U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 261237 
Storm Prediction Center ac 261235 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0735 am CDT Wed Sep 26 2018 

Valid 261300z - 271200z 

..there is a slight risk of severe thunderstorms northeast 

Scattered damaging winds are possible across the northeast states 
between about 2 to 10 PM EDT. A couple tornadoes may also occur, 
mainly centered on western New England between 3 to 8 PM EDT. 

No change to categorical delineations this outlook, but have 
expanded tornado probabilities farther east/north. 

A shortwave trough over the western Great Lakes will rapidly eject 
across southern Ontario and Quebec through this evening. While 
stronger 500-mb height falls will primarily remain north of the 
international border, weak falls will likely extend as far south as 
PA. Coincident with the mid-level trough, the peripheral belt of 50+ 
kt 500-mb southwesterlies will overspread western/central PA to most 
of ME by late afternoon. 

Morning cloud coverage along with ongoing scattered convection from 
western New York to southern Ohio will hinder destabilization. But the moist 
air mass characterized by mid-upper 60s boundary-layer dew points 
coupled with pockets of modest heating should result in a plume of 
MLCAPE this afternoon from 500-1500 j/kg. A few broken bands of 
convection will likely develop by early afternoon along an 
eastward-moving cold front in the Lee of the lower Great Lakes in 
NY/PA. Elongating hodographs suggest some tendency to maintain 
semi-discrete supercell structures during the late afternoon to 
early evening from the Adirondacks/Hudson Valley towards western ME. 
Scattered damaging winds are the predominant hazard but a couple 
tornadoes are also possible. 

With southern extent into the mid-Atlantic states, weaker low and 
deep-layer shear suggest that multicell clusters should be 
increasingly dominant. Locally strong wind gusts capable of isolated 
to perhaps scattered wind damage are anticipated across the Piedmont 
before weakening over the coastal plain. 

.Grams/Peters.. 09/26/2018 


Mesoscale Discussion

acus11 kwns 252221 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 252221 

Mesoscale discussion 1501 
National Weather Service Storm Prediction Center Norman OK 
0521 PM CDT Tue Sep 25 2018 

Areas affected...lower Michigan...Northwest Ohio...Indiana...east 
central Illinois 

Concerning...severe potential...watch possible 

Valid 252221z - 260015z 

Probability of watch issuance...40 percent 

Summary...thunderstorm activity will spread into, and continue to 
develop across, much of the region into the 8-11 PM EDT time frame, 
accompanied by at least some risk for severe wind gusts. It is not 
yet clear that this will require an additional severe weather watch, 
but trends are being monitored for this possibility. 

Discussion...thunderstorms are beginning to initiate in the warm 
sector, across parts of lower Michigan southwestward across Indiana 
into east central Illinois. This is ahead of the forced line of 
storms now spreading into/through the Interstate 55 corridor of 
northeast and central Illinois. 

The northwestern periphery of Richer boundary layer moisture return, 
characterized by dew points around 70, currently extends along the 
Interstate 70 corridor of central Illinois into Indiana, and 
northward, roughly near and east of the Interstate 69 corridor 
across central Indiana into northwestern Ohio/southeastern lower 
Michigan. This moisture, coupled with daytime heating, appears to 
be contributing to cape on the order of 1000-2000 j/kg. This is in 
the presence of generally strong deep layer shear beneath 40-50 
southwesterly mid-level flow. 

Pre-frontal low-level wind fields are somewhat weak and veered 
resulting in generally modest low-level hodographs across much of 
the region. This may be at least one mitigating factor to a more 
substantive severe weather risk into the 01-03z time frame. 
However, in the lingering moderate boundary layer instability, the 
environment may remain conducive to convection capable of at least 
producing localized potentially damaging wind gusts and perhaps some 
hail into/through the evening hours. 

.Kerr/Hart.. 09/25/2018 

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


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