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Hoosier

2017 Short/Medium Range Severe Thread

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Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1140 PM CST Sat Dec 02 2017

   Valid 041200Z - 051200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM FAR EASTERN
   OKLAHOMA TO THE MID MISSISSIPPI VALLEY...

   ...SUMMARY...
   A few thunderstorms may produce strong, gusty winds from parts of
   the southern Plains to the mid Mississippi Valley Monday evening.

   ...Far eastern Oklahoma to the mid Mississippi Valley...
   A broad zone of cyclonic mid-level flow will expand eastward across
   the central US Monday, as a potent impulse lifts from the central
   Plains northeastward to the upper Great Lakes and another dives
   south across the Dakotas. The resultant surface pattern will feature
   a deepening cyclone lifting northeast across the upper Mississippi
   Valley with an attendant cold front sweeping east towards the Ohio
   and Tennessee Valleys overnight.

   Ahead of this front, modest low-level moisture (e.g., surface dew
   points in the upper 50s and lower 60s) will stream
   north/northeastward during the day. Surface heating will not be
   particularly impressive across the pre-frontal warm sector, but a
   narrow corridor of surface-based CAPE around 250-600 J/kg will
   likely materialize from eastern Oklahoma to parts of the mid
   Mississippi Valley. In turn, a thin/low-topped band of convection is
   expected to develop by the evening hours and accelerate eastward
   with the front. Despite the meager buoyancy, considerable
   low/mid-level flow upwards of 40-60 kt may support a few stronger
   gusts in the heartiest of convective elements, primarily during the
   evening hours. Additionally, the strength of low-level shear
   suggests a tornado cannot be ruled out. In turn, marginal
   probabilities have been introduced.

   ..Picca.. 12/03/2017

 

swody2_severeprob.png.bc2b26a36ce3021b7896f20ce7d89ace.png

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New 3km NAM came in with slightly better surface instability compared to previous runs from eastern IA down into MO.  HRRR, and HRRRx have been showing relatively decent low-level cape as well, so perhaps the 3km is catching up a bit.  We're still talking about cape in the 500-800J/kg range, so nothing too exciting.  As Hoosier pointed out the strong wind fields right off the deck could easily get mixed down with any of the storms.  Always love to see the rapidly strengthening surface low as well.

3km NAM at 01z.

27yz8zd.jpg

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13 minutes ago, cyclone77 said:

New 3km NAM came in with slightly better surface instability compared to previous runs from eastern IA down into MO.  HRRR, and HRRRx have been showing relatively decent low-level cape as well, so perhaps the 3km is catching up a bit.  We're still talking about cape in the 500-800J/kg range, so nothing too exciting.  As Hoosier pointed out the strong wind fields right off the deck could easily get mixed down with any of the storms.  Always love to see the rapidly strengthening surface low as well.

3km NAM at 01z.

27yz8zd.jpg

I think there's a decent chance the risk area gets expanded northward.  The NAM(s) might be fine to use for the convection/evolution, but they once again look too cool with temps tomorrow afternoon... which means that the temps tomorrow evening are likely to be higher than those models suggest.  Higher temps/somewhat better near sfc lapse rates = easier to mix down some severe gusts.  

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As others have already mentioned, with low cape, nice wind fields and mid-upper 50 DP's, could see some isolated severe wind threat in portions of IA/IL/MO.

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23 minutes ago, Hoosier said:

No northward expansion on the day 1 outlook... kinda surprising.

That was a pretty lazy interpretation of the event by those forecasters.  Pretty disappointing.  I wouldn't be surprised if in later outlooks a slight is needed from parts of eastern MO up into central IL.  

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Forecast soundings in the Mississippi River area actually look decent later.  Potential is there in eastern IA/northwest IL for something greater than suggested, imo.

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Front timing has slowed down by 1-2hrs compared to yesterday's model guidance.  This places areas like Cedar Rapids, Fairfield IA, and Kirksville MO back in the game.  Nice to see the bump in probs on the new outlook.

EDIT:  Just realized this will be (hopefully) the last time we go without the aid of the more advanced GOES-16 data during a severe event.

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That rapidly deepening surface low raises a few red flags. It's one of the things that was missing in many of the higher-category outlooked events that underperformed this past spring. Could be one of those days where "just enough" instability in spots gets the job done.

Not much precip around on radar, although still overcast across the region there are some thin spots over IA/IL.

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Line has formed quite nicely from Iowa down into Missouri.  The storms popped earlier than what most of the guidance had been indicating, so that favors areas much further west than what was shown yesterday and previous days.  Big question is how long will the storms maintain themselves before the instability axis craps the bed this evening.  

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1 hour ago, Hoosier said:

Not 100% certain but I think 12/23/15 was the last time there was a December slight risk in any part of Wisconsin.

You were correct (as well as the hrrr) in the need to expand the slight farther north.

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Ho hum....how soon before these cells outrun the meager instability axis or will kinetics take over to provide a lifeline?  Not expecting much by the time these things get to IN.  And unless I am mistaken the current low pressure over nw WI is not as deep as previously modeled for that location.

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7 minutes ago, Indystorm said:

Ho hum....how soon before these cells outrun the meager instability axis or will kinetics take over to provide a lifeline?  Not expecting much by the time these things get to IN.  And unless I am mistaken the current low pressure over nw WI is not as deep as previously modeled for that location.

There might be less surface based instability.  I was pulling HRRR forecast soundings earlier that had about 500 J/kg in eastern IA and mesoanalysis only has a couple hundred J/kg.

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Small plane crash on approach at RFD about an hour ago, media reporting pilot was unable to maintain control due to wind. Obs at rfd around 6pm says 20mph gusting to 30mph, so nothing too crazy.


Sent from my iPhone using Tapatalk Pro

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1 minute ago, krfd said:


Small plane crash on approach at RFD about an hour ago, media reporting pilot was unable to maintain control due to wind. Obs at rfd around 6pm says 20mph gusting to 30mph, so nothing too crazy.


Sent from my iPhone using Tapatalk Pro

Could've been significant shear on descent though. 

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1 hour ago, krfd said:


Small plane crash on approach at RFD about an hour ago, media reporting pilot was unable to maintain control due to wind. Obs at rfd around 6pm says 20mph gusting to 30mph, so nothing too crazy.


Sent from my iPhone using Tapatalk Pro

Depends on the plane, the pilot, the wind direction and the objects around the runway that could deflect or "tunnel" the wind.  Just because the reported wind was 20 gusting to 30 doesn't mean a gust to 40 can't happen in the pilots "little" airspace.  Or as Hoosier said, depending on the condition, a small vortex of gusts shifting aft of the aircraft reduces true airspeed by 10 15 mph, your on the ground. A 30mph crosswind landing in just about any aircraft presents challenges.  20-30 crossing the runway at certain angles exceeds the ratings of many small private aircraft.   In my early days of flying I brought a 172 down in a 20mph crosswind about 70 degrees off the nose and damn near flipped it.  Had an unexpected gust of 40 about 20 feet from my flare point.  Had to cram the pedal into the wind, jammed the power full and cross controlled the wings to keep lift equalized.  Then wished I had an extra pair of pants in the cabin!  Lesson learned and that's what flying's all about. I personally will go to one of my pre-planned alternate airports with the winds 30 degrees or less off of the nose on final if I feel uncomfortable in a strong crosswind landing scenario now.  It surprises most people how vulnerable, especially light private aircraft, are to light to moderate winds you would enjoy flying a kite in :D

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