Early May (1st-7th) Severe Chances

[Stebo]

Figured I would start this thread as SPC has put out a day 5 risk area for a decent portion of the region, see below:

With the following discussion:

MODEL CONSENSUS IS THAT UPPER PATTERN WILL HAVE TRANSITIONED TO A

QUASI-ZONAL PROGRESSIVE BY TUESDAY /DAY 4/. MODIFIED CP AIR WILL

GRADUALLY RETURN NWD THROUGH THE PLAINS...MS AND OH VALLEYS BENEATH

MODERATELY STRONG FLOW ALOFT. POTENTIAL FOR A HIGHER COVERAGE OF

SEVERE STORMS MAY EVOLVE OVER ERN PORTIONS OF THE NRN PLAINS INTO

THE UPPER MS VALLEY DAY 4 DOWNSTREAM FROM AN EJECTING SHORTWAVE

TROUGH. HOWEVER...MODELS DIFFER ON AMPLITUDE AND TIMING OF THIS

FEATURE WITH THE ECMWF BEING SLOWER AND EJECTING THE IMPULSE NORTH

OF THE MORE UNSTABLE WARM SECTOR. THERE IS ALSO UNCERTAINTY

REGARDING HOW EARLY WARM ADVECTION STORMS WILL AFFECT NWD BOUNDARY

LAYER RECOVERY.

SEVERE THREAT WILL LIKELY SHIFT A LITTLE FARTHER EAST TOWARD THE

GREAT LAKES AND OH VALLEY WEDNESDAY /DAY 5/ AS ANOTHER IMPULSE MOVES

THROUGH THE TROUGH AND STORMS DEVELOP ALONG AND JUST AHEAD OF A COLD

FRONT SUPPORTED BY A MOISTENING WARM SECTOR.

This to me right now looks more like a wind damage threat than anything, the wind fields are unidirectional . However there is decent speed shear and a good amount of instability that will be moving Northward into the region starting on Tuesday and into Wednesday.

DTX's thoughts on the potential

A STRONGER LOW PRESSURE SYSTEM WILL SLOWLY TRACK THROUGH SOUTHERN

CANADA/NORTHERN GREAT LAKES REGION DURING THE MIDDLE TO LATE PART OF

THE WEEK. THIS LOW WILL PULL WARM AIR AND GULF MOISTURE INTO THE

CENTRAL GREAT LAKES REGION DURING THIS PERIOD. WHILE THIS WILL MEAN

HIGH TEMPERATURES WILL WARM INTO THE 60S ON TUESDAY...WITH 70S

EXPECTED WEDNESDAY THROUGH FRIDAY...IT ALSO MEANS A CHANCE FOR

THUNDERSTORMS DURING THIS PERIOD AS SEVERAL SHORTWAVES TAKE

ADVANTAGE OF THE WARM MOIST AIR ADVECTING INTO THE REGION.

WHILE TRYING TO PIN DOWN SEVERITY OR TIMING OF THE STORMS IS

PROBLEMATIC THIS FAR OUT...CURRENT MODEL RUNS INDICATE THE STRONGEST

STORMS SHOULD OCCUR WEDNESDAY AFTERNOON/EVENING AS MLCAPE VALUES TOP

OUT TO AROUND 1500 J/KG WITH DECENT SPEED SHEAR AND THE BEST

DYNAMICS AS A VIGOROUS SHORTWAVE TRACKS THROUGH THE NORTHERN GREAT

LAKES WITH A 100 KNOT JET STREAK ACROSS THE NORTHERN GREAT LAKES.

HOWEVER THE SOUNDING IS FAIRLY UNIDIRECTIONAL WITH PWAT VALUES

AROUND 1.5 INCHES AND LONG SKINNY CAPE...WHICH WOULD INDICATE THE

STORM MODE WOULD BE LINEAR WITH HEAVY RAIN. DESPITE THIS SETUP THE

TIMING AND NATURE OF THESE FEATURES COULD CHANGE SIGNIFICANTLY WITH

SUBSEQUENT MODEL RUNS.

GRR's thoughts, highlighting some uncertainty on timing which I agree with. I think there could be chances both Wed and Thurs.

THE MAIN CHALLENGE DEALS WITH THE TIMING AND STRENGTH OF THE

THUNDERSTORMS NEXT WEEK.

A SOUTHWEST FLOW OF WARM AND MOIST AIR WILL DEVELOP MONDAY AND

CONTINUE THROUGH TUE. PWAT VALUES CLIMB TO AROUND 1.5 INCHES...WELL

ABOVE NORMAL VALUES FOR THIS TIME OF THE YEAR. A LLJ ARRIVES WED AS

INSTABILITY BUILDS. THIS SHOULD LEAD TO FAIRLY WIDESPREAD

THUNDERSTORM DEVELOPMENT. A SURFACE TROUGH PULLS IN FOR THU WITH

THUNDERSTORMS FORMING OUT AHEAD OF THIS FEATURE. THE GFS IS FASTER

THAN THE ECMWF WITH THIS TROUGH AND ASSOCIATED MID LEVEL JET

STREAK...IMPLYING THE MORE ORGANIZED STORMS WOULD BE WED NIGHT. THE

ECMWF SEEMS TO FAVOR THU FOR THE ARRIVAL OF THE MID LEVEL DYNAMICS.

FOR NOW WE WILL HOLD ONTO THE RISK FOR STORMS...MAINLY FOR WED INTO

THU.

MKX's thoughts

SURFACE LOW OVER DAKOTAS AND MINNESOTA TUESDAY AND WEDNESDAY WILL

KEEP SOUTHERLY WINDS AND WARM TEMPERATURES...WITH HIGHS NEAR 70

TUESDAY AND MID 70S WEDNESDAY AND THURSDAY. UPPER TROUGH WEST OF THE

REGION WILL ALLOW SOUTHWEST FLOW WITH UPPER LEVEL DISTURBANCES

MOVING IN SOUTHWEST FLOW ACROSS THE REGION AND BRINGING CHANCE OF

SHOWERS AND THUNDERSTORMS WEDNESDAY THROUGH SATURDAY. BEST CHANCE OF

STRONG THUNDERSTORMS TUESDAY NIGHT THROUGH WEDNESDAY NIGHT WITH BEST

CHANCE OF SEVERE WEATHER WEDNESDAY AFTERNOON WHEN CAPE...BULK SHEAR

AND OTHER SEVERE INDICIES ARE AT A MAXIMUM FOR THE ENTIRE WEEK.

ADDITIONAL UPPER SHORT WAVES WILL CONTINUE UNSETTLED WEATHER THROUGH

SATURDAY WHEN UPPER LEVEL RIDGE STARTS TO BUILD OVER THE REGION.

These fast moving zonal patterns can yield nicely when you get the instability to coincide with the ripples in the flow. Should be interesting to watch this one play out.

[IWXwx]

What a coincidence. I will be at the local Walgreen's Wednesday manning a booth on supplying information about weather radios and programming them for the public..

[SpartyOn]

D5 disco means business. There must be good model agreement.

[wxman1952]

Sounds good to me. I'm a bit north on the NE fringe, but I'll take anything other than this cold we've been in.

[ChiWxWatcher]

What are the main threats looking like currently?

[frostfern]

The possibility is there, but it's really hard to pinpoint the exact timing and location of convection in this kind of pattern.

There's often a diurnal cycle to MCS development superimposed on the timing of the triggering synoptic waves. Often a nocturnal MCS will form first on the nose of the initial 850 LLJ surge, being mostly elevated instability with mainly a heavy rain and isolated hail threat. Then subsequent severe convection will fire the following afternoon/evening on the southern outflow boundaries set up by the initial MCS.

You really gotta have the shear parameters and jet/synoptic forcing correlating well with the diurnal cycle to get a widespread severe outbreak. It seems to happen so often that great shear and instability is in place, but the jet streak will be in the wrong place with the right-exit or left-entrance, rather than left-exit or right-entrance region overhead, and as a result the cap remains largely in place. Or the jet dynamics might be perfect, but only after a previous nocturnal/morning MCS wiped most of all the instability.

In any case, with no major surface low development, just a series of waves rippling through the SW flow, I don't see a whole lot of directional shear for supercells, except possibly isolated ones near surface boundaries.

[Jim Martin]

From looking at the 12z GFS... just does not seem overly impressive of setup for Wednesday, at least from what I see.

[andyhb]

Unreliable NAM likely at 84 hrs out, but the 12z run shows some pretty decent parameters on Tuesday across a rather large section of the Upper Midwest.

[nwohweather]

Lots of instability with this one and with that strength in the jet we could see something solid if we can get a little twisting of the winds. Still I don't mind a derecho to kick off may

[andyhb]

SREF catching on at 87 hrs.

[WolfSpider]

frost you really nailed it. In summary it's all a matter of everything setting up just right, and on top of that, in the right places. As SPC is highlighting, the probability of that happening is as of now greater than 0.

The media is really picking up on the SPC outlooks the last several years, especially when there is a mdt on d3 down to d1, or in the case of our area, even a slght. I can't outright blame the media in general, severe weather in the extremes has been anomalously outside of the traditional tornado belt in the last decade. Plus now we have large numbers of the general public reading their local tv met's blog and following them on facebook, getting exposed to more technical discussion and build up to the chances for events. The media attention is good and bad. Good in that the public is in a general state of awareness, bad in that frequent busts result in negative conditioning. In this particular case, so much has to come together, all the way leading up to initiation on d0, that bust or verification won't really be able to be well forecasted until just before go time. Excessive media hype is to be expected - if model trends continue and SPC keeps us outlooked. I will be watching to see if the media makes any effort to say in laymans what you did - with a setup like this there is a chance, but no slam dunk, just stay aware.

First post here.

I agree, the media doesn't explain well enough about the model differences and how things can change. Sometimes they can even over hype stuff while the general public doesn't understand forecasting.

[Minnesota Meso]

Based on what I see on the 28/12z ECMWF and GFS, it seems to me that the severe weather threat will move north the next 7 days or so. It could impact the Upper Mississippi Valley on Tuesday and than move into the western Great Lakes on Wednesday, including southern WI and Northern IL. From there it could affect the central great lakes area south into the OH valley area. The ECMWF shows it also could impact the Upper Mississippi Valley area next weekend.

I am moderately surprised that more attention hasn't been paid to this potential threat on the various different threads. Seems strange to me how quiet the conversation's have been about this.

[WolfSpider]

Based on what I see on the 28/12z ECMWF and GFS, it seems to me that the severe weather threat will move north the next 7 days or so. It could impact the Upper Mississippi Valley on Tuesday and than move into the western Great Lakes on Wednesday, including southern WI and Northern IL. From there it could affect the central great lakes area south into the OH valley area. The ECMWF shows it also could impact the Upper Mississippi Valley area next weekend.

I am moderately surprised that more attention hasn't been paid to this potential threat on the various different threads. Seems strange to me how quiet the conversation's have been about this.

I've also noticed that; maybe people aren't talking about it because outbreak hasn't been mentioned?

[Geos]

I've been following the talk on this potential severe outbreak. Got caught up in the one today. I would say this area has a better shot at severe weather as opposed to that threat a couple weekends where the general area around La Crosse had a tor con value of 7 forecast. That was a total bust for the Upper Miss. River Valley. Thought there would have been severe weather further north this spring due to the earlier warmth, but this month changed around at this latitude. Last year, I didn't get a severe thunderstorm warning until May 22nd, which is a bit later then usual.

Usually the southerly and southwesterly flow severe threats have the best chance at coming to fruition around here.

[Minnesota Meso]

I've also noticed that; maybe people aren't talking about it because outbreak hasn't been mentioned?

The SPC has been looking at it the last two days or so.

[andyhb]

I've also noticed that; maybe people aren't talking about it because outbreak hasn't been mentioned?

I'd say it's more because we have been following the threats from the 26th to today for the past little bit.

[Minnesota Meso]

I've been following the talk on this potential severe outbreak. Got caught up in the one today. I would say this area has a better shot at severe weather as opposed to that threat a couple weekends where the general area around La Crosse had a tor con value of 7 forecast. That was a total bust for the Upper Miss. River Valley. Thought there would have been severe weather further north this spring due to the earlier warmth, but this month changed around at this latitude. Last year, I didn't get a severe thunderstorm warning until May 22nd, which is a bit later then usual.

Usually the southerly and southwesterly flow severe threats have the best chance at coming to fruition around here.

I agree, the last event was a bust, but there always a chance IMO of a north or north easterly wind affecting that system. This whole set up doesn't show that until the cold front arrives, this set up looks completely different than the last one. The greatest threat area is still very questionable at this time, but the overall pattern looks interesting indeed.

[BowMeHunter]

who's the goof chaser from the chicago area with a 2012 Jeep Patriot?

[Geos]

who's the goof chaser from the chicago area with a 2012 Jeep Patriot?

You see that on TV? I wonder if Thundersnow was out today.

[BowMeHunter]

You see that on TV? I wonder if Thundersnow was out today.

I didn't see nothing on TV. We recently purchased a Patriot and while I was doing research about them on a forum a chaser from the burbs of chicago was ****ting dumb all down his leg on that forum.

[Hoosier]

I'd say it's more because we have been following the threats from the 26th to today for the past little bit.

This.

I haven't really looked at the setup, but just looking at the CAPE and shear progs (gotta have that for severe weather after all) it looks like there is enough of both to suggest a possible substantial severe threat.

[Minnesota Meso]

I can't wait for twister data to come in, and I can't post what the GFS shows for 0z Wed, and 0z Thursday, but by 0z Wednesday the 60 dew point line almost gets to International Falls MN. By 0z Thursday the 65 dew gets all the way into northern WI. The precip charts are showing rain, so I assume the cap will break. This just got very interesting indeed. I really don't pay attention to the NAM untill 24hrs out of the event (regional model vs global model.) Very interesting indeed.

[Geos]

I can't wait for twister data to come in, and I can't post what the GFS shows for 0z Wed, and 0z Thursday, but by 0z Wednesday the 60 dew point line almost gets to International Falls MN. By 0z Thursday the 65 dew gets all the way into northern WI. The precip charts are showing rain, so I assume the cap will break. This just got very interesting indeed. I really don't pay attention to the NAM untill 24hrs out of the event (regional model vs global model.) Very interesting indeed.

Great... that means it's going to be unbearably humid down here! I know the EURO is showing convection down this way by early to mid afternoon.

[Minnesota Meso]

Great... that means it's going to be unbearably humid down here! I know the EURO is showing convection down this way by early to mid afternoon.

Yeah I know, I've been expecting this based on how warm the GOM has been since around Jan. or so.

[Geos]

Yeah I know, I've been expecting this based on how warm the GOM has been since around Jan. or so.

I find the NW and W flows during the summer the most pleasant!

NAM dewpoint data from Twister Data is showing 60° dewpoint line flirting with this area with a SE flow. 65° line barely makes it into SCentral MN for May 2nd.

[Minnesota Meso]

I find the NW and W flows during the summer the most pleasant!

NAM dewpoint data from Twister Data is showing 60° dewpoint line flirting with this area with a SE flow. 65° line barely makes it into SCentral MN for May 2nd.

I don't trust the NAM this far out...It's my least favorite model, lets see what the Nam shows when it's in it's wheel house...something like 6- 24 hrs before the event.

[andyhb]

I can't wait for twister data to come in, and I can't post what the GFS shows for 0z Wed, and 0z Thursday, but by 0z Wednesday the 60 dew point line almost gets to International Falls MN. By 0z Thursday the 65 dew gets all the way into northern WI. The precip charts are showing rain, so I assume the cap will break. This just got very interesting indeed. I really don't pay attention to the NAM untill 24hrs out of the event (regional model vs global model.) Very interesting indeed.

Need to watch for how much the sfc winds back/veer and how strong the LLJ is.

[Stebo]

When the day 4-8 risks get released later this morning, I expect the day 4 to be pushed West some, and a day 5 issued in close proximity to the current day 5, things have slowed up some today in the models. Thursday looks to actually be the better day now with respect to severe weather for the region.

[andyhb]

Lack of an area on the new Day 4-8 probably stems directly from the 00z Euro flattening the trough almost completely after Tuesday.

Actually, the part Dial mentions about model variability is probably about as obvious as it can get, all four models (Euro, NAM, GFS and CMC) have substantially different evolutions of the trough beyond D3. I can see problems with veering LL winds on Wednesday/Thursday and also with stronger verbatims like the NAM, the UL flow turns awry from the wave becoming too negatively tilted. There still is instability/moisture all over the place across the spectrum though.

On another note, seeing the primary belt of upper/mid level flow this far north in late April/early May is making me wonder whether the folks north of the border may see some more active svr wx patterns come later Spring/early Summer (or perhaps even earlier). Seeing the overall pattern has been somewhat reminiscent of 2007 at times only heightens this possible notion, and also my concern considering that much of my extended family lives in Portage la Prairie and Winnipeg (Which Elie is smack dab in between), also for Ontario possibly as well.

[Stebo]

GRR has a great writeup this morning as to what has changed that would explain the removal of the risk past day 3.

THE MAIN ISSUE IS THE POTENTIAL FOR CONVECTION MOST OF THIS FORECAST

PERIOD. THE SHORT STORY I AM UNIMPRESSED WITH POTENTIAL FOR

ORGANIZED SEVERE STORMS THIS WEEK.

THERE ARE SOME ISSUES WITH HOW THE ECMWF AND THE GFS HANDLE THE

UPSTREAM SYSTEMS... PARTICULARLY THE SYSTEM CURRENTLY GOING THROUGH

THE ALEUTIAN ISLANDS THAT THEN ROTATES UNDER THE CLOSED LOW IN THE

GULF OF ALASKA IN THE THURSDAY TIME FRAME. IT IS HERE PROBLEMS

BEGIN. THE QUESTION IS DOES THE SYSTEM CLOSE OFF AT MID LEVELS? THE

12Z RUN OF THE ECMWF SAID YES...BUT THE 00Z RUN DOES NOT DO THAT.

THE CHANGE IN AMPLIFICATION CHANGES THE OUTCOME DOWNSTREAM. THE GFS

IS MORE LIKE THE 00Z ECMWF IN THAT IT DOES NOT CLOSE OFF THE SYSTEM.

THE FLATTER UPSTREAM RIDGE DOES NOT ALLOW THE COLD FRONT THROUGH AS

QUICKLY. MORE IMPORTANT THEN THAT IS THIS KEEPS THE UPPER JET WELL

NORTH AND WEST OF SOUTHWEST LOWER MICHIGAN AFTER TUESDAY (JET LIFTS

THROUGH THIS AREA THEN). WITH THE UPPER JET SO FAR AWAY THE RISK OF

ORGANIZED SEVERE STORMS WILL BE MUCH LOWER THIS WEEK.

SO A WEAK COLD FRONT COMES THROUGH MONDAY NIGHT... THEN THAT PUSHES

BACK AS A WARM FRONT WEDNESDAY. THEN THE COLD FRONT FROM THAT SYSTEM

STALLS WEST OF HERE UNTIL FRIDAY. AT THAT POINT THE COLD FRONT

SLOWLY FALLS THROUGH THE AREA WITH ONLY WEAK UPPER JET SUPPORT. THE

DEEP LAYER SHEAR STAYS UNDER 30 KNOTS THROUGH THE WEEK. THE ECMWF

STALLS THE FRONT WEST OF HERE LIKE THE GFS WED NIGHT INTO THURSDAY

BUT BRINGS IT THROUGH THURSDAY NIGHT ONLY TO STALL IT OVER I-94

FRIDAY. ONE WAY OR THE OTHER THE UPPER JET IS IN CENTRAL ONTARIO

FRIDAY. SO... EVEN THROUGH WE HAVE CONSIDERABLE INSTABILITY AND SOME

MOISTURE (NOT DEEP MOISTURE)... THERE IS NOT MUCH FORCING. SO OUR

BEST SHORT AT RAIN IS WEDNESDAYS WITH THE WARM FRONT. I COULD SEE

SOME ISOLATE STRONG STORMS BUT WITH WEAK SHEAR THESE WOULD BE MORE

PULSE TYPE EVENTS.