern. There may be a similar situation off the higher terrain in southwestern Newfoundland.

 

 

Thunderstorm Outlook for Day 2

 

Daytime heating air mass type thunderstorms are possible mainly over Maine tomorrow afternoon but with the slight winds aloft from the west, some of these cells could reach into western New Brunswick in the evening. The risk remains very low as mixing in the low levels may inhibit development. A ridge of high pressure  sinks south into Newfoundland and Labrador tomorrow with no convective weather expected.

 

 

 

Roberta McArthur

 

 

Convective Outlook Valid for Today and Tomorrow

Regional Impacts

 

Slight risk of a non-severe thunderstorm over southern portions of the interior of Newfoundland later this afternoon.

 

 

Convective Discussion

 

Over Newfoundland, an area of surface based instability is expected to develop later this afternoon over unpopulated interior areas of the island. This is supported by a weak disturbance embedded in the strong northwesterly flow aloft. One condition for the destabilization to occur would be for the current cloud deck to break-up/scatter out which should happen beginning over the west coast during the early afternoon, then later over inland areas.   

Warm and dry conditions will prevail over the Maritimes today.

 

 

Thunderstorm Outlook for Day 1

 

 

Thunderstorm Outlook for Day 2

 

Hot and somewhat more humid conditions are in store for Friday across the Maritimes. Some of this (modified) airmass will try to push across Cabot Strait into southwest NF. Temperatures profiles over NB are supporting the potential for the development of an isolated thunderstorm, especially with the help of forcing from hills and  where forest and other vegetation may provide the extra moisture and energy needed.

 

Convective Outlook Valid for Today and Tomorrow

Regional Impacts

isolated non-severe thunderstorm over western Labrador this afternoon.

 

Convective Discussion

Aloft this morning there is an east-west oriented ridge over the Great Lakes and the Maritimes. To the north, a frontal band extends from Labrador westward to northern Ontario and the eastern Prairies. At the surface, rain continues fall along  a warm front which is moving towards the Labrador coast and the Great Northern Peninsula of NF. Moist and somewhat unstable airmass will prevail over the rest of Labrador this afternoon and into the evening. Based on the soundings from YZV and LG4, the expected daytime high (low to mid-twenties) should be enough to produce an isolated cell, especially over southernmost areas. Very strong winds aloft may not permit much vertical development, but considering the amounts of moisture brief moderate showers are a good possibility along with winds mixing to the surface and giving gusts between 50-70 km/h.

 

Thunderstorm Outlook for Day 1

 

 

Thunderstorm Outlook for Day 2

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

Slight risk of a thundershower over Southeastern Labrador

 

Convective Discussion

The tail of a weak departing trough currently over the south Labrador coast could provide enough forcing for showers and a weak thundershower to develop until mid-afternoon. This region is on the southern periphery of pool of cold air aloft. Forecast guidance is suggesting a small area of surface-base CAPE in the 100-200 J/kg range, and the lighting guidance is also indicating a chance of lightning over the same area.    

 

Thunderstorm Outlook for Day 2

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts:

 

Labrador: a chance of thundershowers this afternoon into this evening with gusty winds (weak dry microburst)

 

Offshore waters south of  Nova Scotia and Newfoindland: embedded convection with heavy downpours.

 

Convective Discussion

A deep cold trough is moving into western Labrador with some associated PVA and a left exit region from the upper jet west of the trough. There’s also a little cold advection. Near the surface there is a capping inversion that will weaken this afternoon. The main limiter here is moisture and CAPE. PWAT is only 15 mm, dew points are pretty low, and if the upper lift is strong enough you might get 200 J/kg. There’s moderate deep layer shear as well. There’s a chance for a weak dry microburst, which will probably be the major impact. Conditions will improve overnight.

 

For the offshore waters, it’s a continuation of the rationale from yesterday, with an inverted trough as the trigger for a saturated moist adiabatic profile. Shear is moderate near the trough, but mainly less than 35 kts. So mainly rain and some gustry winds.

 

 

 

 

Thunderstorm Outlook for Day 2

 

 

There may be some convection overnight and into Tuesday morning just south of Labrador, but there’s only a slight chance of it reaching towards the Strait.

Convective Outlook Valid for Today and Tomorrow

 

Thunderstorm Outlook for Day 1

 

Regional Impacts: None for land areas of the Atlantic Provinces. A chance of embedded convection with occasional heavy showers for Nova Scotia slope waters spreading northeast towards Sable later today and approaching southeastern Newfoundland overnight.

 

Convective Discussion

There is a ridge extending from the Great Lakes ENE into  the South Labrador Sea, with a second ridge southeast of Newfoundland into the mid Atlantic. Between these two features is a northeast-southwest trough over Newfoundland.  Above the western ridge is a nearly stationary upper trough extending from Lake Erie along the St. Lawrence river towards southern Labrador. Southwest of Newfoundland the surface trough is intensifying as an inverted trough slowly moving toward the Avalon Peninsula tonight and  on Monday.

 

The basic story is that near and under both ridges things are stable under subsidence inversions around 700 to 850 mb, with very little precipitable water and near zero CAPEs, and with wind shears around 40 kts over the Maritimes, and for western Newfoundland approaching 70 kts. So little water, negligible CAPE, and high shear should kill any chance of thunderstorms.

 

For the Scotian Slope and approaching southern Newfoundland things are a bit less definitive. The aforementioned inverted trough is forecast to intensify and extend towards the Avalan. There’s deep moistire and PWs  in the 35-45 mm range, an almost tropical moist adiabatic profile. And the trough as a possible trigger. Shear is weak near and south of the trough, but just north it increases to the 30-40 kt range. My best guess is elevated weak convection with some heavy showers, extending to south of Newfoundland overnight, and maybe brushing the southern Burin or Avalon on Monday.

 

 

 

Thunderstorm Outlook for Day 2

See above.

Convective Outlook Valid for Today and Tomorrow

 

Thunderstorm Outlook for Day 1

 

Regional Impacts: no severe weather expected.

 

Convective Discussion: There is a large ridge centred north of Lake Superior and another large ridge east of the Grand Banks, with a weakening trough over the Atlantic Provinces between them. There is some dynamic support, but the ridging from the west is resulting in a fairly strong subsidence inversion capping convection for the Maritimes and western and northern Labrador.  For southeastern Labrador there is a frontal trough, but increasing cloud and low precipitable  water,  as well as a higher inversion at about 700 mb, argues for a few moderate to heavy showers at worst.

 

Over Newfoundland the story is similar with the riding inversion being weaker, but still there between 850 and 700 mb, and without the trough in Labrador. Precipitable water is a bit higher, especially in the southeast, and deep layer shear is okay, but the inversion and relatively low dewpoints for most of the island are consistent with no CAPE likely in the guidance.

 

 

Thunderstorm Outlook for Day 2: no severe weather expected.

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

 

 

Regional Impacts

 

Scattered showers with a slight possibility of embedded thundershowers early this afternoon over parts of the Bay of Fundy, Annapolis Valley, and northern Nova Scotia. The main threat with these will be some localised heavy rain.

 

Convective Discussion

 

A broad trough from a low pressure system over the northern Labrador Sea extends from western Newfoundland southwestward through the Gulf of St. Lawrence, PEI and the Bay of Fundy region this morning. The trough has not seen any lightning over our area this morning but some elevated convection is still possible into this afternoon. This area is in the right entrance of a 110 KT 250 mb jet with a weak low level jet over Nova Scotia. A cold pool lies south of New Jersey where the convection has been firing up this morning but the upper levels remain warmer further north, which is likely the main inhibiting factor. There is still a good amount of precipitable water available from the Yarmouth and Shearwater 12Z soundings of around 40 mm so locally heavy downpours are possible.

This trough will continue to weaken and somewhat stall over Nova Scotia and western Newfoundland into Saturday.

 

Later this afternoon, another trough will slide in from central Quebec into northwestern New Brunswick this evening. The main threat for convection will remain north of the St. Lawrence River extending north possibly reaching southwestern Labrador. Some TCUs reaching 18,000 ft in showers are expected to reach New Brunswick this evening.

 

Thunderstorm Outlook for Day 2

 

 

Regional impacts

 

None

 

Convective Discussion

 

The second trough will continue eastward over southern Labrador and northern Newfoundland with just scattered showers expected.  A sea breeze convergence over the Annapolis Valley and northern Nova Scotia may kick off some TCU in showers but a warm 700-500mb layer will likely inhibit any CB formation. The quasi-stationary trough over the southern maritime marine areas may prompt some embedded convection in the early morning hours Sunday morning.

 

 

Roberta McArthur

 

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

 

 

Regional Impacts

 

Scattered showers and embedded thundershowers for today and this evening over western New Brunswick. Heavy rain and brief strong wind gusts are possible.

 

Convective Discussion

 

A near stationary low pressure system resides over James Bay with a long cold frontal boundary extending south over southern Quebec and along the eastern United States this morning. Most of the dynamics remains further west over southern Quebec into New England. Ahead of the front over New Brunswick today, there is warm advection in the mid to upper levels, limiting the energy possible in embedded thundershowers. It is also expected to remain cloudy today, prohibiting any surface based convection. There is however increasing 0-6km shear, and a low level jet will help destabilize the environmental flow this afternoon. Linear lines of convection may form but will be quite fast moving. There is also ample precipitable water available of 40 to 50 mm which could prompt some localized heavy downpours.

 

Meanwhile over Labrador, a warm front will push northward into central Labrador today. There is a very slight risk of elevated convection overnight in the warm sector over southwestern parts of Labrador.

 

Thunderstorm Outlook for Day 2

 

 

The weakening trough will continue to push into Nova Scotia overnight and stall on Friday. A short wave trough will move from Quebec into northwestern New Brunswick late afternoon. At this time, TCUs are expected. With very high shear but weak CAPE, CBs are not expected at this time.

 

Roberta McArthur

 

 

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Regional Impacts

Brief showers with risk of a thundershower for Terra Nova, Clarenville and Bonavista Peninsula.

 

Convective Discussion

Surface High pressure system continues to build over the Maritimes. High cloudiness ahead of a warm front extending ahead of extratropical cyclone Cristobal will continue to push eastward during the next day reaching Maine by Thursday. A 50kt southwesterly low-level jet will advect some low cloud and moisture, and forecast guidance is suggesting some elevated CAPE and a slight risk of thundershower early Thursday morning.

 

Over Newfoundland today, the air mass is fairly unstable but lacking moisture and diurnal heating. Towering-cumulus giving brief showers are in store for the afternoon, however with the help of a convergence line over east-central portions of the island, one or two of these TCU’s could build high enough to have glaciated tops. A slight risk of a thundershower is indicated on the western edge of a deck of strato-cumulus covering southeastern regions of the Island

 

Thunderstorm Outlook for Day 2

 

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

 

Convective Discussion

There is a slight risk of thundershowers this afternoon and evening across SE NB, NRN/ERN NS, Cape Breton and PEI. There not a lot of support, but a fairly potent short wave trough will be moving through later today. An approaching 250 mb jet will place this areas near the left exit region which may support some development. CAPE values will be low, but there will be increasing shear as the day progresses (upwards of 40 knots). Freezing levels are quite low (6-7,000 feet) as well. Expect numerous TCUs to form (15,000-20,000 feet) with ISOL thundershowers as well. Given the low freezing levels, small hail/ice pellets is a possibility in some of the stronger echoes. Also gusty winds can be expected in some TCU/CBs. OTWZ nil sig weather is expected.

 

Regional Impacts

MRTMS: ISOL thundershowers possible this afternoon and evening.

 

Thunderstorm Outlook for Day 2

Convective Outlook Valid for Today and Tomorrow

Thunderstorm Outlook for Day 1

Convective Discussion

There is a slight chance of a thundershower across parts of the Maritimes today. The only issue will be the lack of sunshine as there is a lot of cloud cover. CAPE values will be low (in the 200-300 range) and the shear is relatively weak (roughly 30 knots). There does appear to be some vorticity advection which could aid in the formation of TCU and an isolated thundershower or 2. Cloud tops will be relatively low as well.

There is also a risk of a thundershower over western Newfoundland this afternoon and evening. An upper level trough, currently over the eastern Maritimes and Gulf of St. Lawrence area will gradually approach the island today. There has been history with some isolated thundershowers just ahead of it where there is some slight divergence aloft and associated PVA. Some locally heavy downpours are possible if the odd cell forms this afternoon.

Regional Impacts

Maritimes: slight chance of a thundershower this afternoon and into early this evening.

Newfoundland: slight chance of a thundershower this afternoon and evening.

Thunderstorm Outlook for Day 2

Convective Outlook Valid for Today and Tomorrow

Thunderstorm outlook for Day 1.

 

Convective Discussion

A slight chance of embedded convection today and tonight as a low currently south of Nova Scotia tracks towards NFLD tonight.  If any convection does get initiated, it would have to go through a significant warm nose before reaching a level of positive buoyancy at which 200 to 300 J/Kg is possible which is not significant.  However, a few cells passed over Sable island early this morning bringing near 60 mm over the island in a span of 3 hours (34 mm was reported in 1 hour).  There was no lightning detected on these cells but due to the amount of rainfall over a short period of time, this does indicate that there could be some convective activity.  This area of convection is expected to reach eastern Newfoundland later today so if the cells maintain their strength, it could result in some significant rainfall over a small period of time for parts of eastern NFLD.

 

Regional Impacts

Maritimes…

A slight chance of embedded convection over eastern NS.  No impacts expected.

 

Newfoundland…

A chance of embedded convection this afternoon and tonight over NFLD. Locally heavy downpours possible.

 

 

Thunderstorm outlook for Day 2.

 

 

Convective Discussion

Some instability expected over southeastern NB Monday afternoon but a capping inversion at around 11,000 ft could limit the vertical extent and thus hamper convective activity.  If this capping inversion is less significant than currently expected then non-severe thunderstorms are possible.

 

Regional Impacts

Maritimes…

None at this time.

 

Newfoundland…

None.

 

Barrie MacKinnon

Program Supervisor – Operations Support

Atlantic Storm Prediction Centre

Meteorological Service of Canada

45 Alderney Drive, Dartmouth

Nova Scotia, B2Y 2N6

Weather - Environment Canada

(902) 497-9317

 

Surveillant de Programme - Soutien aux Opérations

Centre de prévision des intempéries de la région de l'atlantique

Service météorologique du Canada

45 promenade Alderney, Dartmouth

Nouvelle-Écosse, B2Y 2N6

(902) 497-9317

 

Convective Outlook Valid for Today and Tomorrow

Convective Discussion

Generally a chance of embedded/elevated thundershower activity today. A low pressure system with lots of cloud and rain will move across the region today and Sunday. Rain training is possible today as lines of convectively enhanced rain move across localities. Over southwestern NB, brief clearing skies are possible. If this in turn causes temperatures to rise much higher than currently forecast then surface based convection could occur. 40 kts of linear shear and Cape values of 500+ J/kg would result, giving the potential for gusty winds, downpours and small hail. On Sunday the low will give a chance of thundershower activity for eastern regions.

 

Regional Impacts

Maritimes…

A chance of thundershower activity today and Sunday. A slight chance of severe weather for SW NB today.

 

Newfoundland…

Nil.

 

Thunderstorm Outlook for Day 1

 

Thunderstorm Outlook for Day 2