Thursday, July 03, 2008

Thunderstorms possible Saturday evening

Hot weather will be returning to southern MB over the next couple of days with highs approaching 30 degrees Friday, likely reaching the low 30s on Saturday. Saturday will also see increasing humidity levels as southerly winds tap more humid air to our south and advect it north. This will set the stage for possible strong to severe thunderstorms to develop by Saturday evening over the Red River valley as a weak cold front approaches from the west. Behind the cold front, more comfortable conditions will return for Sunday with seasonably warm and less humid conditions.  

49 comments:

  1. Looks like the most significant severe weather threat so far this year. CAPE of 1000 to 3000J/kg, and impressive wind shear (in comparison to what we have seen so far this year). With a fairly strong cap I would expect storms to only fire off at 22 to 24Z (5 to 7 p.m.), and it looks like this late initiation will keep most of Western Manitoba out of play for this event. Definitely something to keep an eye on, as some tornadoes are possible!

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  2. Dewpoints progged to climb into the upper teens to low 20s across the Red River valley Saturday with southerly flow and evapotranspiration contribution. As Scott mentioned, should be well capped most of the day here in the valley, but cold front expected to erode the cap by 21-00z (4-7 pm) Given surface temperatures of 30-32C and dewpoints of 19-21c, progged soundings show CAPES of 3000-4000 J/kg. Narrow window of opportunity for storms to fire up.. could end up with nothing on it, or some big explosive storms to deal with... in effect, an all or nothing situation. Large hail and damaging winds will be the main threat with any storms given marginal directional shear, however given degree of potential instability, isolated tornadoes possible with any discrete supercells. Could be an interesting end to a hot and humid day tomorrow!

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  3. With the main upper low tracking well to our north for the first time this season.. we will finally tap into deep moisture and warmth at the surface. However with most of the large scale ascent targeting central and northern Manitoba... we are left with mainly surface convergence from the front.

    Judging from past events... it is difficult to break the cap over the red river basin in such cases. The extremely flat, featureless terrain does not provide any focus or trigger for convection to initiate. Sometimes convection is triggered right over the city (differential heating caused by urban infrastructure) or just to our east in upslope terrain (Sandilands/ Steinbach etc). Or if we had more of NW component to the upper flow.. we could tap into some lake breeze convergence or slope effects off riding mtn.

    As so often happens, the front will stall out to our southeast as the upper low loses definition. A potent but compact trailing shortwave will interact with very deep moisture/instability just to our south. Large MCS is likely to form over the Dakotas and Minnesota bringing copious rain to them. GFS advertises very cool 850 mb temps over us and with debris cloud present... highs could struggle to reach the low 20's by early next week.

    Seems like this pattern has occurred frequently in recent years... with a dry frontal passage for us ... and widespread precip as the front moves down into the midwest. Interesting because it is more a question of dynamics than moisture. Let's see what happens this time ^.~

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  4. My conclusion is "If the cap breaks look out!!"

    I personally don't think the cap will have any trouble breaking. SPC has issued a slight risk, and I think the most favourable shear is farther North anyway. This situation looks quite dangerous. If storms can become surface based early, they could easily produce tornadoes once they get around Winnipeg, and slightly farther East. A strong jet streak will give good speed shear increasing from 35 to 45 to 60 (850 - 700 - 500mb). Directional shear is only present in Eastern Manitoba. Helicity also looks good in Eastern Manitoba. CAPE of 1000 to 3000J/kg (highest CAPE in and around the RRV).

    Looks most hazardous in South-Eastern Manitoba (good for me!). The front is going to be in very good position for early evening initiation just West of Winnipeg, and with those strong upper-air profiles storms should really make good time moving East. I hope Environment Canada doesn't downplay this event, as it could end up being very dangerous.

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  5. Do you think all this cloud cover from all the decaying thunderstorms from last night will limit convection????

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  6. Not really...The temperature at my house is still rising at almost two degrees per hour, and there appears to be a number of breaks in the cloud cover which will give plenty of heat to get us into the high twenties.

    EC has issued a very positive outlook stating that last nights left-over convection may be the focus for new storm formation today.

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  7. Dew points climbing nicely across Red River valley this afternoon with values of 18-20C appearing. Debris cloud and brisk southerly valley wind keeping a lid on temperature rise.. but still seeing mid-upper 20s most areas as of 1 pm. Should be seeing max temps of 30-32C by late afternoon. Warm mid level temperatures will keep a lid on convective initiation until trof passes through.. possibly not developing until east of Winnipeg/Red River valley. Will have to wait and see what happens..

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  8. Line of convection starting to develop between Brandon and Portage ahead of surface trough line. Still could see some decent storms here in the Red River valley by this evening with dewpoints in the 20 degree range although if we could get some additional solar heating, things could get a little more interesting..

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  9. Convection along the cold front continues to get squashed by the cap. With so much debris cloud present and temps failing to reach 30 C ... it will be tough to break through the mid level warmth. However...models like th RUC still advertise the cap to break in early evening bringing in 750 MB temps of 8 C.

    There is also a potent lobe of vorticity extending down from the main upper low to provide PVA/ large scale ascent. Some models show extremely deep moisture getting drawn up with 850 MB Td's of 17 C (!!!). This could turn into more of nocturnal MCS event with widespread precip for NW Ontario. Let's see.

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  10. Severe thunderstorm watch issued for areas just east of the Red River.. not including Winnipeg. Watch indicates possibility of storms in Red River valley this evening, but greatest threat of severe weather is expected east of the river.

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  11. There is a tornadic cell just South of Devil's Lake...the cap is breaking!

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  12. talk about WOW

    from my area of the city, I can see what looks to be 3 supercellular cells-one to the SW, one to the NW, and one directly to my West. I am currently under the front of one of them, and got pictures!

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  13. some tornado watches issued now i see. is the threat basically over for winnipeg now?

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  14. Tornado Warning for as close as Lockport and Selkirk
    I 'm going to post some pictures of the supercells from my balcony and post them on Canadianweather.org

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  15. Yup, Winnipeg's threat is over now it seems.

    Though if you live in East Winnipeg, you have a great photo oppurtunity right now.

    I can see 3 supercells from my house, one with mamatus overhead!

    Watch out if you live East of Winnipeg! Very dangerous situation!!!

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  16. yeah, great view here in east transcona, some great mammatus with the setting sun...

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  17. Just got out of the movie theater.. and WOW.. what spectacular CB tops to the east! Mammatus with anvil level lightning! Radar shows 50-55K tops.. Steinbach area must be getting hammered!

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  18. Unfortunately not...

    I just called Environment Canada to report gusty winds (40 to 60km/h), heavy rainfall (7.2mm in about 20 minutes), and intense lightning.

    No hail at all, and winds didn't even meet criteria, quite surprising all things considered.

    I also contacted a friend in La Broquerie, and he said there were similar conditions there, but small small hail (5 to 10mm). No much to report, although the clouds looked quite ominous for a non-severe storm!

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  19. Has Winnipeg airport still waiting to reach it's first 30.0C reading for the year???
    Man it sure is taking longer than usual this year!
    And when I mean 30 C I not talking about 29.6 rounding up to 30!!!

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  20. Scott.. Radar shows strongest core of storm moved just south of Steinbach through the St Pierre-St Malo-Grunthal areras.. wouldn't be surprised to hear of large hail or severe wind gusts in those areas..

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  21. The cap remained too strong for a widespread severe outbreak. Cap only broke in localized areas and storms fizzled as they moved away from the forcing mechanism.

    The cell north of Winnipeg probably got triggered by a lake breeze boundary interacting with the cold front. The cells to the south were formed off the pembina escarpment possibly due to some kind of rebound effect or differential heating. A couple cells even tried to form on the east side of city.

    This reaffirms to me how difficult it can be to break the cap... particularly the RRV even if the models are going crazy with tons of cape etc.

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  22. I got some nice wall cloud pictures to post for tomorrow. Those storms remind me of the late August supercells from last summer. They formed in a line, and passed through similar areas (looked very similar on satellite too).

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  23. Interesting pattern for the next few days. Models bring down very cold air with secondary type cold front. 850 MB temps progged to be at 6 C by early tomorrow afternoon. Surface high will drop down into NW Ontario (going to be very cool morning on Monday for areas to our northeast) setting up a very tight thermal and moisture gradient just to our south.

    Another shortwave will pull back the moisture/instability by Monday. Models still hint at some kind of MCS riding the moisture and thermal boundary... could be a good set up for elevated nocturnal convection but for the lack of a well defined southerly LLJ. In any case .. cloud cover and cool temps will make for an almost fall-like period early next week.

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  24. I posted picture of the storm here:

    http://members.shaw.ca/sdk1/pages/pictures.htm

    That was just North of Mitchell, which is about 2 minutes West of Steinbach.

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  25. Rob!
    How much rain are the computer models showing for tomorrow????
    I know a thunderstorms can really boost amounts!
    Any chance of a few severe storms????

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  26. GEM is wettest with this system showing swath of 30-40 mm across Riding Mtns/Interlake areas Monday with another precip max area across Dakotas/nrn MN. In between they give Winnipeg and the Red River valley about 10-20 mm. NAM gives 10-15 mm for Winnipeg while GFS only has 2-3 mm for us, but heavier amounts over the southern valley. This is the problem with trying to forecast precip amounts with a convective system.. too much variability and uncertainty from run to run. Suffice to say that Winnipeg and the Red River valley in general could see 5-15 mm Monday, with heavier amounts of 20-40 mm possible in thunderstorm activity.

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  27. Weren't those Goldeyes something last night? :)

    All seriousness aside, I need a bit of education. I hear the word "cap" used often. Is it what it sounds like, a barrier which keeps plumes of rising moisture from go high into the high atmosphere? If so, what is this barrier, colder, denser air?

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  28. The "cap" is actually a layer of warm air aloft that inhibits an air parcel from rising into the upper levels and developing convective cloud. In order for an air parcel to continue to rise in the atmosphere, it must be warmer than the surrounding environment to maintain its buoyancy. Thus, the colder the air aloft, the greater the potential for an air parcel to continue to rise. If the rising air encounters a layer of air that is warmer then the air parcel itself, then the air parcel loses its buoyancy.. and the lifting process is stopped or "capped". This cap can act like a lid that suppresses convective development until the cap is broken (i.e. the warm layer aloft is cooled) When this happens, this can lead to explosive convective development, similar to what happens when you open a pop bottle that has been shaken.

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  29. I'll take a crack at this question...feel free to correct me Rob and others...

    First of all I would encourage you to visit Wikipedia, as it gives more detail than I or others probably will (it is more technical also, which might be a negative)...Here are those links:

    http://en.wikipedia.org/wiki/Convective_inhibition
    http://en.wikipedia.org/wiki/Capping_inversion

    I will use my favourite analogy for this...A lid! In most simple terms a cap is like a lid on the atmosphere. If the lid is on, none of the warm, moist air can escape...on the other hand if the lid is removed (or in some meteorological cases ripped off!), storms can erupt and quickly become severe. And to answer your other question, yes it is what is sounds like...a cap, or lid on the atmosphere. A cap can be caused by a couple things (this is where you might need to correct me)...Saturday I believe was mostly due to warmer mid-level temperature (700mb or about 10,000ft). Temperatures there were in the low teens, while temperatures between the surface and there didn't decline very quickly. Because of the generally small temperature change in such distance rising air stalled out part way, or didn't have a big enough push to go higher. This is explained in better detail in the capping inversion wikipedia page by the way...The other reason I can think of for capping is moisture difference. If the air doesn't become drier quickly by the mid-levels then the atmosphere may be more stable. For a very unstable atmosphere it is not only good to have a strong temperature gradient between low and high altitude, but also a good moisture gradient. Lots of moisture at the low-levels, and little moisture at the mid/high levels.

    Capping is measured in -J/kg, essentially the opposite of CAPE (if you don't know what CAPE is...its the exact opposite of CINH or capping). CAPE measures how unstable the atmosphere is in +J/kg...but that is another explanation. If the CINH is -250J/kg, for example, then severe weather is unlikely, because air cannot rise freely. On the other hand, if there is no CINH then a major severe weather outbreak may be looming.

    I hope that answered you question, you sound like you already have a general idea of what it is. Once again to others... feel free to rip apart that explanation as I am still learning too!

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  30. Sorry Rob,

    I was typing as you posted your comment...

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  31. Elevated MCS formed on cue in SE Sask/SW Manitoba. Has a decent SE LLJ to work with as it rides the thermal and moisture boundary. Right now it appears to be moving ENE.. corfidi vector method suggest the MCS could curve ESE.

    The low level jet will be nosing into the Winnipeg region by midday and depending on exactly where the best forcing/lift from shortwaves occurs.. could be in for some heavy precip.

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  32. Looks like we got the WRF solution for rainfall, with the heaviest precipitation sliding through Southern Manitoba and North Dakota.

    The WRF and RUC are also pointing towards the potential for severe storms in the Western RRV later this afternoon and evening. If sufficient clearing can occur to get highs into the high teens/low twenties we may see some storms fire along the occluded/cold front. The RUC has the most significant solution, with SBCAPE reaching 2500J/kg between Boissevein and Emerson. Lift Indexes of up to -10 are forecast by the RUC for areas in North-Eastern North Dakota...and all this with highs in the low twenties!

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  33. Line of convection starting to develop near Deloraine along cold front/dry line. Temps in SW MB have climbed into the low 20s with some sunshine along with dew points in the 17-18C range giving CAPES in the 2000+ range. Better shear south of the border, but sufficient shear together with degree of instability and frontal forcing may trigger some locally strong storms along the MB/ND border as the WRF suggests.

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  34. A storm looks to turn severe momentarily directly South of Brandon, sliding across the International border. I am afraid it might drift out of the instability axis, and therefore commit "meteorological suicide".

    NEXRAD in Minot is showing some possible rotation on the cell, and it looks well organized. If it can make it farther East without dying it will encounter an area of very high helicity values.

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  35. Tornado on the ground!! Around Turtle Mountain Provincial Forest!!

    That cell looks impressive, but will it sustain itself?

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  36. Things could get interesting for the Southern Red River Valley.. An area of clearing formed off the Pembina escarpment reaching just southwest of Winnipeg. Models suggest the the high Td's (surface: 18, 850 MB: 13) will continue to nudge up before the trough passes. High SB CAPE may sneak a little bit further north if trend continues. There is still one last blob of vorticity/lift that will rotate thru... so who knows.

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  37. I agree Daniel...

    The trough is now around Killarney, and will enter the RRV in the next couple hours. It is only taking temperatures in the high teens/low twenties to provide sufficient instability for these storms. At my house it is currently 18/16 (Temp/DP) and the temperature is rising fairly quick as that clear slot moves in. The RUC is pointing towards temperatures in the low twenties, with DPs in the high teens by early evening (in the RRV). As long as things aren't capped, we should see something fire up.

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  38. Damage is being reported with the tornado South of Deloraine. Cottages and boats along a lake there are reported destroyed.

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  39. That large tornado is now on the ground in North dakota according to the National weather service!!

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  40. Look like we will get some thunderstorms here in the city in the next hour or so as there is a small line of thunder developing as we speak!!!

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  41. Thanks for your explanations about "capping." Thinking about it now, it makes perfect sense, that warm air isn't going to rise in warm air! I've heard of CAPE before, but not CNIH.

    We're getting close to the third anniversary of a monster storm we had here on July 16, 2005. That was a scary one. I guess the CAPE values were high that day.

    I know Rob is a meteorologist, are you one too, Scott?

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  42. No I am not a meteorologist, just a weather enthusiast (hence the comment about my statements possibly needing correction)...

    I like Rob maintain a weather website, but that is about all.

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  43. For what it's worth, I measured 20mm of rain yesterday.
    Chris in Westwood

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  44. Only 7mm measured in East Transcona.

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  45. Here we go again....

    One last vestige of vorticity from yesterdays system will brush us to the NE on Wednesday. Look for garden variety showers maybe forming in the interlake and brushing by us as they move down into NW Ontario. The disturbance will have little moisture/instability to work with.

    Then attention will turn to Thursday night. Yet another upper low from the pacific will drop SE into the prairies. Again moisture and instability will be pulled to the NW. Looks like surface warm front will set up along Manitoba/N Dakota border on Thurs nite. Good LLJ linkage tapping deep moisture will intersect warm front.

    First guess...convection may initiate in SE Saskatchewan late Thurs and congeal into an elevated MCS that will ride east just north of the warm front. Convection could be severe as 850 Td's are progged at 15 C.

    To add insult to injury.. we might get clipped by the deformation axis under the upper low (steady rain) while our neighbours in the midwest bask in a steam bath of 24 C td's. I have 'active weather' fatigue... will a quiet summer like regime ever take hold this year?

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  46. Today looks like there will be a threat for isolated strong wind gusts/large hail/cold core funnels, as per the PASPC discussion. Perhaps a similar situation to a week ago, where numerous storms formed and produced tornadoes in a weakly unstable atmosphere?

    Thursday night looks very interesting for Southern Manitoba. A cold front/dryline/occluded front will slide through South-Eastern SK and South-Western MB. This will happen in the late afternoon/evening likely triggering severe storms. As the night wears on MLCAPE constantly between 2000 and 2500J/kg will follow the front East. Severe storms from South-West MB will likely maintain intensity as they sweep East. By the way, tornadoes will be possible with these storms given the helicity over 400m^2/s^2.

    Then storms will continue to maintain their intense figure as they move through the RRV. MLCAPE will stay about the same, with isolated readings close to 3000J/kg. Helicity will increase further to exceed 600m^2/s^2 across the entire valley. As the warm front punches into South-Eastern Manitoba late Thursday/Early Friday, storms will continue. MLCAPE will stay above 2000J/kg, and helicity of over 800m^2/s^2 will overlap the entire warm sector...with isolated helicity reading approaching 1000....

    Right now the only limiting factor I see is capping, however given the extreme instability and intense storms moving East I am not convinced capping will be an issue. If storms remain surface based as they move East overnight, there will be an extreme tornado risk, given the crazy helicity values and good shear.

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  47. Capping will be an issue south of the warm front overnight Thursday. 700 MB temps of 12 - 14 C will poke into extreme S Manitoba as the 700 MB thermal ridge builds in. That is why this is looking like an elevated event north of the front.

    There certainly is good directional shear/helicity to work with (see my post at canadian weather) but it will occur in an environment of elevated instability (rooted at ~ 850 MB) so LCL's will be high. Elevated storms can still be very severe and with 850 MB Td's in the mid to high teens there could be some rough cells.

    A few cells might remain more surface based and 'ingest' lower LCL's right along the warm front so you cannot rule an isolated tornado. But such storms will be hindered by a robust cap.

    The best chance for surface based convection to me looks like SE Saskatchewan and extreme SW Manitoba on Thursday afternoon and Central/Southern Minnesota on Friday afternoon.

    On Thursday.. I would watch the junction between the surface low, inverted trough and warm front. 700 MB thermal ridge will be south of the border with temps of ~ 8 C so cap has decent chance of breaking. Lots of SB CAPE to play with (1500-200 J/Kg). Looks like the cold front will not come into play as the warm sector will stay just south of the international border.

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  48. Yep.. looks like another severe weather threat tomorrow evening. I've started a new blog post for discussion on this event. Good dynamics but moisture may be limited for a good part of the day before things can get going. This would be a scarier setup if we were looking at more typical summer heat and humidity values than we're getting this year.

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  49. "I like Rob maintain a weather website, but that is about all."

    Thanks Scott. Well, all I can say is you've reached a new level in "weather enthusiast" because you sound like you're up with the pros.

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