Tag: Thunderstorms

Elsewhere in Weather News: June 1st, 2013

Matthieu Leave a comment

This past week featured a number of severe weather events across the Great Plains of the United States. At least 98 tornadoes had been reported since Monday as of Friday evening.

image

Extreme Rotation was Observed as a Supercell Thunderstorm Approached Oklahoma City on Friday

This series of days with severe thunderstorms and tornadoes was caused by similar atmospheric conditions to what was in place around the time of the Moore, OK tornado. These conditions revolve around four properties that forecasters look for when assessing the potential for thunderstorms, the properties being moisture, instability, shear, and a triggering mechanism. Over the past week copious amount of moisture were present over the US Plains, noted by very humid conditions at the surface. These hot and muggy conditions caused the lower atmosphere to become very unstable generating very high levels of instability. A large dip in the jet stream, called a longwave trough, came onshore from the Pacific Ocean earlier this week putting the plains under a strong jet stream causing wind shear. And finally a series of fronts and drylines offered triggering mechanisms for storms throughout the week. These four ingredients came together perfectly to generate many severe and indeed tornadic storms all over the United States.

The week was capped (no pun intended) off on Friday by a complex of supercell thunderstorms that passed through Oklahoma City causing widespread damage and at least 5 deaths. The rotation in these thunderstorms (seen as the bright colour in the image above) has been referred to by many meteorologists as the strongest they have ever seen. In addition to several tornados touching down in the Oklahoma City Metro area, hail as large as softballs pelted down in some areas while nearly the entire OKC metro area received between 6 – 10” of rain causing widespread flooding with water as deep as 4’ in some places. Flash flood warnings continue this morning for much of eastern Oklahoma state with widespread overland flooding ongoing.

Get Ready For More Wet Weather

Brad Leave a comment

Southern Manitoba is on tap for another complicated significant precipitation event for the second half of the week as another strong low stalls over North Dakota. With a good moisture feed northwards from the Gulf of Mexico, significant convection will fire up through the Northern Plains and wrap around the low into Southern Manitoba. If this sounds like last week’s system…well, it is. Scott had a keen eye to mention it in Monday’s post and this system certainly has the potential to have a significant impact on some communities in the Red River Valley, especially those near the western escarpment who were deluged with up to 8 inches of rain on the May long weekend.

So just how much rain will fall and when? Read on to find out.

Today & Tonight


20°C / 14°C
A mix of sun and cloud. Increasing cloud overnight.

Today will be a fairly nice day with a mix of sun and cloud as we push to a high of around 20°C. There may be some fog patches through the Red River Valley this morning – and some may be dense – but they should burn off by mid-morning. Winds will remain fairly light out of the west at around 10–15km/h.

Clouds will begin to push into the Red River Valley from the south overnight as thunderstorms fire up in North Dakota. There may be a scattered shower or two overnight, but the main area of rain should stay south of Winnipeg until Thursday morning.

Thursday & Friday

The main event begins Thursday morning as rain pushes up from North Dakota into Southern Manitoba. This area of rain may have thunderstorms embedded in it and will likely produce moderate accumulations as it pushes northwards. This band will continue to push northwards until it’s fully developed along the deformation zone resting along a line through the Manitoba Parkland and Interlake regions. It will likely rain through most of the day across all regions of Southern Manitoba before tapering off Thursday night. The temperature, for those who still like to know despite it raining all day, should climb to around 19–20°C.

Thursday

20°C / 14°C
Rain beginning in the morning & tapering off overnight. 15-25mm generally.
Friday

20°C / 9°C
Rain or thunderstorms beginning by midday, tapering off early Saturday. 10-20mm generally.

It’s likely we’ll get a 6–12 hour reprieve from the rain sometime overnight Thursday into Friday morning; the exact timing of this break will depend on two particular things:

  1. When the moisture first arrives in Southern Manitoba
  2. The exact placement of the low pressure centre in North Dakota.

It may happen a little bit earlier or later, and if the low ends up further south than currently forecast, the break may not occur for places in the Northern Red River Valley, just the south. But at this point it does look likely that we’ll see a reprieve from the rain for a little bit.

On Friday morning the rain will start up again as significant instability develops along a trowal[1] laying across the Red River Valley. There’s a decent chance that we’ll see a thunderstorm or two; whether or not they’ll be severe is too early to tell, but there may be a threat for hail or even an isolated, weak tornado formed from the steep low-level lapse rates and vorticity rich environment near the upper low. At the moment, it looks like the best bet for any thunderstorm activity would be close to or just south of the U.S. border, but we’ll keep an eye out and post updates below. The rain/thunderstorm activity will gradually move southwards and eastwards through the day and night. It won’t be until Saturday morning that all the precipitation leaves the province as the upper low finally pushes eastwards.

The Convective Element: Heavy Rain

The biggest unknown right now is exactly how convective things will be. As mentioned above, the storm-total rainfall accumulations for this event will likely sit around 30–50mm (1.5–2”) for most areas in Southern Manitoba. Any convection or thunderstorms have the potential to dramatically increase that amount as does any upslope flow along the escarpment.

Storm Total Precipitation

Estimated storm-total precipitation by Saturday morning.

Two areas of concern exist. First, over the western Red River Valley and SW Mantioba northward into the Interlake; as mentioned before, generally 30–50mm are expected, but there’s potential for significant embedded convection to exist within the area of rain, and localized accumulations of much greater than that are possible. At this point, it looks most likely that the worst-case scenarios would top out at around 3“ total accumulation (75mm) by Saturday morning, but this early it’s impossible to say. The second area of concern are for the upslope environments of the western escarpment of the Red River Valley and Parkland regions. A strong north-easterly wind will be in place through the duration of this event which will provide additional lift and enhanced precipitation to communities close to these features. With the last system, which was similar to this one in many ways, rainfall accumulations were about 5–8” along the escarpment, depending on location. Any people living in regions that were hit hard by the last system may want to make preparations for another significant rainfall event. Again, at this point it’s only safe to say that more than 4” is possible in these locations, not that they will for sure get higher amounts.

We’ll keep a close eye on this system as it develops. Being convective in nature it will be inherently complex, but we’ll provide updates below. Live in Southern Manitoba and have a question about this system? Ask in the comments below and we’ll be happy to try and get an answer for you!

  1. A trowal is a Trough of Warm Air Aloft and is an mid-to-upper atmospheric feature that wraps from the fronts of a low around the northern side of the low and commonly produces precipitation.  ↩

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. ↩

Cool Blast Across Manitoba

Brad Leave a comment

A powerful low pressure system moving through the Interlake will bring significantly cooler weather to Manitoba over the next couple days. Read on to find out where the rain will be and how cool it’s going to get.

GFS Forecast for this evening

GFS 850*mb* temperature forecast for this evening. I’ve drawn on the strong high over the Western Prairies that is helping our low pressure system pull down cool Arctic air. Cold front and warm fronts have been drawn in blue and red, respectively.

As the low pressure system passes to our north today, it will drag a cold front through the Red River Valley over the course of the afternoon. Preceding this front will be an area of rain, and while a majority of the thunderstorm activity will be through the Interlake, closer to the low, there will likely be some isolated embedded thunderstorms through the Red River Valley.

Accumulations will generally be higher the further north from the international border you go; while up to 25-50mm will likely fall through the Interlake, only 2-5mm are expected over the southern RRV. Any location in the RRV that sees a thunderstorm could easily receive between 0.5 to 1” of rain. Here in Winnipeg, we’ll probably see between 5-10mm with some locally higher amounts if a thunderstorm rolls through by the end of the day.

On the backside of this system a strong northerly wind at all levels will drag down the most potent shot of cool, Arctic air we’ve seen in quite a while. While winds will be out of the south in the morning, gusty northerly winds will kick in quickly behind the cold front, with sustained speeds between 40-50km/h and gusts as high as 60-70km/h. 850mb temperatures approaching 0°C will push down into the Interlake region tonight. This will bring with it the chance for the first significant lake-effect shower event of the “fall”1 season. When looking for the generation of lake-effect showers, there’s a few things to look for:

  1. A temperature difference from the surface to 850mb of at least 13°C.
  2. Less than 60° of wind shear between the boundary layer and 700mb; preferably less than 30°.
  3. At least 100km of fetch which the air travels over open water.

Given that lake temperatures are still sitting at 22-24°C, we’ll certainly see plenty of clouds develop and move south off the lakes. Whether or not lake-effect showers develop will depend precisely on the winds, but it looks fairly favourable right now with winds forecast to be from the NW with minimal directional shear. We’ll likely see some streamers develop overnight, providing showers or drizzle to communities in lee of the lakes. Current forecasts put these narrow bands of precipitation between Portage & Winnipeg and just east of Winnipeg, but we’ll have to see what the actual wind direction ends up being as to where they’ll go. Right now it looks like there won’t be enough of a westerly component to the wind to bring them into Winnipeg.

If the surface winds end up too westerly, or the 850mb temperatures don’t cool off quite as much as forecast, then no streamers will form. It will be interesting to see what happens!

In the end, it could certainly be worse than it’s going to be. The coldest air will be over us during the overnight period, and we’ll likely have cloud cover as well which will help keep things a little warmer. By the time Thursday afternoon rolls around, the cold air will already be exiting the region, so we won’t end up dealing with temperatures quite as cool as many other areas across the Prairies will.

  1. It’s not fall yet! ↩