U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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acus01 kwns 212002 
swody1 
Storm Prediction Center ac 212001 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0301 PM CDT Thu Sep 21 2017 


Valid 212000z - 221200z 


..there is a marginal risk of severe thunderstorms over a portion 
of the Southern Plains... 


..there is a marginal risk of severe thunderstorms over a portion 
of the upper Midwest... 


... 
A marginal risk for a few large hail events exists over a part of 
the upper Midwest tonight into Friday morning. Isolated strong to 
severe wind gusts are possible from West Texas into southcentral 
Kansas between 4 and 9 PM CDT. 


No changes needed with either of the marginal risk areas. Previous 
forecast reasoning remains valid for this outlook issuance. 


..West Texas through southwest/south-central Kansas... 
Observational data indicated gradual weakening of surface-based 
inhibition, given ongoing strong diabatic heating in vicinity of the 
dryline. Trends in visible satellite imagery showed cumulus clouds 
forming within this environment across western Texas into southwest OK, 
and this development will persist as additional surface heating 
supports further weakening of the cap. Meanwhile, water-vapor 
imagery showed a shortwave trough tracking from southwest to 
west-central Texas this afternoon, as also indicated by backing 850-mb 
winds across the Southern Plains, and veering 500-mb winds across 
southwest Texas with the trough passage. Attendant forcing for ascent 
should prove favorable in aiding eventual stronger updrafts across 
this marginal risk area later this afternoon into the early evening. 
Weak deep-layer shear and dcape values exceeding 1500 j/kg suggest 
locally strong downburst winds should be the primary severe threat. 


..FL Panhandle into northern Florida/extreme southern Georgia... 
Weak deep-layer shear will limit organized storm development, though 
moderately strong buoyancy and sea breeze/convective outflow 
boundary mergers suggest additional storms will be possible. High 
precipitable water values and high dcape suggest locally strong wind 
gusts will be the greatest threat. The lack of storm organization 
precludes the introduction of severe probabilities. 


..eastern NC... 
Forcing for ascent and cold 500-mb temperatures (-11 to -12c) 
attendant to a nearly stationary mid-level circulation centered over 
eastern NC today combined with steep low-level lapse rates and 
precipitable water exceeding 1.5 inches suggest additional storms 
will be possible into the early evening. Weak deep-layer shear will 
limit storm organization, though dcape exceeding 1000 j/kg across 
southeast NC and high precipitable water could support locally 
strong winds. 


.Peters.. 09/21/2017 


Previous discussion... /issued 1129 am CDT Thu Sep 21 2017/ 


..West Texas through southwest Kansas... 


Insolation will result in a corridor of strong instability 
(2000-3000 j/kg mlcape) from the eastern portion of West Texas through 
central Kansas where steeper lapse rates associated with the elevated 
mixed layer overlap western fringe of Richer low-level moisture. 
Other than what appears to be a weak vorticity maximum moving 
through northern Mexico and southwest TX, forcing aloft will remain 
weak. However, deeper mixing and convergence in vicinity of the 
dryline should result in at least isolated multicell storms, a few 
of which could produce downburst winds through early evening. 


..upper Midwest... 


Strong low-level Theta-E advection will be underway today resulting 
in northward destabilization into the upper Midwest. However, the 
combination of rising heights and northeast advection of warm air 
within base of the elevated mixed layer should effectively cap the 
atmosphere. Therefore, thunderstorm initiation is not expected until 
later tonight, most likely across northern Minnesota just north of a warm 
front, where isentropic ascent will increase in association with a 
strong southerly low-level jet. These storms will be elevated, but 
instability should be sufficient for at least a marginal threat for 
large hail. 


$$ 

Mesoscale Discussion


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acus11 kwns 202017 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 202017 
wiz000-ilz000-iaz000-mnz000-202215- 


Mesoscale discussion 1662 
National Weather Service Storm Prediction Center Norman OK 
0317 PM CDT Wed Sep 20 2017 


Areas affected...northeast Iowa through central Wisconsin 


Concerning...severe potential...watch possible 


Valid 202017z - 202215z 


Probability of watch issuance...40 percent 


Summary...storms may become capable of producing a few strong to 
locally damaging wind gusts and hail this afternoon into early 
evening from northeast Iowa into southern and central Wisconsin. 
Trends will continue to be monitored. 


Discussion...cold front extends from western WI through northeast 
and southwest Iowa. Upper 60s f dewpoints have advected through 
pre-frontal warm sector which, in conjunction with diabatic warming, 
have boosted surface temperatures to 85-90 resulting in 2000-3000 
j/kg MLCAPE. Despite weak forcing aloft and a gradual weakening of 
the low-level jet in wake of a shortwave trough lifting north 
through Manitoba and western Quebec, convergence and a destabilizing 
boundary layer are sufficient to initiate storms along the front 
this afternoon. While the stronger winds aloft remain Post frontal, 
moderate flow with 40-50 kt in the 700-400 mb layer resides in 
frontal zone, but vertical shear remains modest and supportive of 
mostly multicells and possibly some marginal supercell structures. 
Primary limiting factor for a more robust threat is the weak 
mid-level lapse rates with warm temperatures aloft (-6 to -7 c at 
500 mb). Nevertheless, overall parameter space appears sufficient 
for at least a modest risk for isolated strong to damaging wind 
gusts and some hail. 


.Dial.. 09/20/2017 


..please see www.Spc.NOAA.Gov for graphic product... 


Attention...WFO...grb...mkx...dvn...arx...dmx... 


Latitude...Lon 43339151 44479055 44898969 44348877 43398889 42608981 
42059139 42529221 43339151