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Severe Weather Threat May 7th - 13th


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We're dealing with another radar hole...

TORNADO WARNING

NATIONAL WEATHER SERVICE ABERDEEN SD

808 PM CDT SUN MAY 8 2011

THE NATIONAL WEATHER SERVICE IN ABERDEEN HAS ISSUED A

* TORNADO WARNING FOR...

CENTRAL LYMAN COUNTY IN CENTRAL SOUTH DAKOTA...

* UNTIL 845 PM CDT

* AT 805 PM CDT...TRAINED WEATHER SPOTTERS REPORTED A TORNADO 8

MILES SOUTHWEST OF PRESHO. DOPPLER RADAR SHOWED THIS TORNADO MOVING

NORTHEAST AT 15 MPH.

* THE TORNADO WILL BE NEAR...

PRESHO AROUND 815 PM CDT...

THIS WARNING INCLUDES INTERSTATE 90 BETWEEN MILE MARKERS 221 AND 240.

My chaser friend is driving back from California today..He actually just passed through Presho and watched this storm explode right in front of him on the interstate.

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If anyone wants a really detailed description of the situation, at least as it pertains to much of the Upper Midwest, I suggest you read LSX's AFDs. They are almost too detailed. They explain their thinking that goes into the pops for each of the next three or four days. Apparently the models have been hinting at an MCS developing Tuesday night in NW Minnesota that will sweep SE, probably toward Minneapolis or LaCrosse. Although a long shot, I wonder how far such a feature could make it before it peters out Wednesday morning.

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I dont know if I see any sfc based convection tomorrow in IL. The 0z NAM shows the whole state very capped, would be a big waste of a nice looking low-level hodograph in parts of the area.

It's interesting that the complexes expected to form tonight in eastern Nebraska and NE Iowa seem to no longer be there on the NAM. There was at least very good agreement on showers and storms developing over NE Iowa late tonight and early tomorrow morning and then pushing east.

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It appears the 0z NAM develops no precip that moves east of the Mississippi River before tomorrow night, anyway. Hopefully that's just problems with the NAM.

It looks like it develops something near Moline but it doesn't do much with it.

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It's interesting that the complexes expected to form tonight in eastern Nebraska and NE Iowa seem to no longer be there on the NAM. There was at least very good agreement on showers and storms developing over NE Iowa late tonight and early tomorrow morning and then pushing east.

Which is a dramatic reversal and not to be believed until more models show that. If it's incorrect with the early stuff then its late day evolution may be suspect.

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Which is a dramatic reversal and not to be believed until more models show that. If it's incorrect with the early stuff then its late day evolution may be suspect.

Second this. It appears the complex over SD is developing a cold pool per radar loops, and with the LLJ now kicking, we should see this stuff propagate along the instability gradient (warm front) through the night.

Edit: Sioux Falls, SD VAD wind profiler is detecting a powerful SSE LLJ of 50-60KT at 2-4kft.

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Thursday could end up being a chase day around here if the NAM ends up being correct....huge differences regarding what happens with the wave between the two models. The 0z NAM has a closed off/neg tilted ULL over north central KS at 84hrs with a sfc low in southeast NE where the GFS has it a strong but open wave over northeast NE/southeast SD and sfc low over southern IA. This will be interesting to watch.

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Thursday could end up being a chase day around here if the NAM ends up being correct....huge differences regarding what happens with the wave between the two models. The 0z NAM has a closed off/neg tilted ULL over north central KS at 84hrs with a sfc low in southeast NE where the GFS has it a strong but open wave over northeast NE/southeast SD and sfc low over southern IA. This will be interesting to watch.

GFS has been just pathetic lately--mainly the last month or so. It does continue to trend stronger with the secondary ejecting PV anomaly--what will likely eventually happen is once it reaches a critical strength, it will deepen rapidly over the plains and stall more similar to the NAM/Euro as opposed to remaining an open wave/progressive. Wednesday looking potentially significant over the southern plains.

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I'm looking at that small complex of storms just west of Sioux Falls. It looks like it's starting to take the form of a bow echo. Could this be what the ARW was hinting at to eventually cross the Mississippi River late tomorrow (now this) morning into afternoon?

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GFS has been just pathetic lately--mainly the last month or so. It does continue to trend stronger with the secondary ejecting PV anomaly--what will likely eventually happen is once it reaches a critical strength, it will deepen rapidly over the plains and stall more similar to the NAM/Euro as opposed to remaining an open wave/progressive. Wednesday looking potentially significant over the southern plains.

I will be curious as to what the 12z NAM does with thursday evening.

And here is the fcst sounding for Watongo, OK for weds evening at 0z. Pretty cool seeing the increase in low-level moisture and backing of the sfc winds from 21z to 0z.

pretty nice looking sounding/hodo besides the veer-back-veer wind profile which some have discussed quite a bit on here.

http://beta.wxcaster.com/cgi-bin/parse_skewt_trace_all.cgi?model=NAM&STATIONID=JWG

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I'm looking at that small complex of storms just west of Sioux Falls. It looks like it's starting to take the form of a bow echo. Could this be what the ARW was hinting at to eventually cross the Mississippi River late tomorrow (now this) morning into afternoon?

It should continue for awhile but it wont make it over this way, it already looks like its turning a tad more south and will ride the the northern fringe of the instability axis as csnavywx pointed out. You can also see the surging outflow out in front of it on radar and also see the highest dBZ's at both the surface and aloft are on the southern end closer to the better instability.

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0z GFS shows 5000 j/kg of MLCAPE in southern WI/northern IL on tuesday with dew points between 70-73 degrees and LI's >-12.

Question is whether anything can pop. Some model runs have been trying to scrape northeast IL with precip Tuesday night/Wednesday morning.

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Day 1 slight risk for IA/WI/IL...no mention of current SD MCS, but possible afternoon/evening development.

Day 2 slight risk for the OV/SE mentions the current SD MCS which will push SE.

Lack of agreement FTL.

Was a little surprised that the day 1 slight wasn't expanded farther east. With steep lapse rates/elevated instability nosing in, think we'll be able to pull off more of a hail threat.

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Day 1 slight risk for IA/WI/IL...no mention of current SD MCS, but possible afternoon/evening development.

Day 2 slight risk for the OV/SE mentions the current SD MCS which will push SE.

Lack of agreement FTL.

For the Day 2 Slight Risk I believe they were talking about the impulse associated with the current Dakotas MCS and not the actual MCS...I could be wrong though.

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Pressure dropped 3 hpa in 25 mins at Huron. Pressure gradients/falls like that are obviously not typical in any way on the synoptic scale.

KHON 090719Z AUTO 02019G26KT 10SM -RA BKN070 BKN095 18/11 A2949 RMK AO2 PK WND 36026/0714 WSHFT 0659 PRESFR P0003

KHON 090655Z AUTO 31019G28KT 10SM -RA OVC090 18/12 A2958 RMK AO2 PK WND 29028/0649 WSHFT 0636 LTG DSNT SW SLP010 P0006 T01830117

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Looking ahead to Wednesday's event, I'm noticing a really nasty veer-back-veer profile over much of the dryline on most of the models. The GFS is by far the most favorable-looking in this regard, as it keeps the trough more neutral-tilt and doesn't dig as much, which prevents extreme backing of the 200-300 mb flow. The NAM and ECMWF are both atrocious, though, with as much as 45° of backing with height in the 500-250 mb layer over the relevant stretch of TX/OK/KS.

This all leads me to take a very conservative approach at this point. After noticing the upper-backing issue, one of the events that immediately came to mind was 2009-04-26, a case with much more bark (read: hype) than bite. There were still a couple tornadoes, but by and large, the HIGH risk area saw messy, strung-out dryline thunderstorms with borderline/intermittent supercell structures. To be honest, the trough in this case is even sharper and more meridional by 00z Thu, and the backing between H5 and H25 is even more pronounced if you buy the NAM/Euro. Yikes. A bit ironic that, after a March and April in which much of W OK saw literally no surface-based supercells due to capping and drought, their first shot of the year might be tainted by an overly-messy and dynamic setup.

Thing is, this is all very sensitive to the exact evolution of the trough, which has not been handled well at all this year (I'm thinking back to the April 14/Tushka event, in particular, which went from being a flat wave to a four-contour closed low inside of the 72-hour timeframe). The threat for a significant outbreak is there, but right now the model consensus favors a warm sector vertical wind profile that would likely mitigate the threat significantly, and that will need to change. Also, I'd note that IMHO, this looks like a classic setup to get overhyped and ultimately disappoint *IF* the shear/VPPGF issues persist over the next few days.

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This thing really took off overnight, I see.

It looks like those cells are being preferentially being created on the SE side of the system now, where the better instability and relative inflow is, so I would imagine that this sucker turns ESE-SE along the instability gradient. This is all contingent on the complex being able to survive the "sun-up syndrome" via a weakening nocturnal component of the LLJ, but there are some lone cells developing well in advance, and also some stuff getting going further south, so it may just hold together.

I was also kind of shocked at no mention by the SPC at all, despite this thing having the chance of plowing into the edge of a very unstable airmass later.

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