Elsewhere in Weather News: May 18th, 2013

Matthieu Leave a comment

Severe Weather Event South of the Border: Possible AWM Chase Sat/Sun

Trough

Negatively-tilted trough on Saturday late afternoon. (NAM) (Map source: Twisterdata)

The same system that will cause Southern Manitoba to have a rainy May long weekend is expected to produce a string of severe weather days across the Central US, starting today, continuing through at least Tuesday. A negatively-tilted longwave trough approaching from the west will help to provide the necessary wind shear, lift, and instability for severe storms to develop. A broad surface low with an extending cold front from Central South Dakota through Nebraska will be in place with a warm front slicing through north-east South Dakota. Another, stronger, low pressure center will be in place in Kansas with an extending dryline all the way down to Texas. These features will offer enough lift for severe storms on Saturday. This, combined with other severe weather ingredients such as a stiff low level jet (LLJ), high amounts of instability, deep low level moisture and low cloud bases is ideal for severe weather and tornadoes. The Storm Prediction Center has issued a Moderate Risk for Nebraska and Kansas with a Slight Risk extending up to North Dakota for their 2 Day Outlook as of Friday night.

Surface Analysis

Surface analysis of the Plains. (Map source: Twisterdata)

As of Friday evening these ingredients appeared to be maximized in the vicinity of north-western Kansas near the Nebraska border. Storms will not be confined to this area though, as strong to severe storms will still be possible into South Dakota and down into Oklahoma, but whether they will be tornadic or not is still to be seen.

The ingredients that will be in place tomorrow in South Dakota:

  • Instability: about 2000J/Kg of MLCAPE (mixed layer)
  • Surface dewpoint: 18°C
  • Shear: 40 knots (0-6km shear)
  • LLJ: 20 knots
  • Lifting condensation levels (cloud bases): around 750m

With the LLJ being only 20 knots, an AWM Chase is still up in the air. If models tend towards a stronger LLJ, the chase will likely be on with a target somewhere in Central South Dakota.

Sunday also offers a significant severe weather threat as the trough continues its trek towards the east. Severe weather risk would extend into Southern Minnesota all the way down to Oklahoma. There is still some uncertainty about Sunday but it does look like another significant severe weather outbreak day for the Plains and Upper Mississippi Valley. Stay tuned in the comments section as the event approaches, updates will be posted.

AWM Chase is On!

Two of the AWM team members will be out chasing this setup along with a student from the University of Manitoba. You can keep up with their chase right here:

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Unsettled Weekend Ahead

Brad Leave a comment

An unsettled weekend lies ahead of us as the atmosphere slowly organizes itself into a major low pressure system to start next week.

North America Satellite Image - Annotated

North America satellite image showing the intense long-wave trough developing over the Rocky Mountains and associated convection with shortwaves pushing northeastwards into the Plains/Prairies. Orange arrows depict the jet stream.

We’ll see several batches of showers/thundershowers over the next few days before a significant area of rain develops through the Dakotas may push into Southern Manitoba for the start of next week. How stormy is it going to get? Lets take a look…

Today & Tonight

Friday

20°C
A few morning showers then mainly cloudy with a chance of afternoon showers.

Today will bring mainly cloudy skies over most of Southern Manitoba ahead of a weak shortwaves ejecting northeastwards from Wyoming towards north-central North Dakota which will be coupled with a decent 500mb jet streak. There may be a few light showers over southwest Manitoba or the Red River Valley this morning as some scraps of overnight convection drift in from North Dakota, then the next slight chance of precipitation will come this afternoon.

Whether or not the afternoon convection materializes is very questionable; if it does, it would likely be driven by a phenomenon called jet coupling1 as the nose of a 70-80kt jet pushes into Southern Manitoba from Wyoming/North Dakota catches up to a weaker 40kt jet sliding across the Interlake. The most likely area for showers would be south of the lakes in the Red River Valley and west towards Brandon and the Pilot Mound area. If any showers form they’ll be relatively weak and short-lived as most convective parameters aren’t all that favourable. We’ll climb up to around 20°C today.

Friday Night

12°C
Cloudy. Showers or thunderstorms possible overnight.

Things will begin to get more organized tonight as the first impulse pushes into Southern Manitoba. An area of showers and thundershowers should develop early in the evening through Montana and North Dakota and advect east-northeastwards into Southern Manitoba. The bulk of the system looks to track along the international border but will extend northwards towards Portage la Prairie & Winnipeg.

There’s uncertainty to exactly how far north these storms will push; the Trans-Canada corridor (including Winnipeg) may miss out on all of the precipitation or may end up seeing the bulk of it if things set up even 50-100km further north. It’s a fairly sensitive situation that we’ll be monitoring and providing updates below if necessary.

The precipitable water values are expected to rise to around 25-30mm tonight, which means that while the storms are not expected to really even approach what would be considered a severe thunderstorm, they may have the potential to produce quite a bit of rain, perhaps locally as much as 1 to 1.5” (25-35mm), in short periods of time under any heavier showers that may develop.

Saturday

Saturday

18°C / 11°C
Cloudy. Showers likely.

On Saturday we’ll likely see leftover convection pushing into the Red River Valley maintained by a weak low pushing northeastwards out of North Dakota. Whether or not this happens will depend on the low being able to initiate and support convection on it’s northern/northwest quadrant overnight, so it’s by no means a sure bet. If it does develop that convection, we’ll likely see some showers/rain in the Red River Valley through the day. Any left over precipitation will pull out of the region by Saturday evening at the latest with up to 10-15mm of rain falling through the day through the RRV and the Whiteshell. Mostly cloudy skies will remain through Southern Manitoba on Saturday night with temperatures dropping to around 10 or 11°C.

Sunday

Saturday

18°C / 11°C
Cloudy. Chance of heavy rain or thunderstorms…or not.

Sunday is a massive wildcard. What up until now has looked like a slam-dunk significant rainfall event for Southern Manitoba has been thrown into question by the latest long-range model runs which have pulled all the precipitation further south into the United States. Interestingly enough, it’s for different reasons:

  1. The GDPS2 pulls the precipitation further south due to a rather bizarre closed high it develops at 500/250mb that interacts the the long-wave trough in really, really bizarre ways. I’ve never quite seen anything like what the GDPS is doing with the upper flow and would say that I would err on the side of the past 4-5 runs which have all, definitively, placed the precipitation further north into Southern Manitoba. Importantly, the position of the main upper low has not changed with the latest GDPS run; it’s actually moved it a little east and a little further north, emphasizing the impact of the closed high it’s developing.
  2. The GFS3 is also pulling the precipitation further south, not because of bizarre acts of physics in the upper-levels, but for a rather boring reason: it’s faster and more progressive with the system. Overall it is now producing a little less precipitation than before and shifting everything further east. Instead of the deformation zone (which marks the northern/western extent of the precipitation) laying west-east through the southern Interlake region, it has the deformation zone laying along a line from Bismark, ND through Sprague, MB.

If the models are right about the rain staying to our south, then I’ll definitely believe it’s for the GFS’ reasons before I give the GDPS honours for a correct forecast. If they’re wrong, though…

If the previous runs of the models turn out to be correct, then we’d see heavy rain or thunderstorms push up into the Red River Valley on Sunday afternoon, pushing as far north as the southern Interlake by evening. The precipitation would then stall out and slowly spread east and westwards across Southern Manitoba overnight. In this scenario, it would also be likely that there would be some embedded thunderstorms on Monday night.

The Start of Next Week

…will play out in one of two ways:

  1. We’ll see heavy rain through the first day or two of the week as multiple shots of precipitation wrap across Southern Manitoba.
  2. We’ll see mainly sunny skies as the deformation zone of this system sets up through North Dakota, locking the precipitation up to our south.

It’s simply too early to tell which will come true; again, we’ll update below when things become a little more clear.

  1. When looking at jets in the atmosphere, meteorologists will look towards the left side of the nose of the jet and the right side of the tail of the jet. These regions are areas of atmospheric lift which can be very vital in forecasting convection. Jet coupling occurs when two separate jets organize in such a way that the right side of the tail of one and the left side of the nose of the other cover the same area. When this occurs, the lift can be increased substantially. ↩
  2. GDPS stands for the Global Deterministic Prediction System; it’s Canada’s long-range forecast model. ↩
  3. The GFS is the Global Forecast System, the US’ long-range forecast model. ↩

A Mixed Bag in Store

Brad Leave a comment

Sunny skies will give way to clouds and a few scattered showers by midday today, but the threat will be short-lived as we move into a more stable, settled pattern for a brief reprieve from the wild weather of late. The calm weather won’t last too long, though, as more showers look to push into southern Manitoba on Friday.

Wednesday

24°C / 8°C
Becoming a mix of sun & cloud with a chance of midday showers.
Thursday

24°C / 12°C
Sunny
Friday

18°C / 12°C
Showers likely.

Today & Tomorrow

NAM Sounding for CYWG valid 18Z Wed May 15, 2013

Sounding for Winnipeg from the NAM valid at 18Z today.

We’ll see a sunny start to the day today, but as we warm up some convection will begin to bubble up by late morning. Widespread showers are not expected; while some marginal instability will be realized, moisture will be a limiting factor and increasingly so through the day. A few scattered showers will likely develop along a trough line extending from Swan River southeastwards through Lake Manitoba and the Red River Valley midday and then die out in the early afternoon as any remaining available moisture mixes out. We’ll climb to a high of 24°C and drop down to around 8°C overnight under clear skies.

Thursday will be a beautiful day as we sit under the influence of a weak ridge. We’ll climb to a high around 24°C again with an overnight low of only around 12°C with increasing cloudiness overnight ahead of the next weather system.

Friday

The main weather story on Friday will be an area of rain or showers that moves into Southern Manitoba ahead of a shortwave pushing through Saskatchewan. An area of showers with possible embedded thunderstorms will develop through western North Dakota and spread northwards on Thursday night into SE Saskatchewan and western Manitoba along a 850mb low-level jet (LLJ). The LLJ looks to maintain it’s strength or even strengthen through the day on Friday, which should make it able to maintain the area of precipitation as it spreads eastwards.

At this point, it looks like showers will push into SW Manitoba overnight on Thursday night and spread into the Red River Valley on Friday morning. The showers should push out of the Red River Valley by mid-afternoon after producing, in general, around 5–10mm of precipitation. We’ll be left with a chance for some lingering showers in the afternoon/evening.

The Long Weekend

Continuing in a long and storied history of May long weekends in Winnipeg, the weather does not look very good for this coming weekend. As the showers push through on Friday, it looks like a potent weather system will be busily organizing itself and preparing to deluge Southern Manitoba.

850mb theta-e values showing a southerly conveyor belt of gulf moisture setting up for the weekend.

A massive upper trough is forecast to move onshore from the Pacific Ocean this weekend, setting up a deep southerly flow that will transport large amounts of Gulf moisture northwards through the Great Plains into the Southern Prairies. A series of shortwaves is forecast to produce multiple shots of moderate rain and/or nocturnal convection. It’s still too early to get into specifics, but at this point it’s looking like it will rain over a majority of Southern Manitoba on Saturday, Sunday and Monday.

Different models currently have vastly different solutions: the GDPS[1] is forecasting absolutely unbelievable storm total precipitation amounts of anywhere from 5–10“ to upwards of 30”. This is impossible but does emphasize both the strength of the system and the amount of convection the GDPS is seeing. The NAEFS[2] is producing around 1–2“ of rain for most of Southern Manitoba. The GFS[3] is producing 2–3” of rainfall, but focused more over SE Manitoba with amounts diminishing westwards through the Red River Valley.

All in all it looks to be quite a rainy weekend. There are still plenty of details to be determined and we’ll take a much closer look at it for Friday’s post. We’ll leave a comment or two below with any interesting developments before then.

  1. Global Deterministic Prediction System (Canadian Long-Term)  ↩
  2. North American Ensemble Forecast System  ↩
  3. Global Forecast System (U.S. Long-Term)  ↩

Warm but Unsettled

Brad Leave a comment

This week will be warm, but somewhat unsettled. A fairly potent weather system will bring rain and possibly thunderstorms to Southern Manitoba on Tuesday.

image

A low pressure system will bring unsettled weather to Southern Manitoba on Tuesday

Monday

Monday
image
Showers in the morning, then becoming Mainly Sunny.
27°C / 13°C

Monday looks to be the nicest day of the week as we potentially experience our warmest day of the year so far. However, before that happens there may be some shower activity this morning along an approaching warm front. Once this front passes skies will clear and temperatures will begin to rise. High temperatures are expected to be in the mid or upper twenties, with skies becoming mainly sunny by the afternoon.

Tuesday

Tuesday
image
Mainly cloudy. Periods of Rain.
20°C / 6°C

Tuesday will be a rather unsettled day in Southern Manitoba. A strong low pressure system will bring rain and perhaps a few non-severe thunderstorms to the southern-most part of the province. The Red River Valley will be in the warm sector of this system in the morning, allowing temperatures to climb up to around the 20C mark before a cold front goes through later in the morning/early in the afternoon. There is a chance we may see some rain or a weak thunderstorm in the morning, but generally speaking the main area of precipitation is expected to pass to our north in the interlake area. Up north of Winnipeg heavy rain will be possible as a stream of moisture is lifted by this system and forms an area of rain. There may be some embedded thunderstorms within the larger area of rain, but they will not be very strong. The Red River Valley may see some wrap around rain on Tuesday night as the area of precipitation over the interlake slumps south. Precipitation amounts from this system will be nearly impossible to predict due to the potential for embedded thunderstorms. In Winnipeg we should see somewhere in the range of 5-10mm, unless a storm goes through in which case we’ll get more. North of Winnipeg upwards of 25mm is possible, but more precise estimates are too difficult to attempt.

Wednesday

Wednesday
image
Mix of Sun and Cloud
20°C / 7°C

Wednesday will see a return to more settled conditions as Tuesday’s system departs. Temperatures will be around the twenty degree mark with a breezy west wind.

Long Range

The rest of the week looks fairly seasonal temperature-wise, with perhaps a chance of light rain on Thursday and/or Friday. A first glance at the long weekend forecast reveals a chance of rain and near seasonal temperatures…but based on past experience, rain on May long weekend is usually a strong possibility…though I probably didn’t need to tell you that!