U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 221621 
Storm Prediction Center ac 221620 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1120 am CDT Sat Sep 22 2018 

Valid 221630z - 231200z 

..no severe thunderstorm areas forecast... 

thunderstorms are possible over parts of the southeast westward to 
south and east texas; however, no organized severe weather is 

In the wake of a midlevel trough now moving over the Canadian 
Maritimes, a surface cold front will continue to drift southward 
across the Southern Plains, Tennessee Valley, and mid Atlantic. A series 
of weakening midlevel vorticity maxima, within a tropical moisture 
plume from northeast Texas to the lower Ohio Valley, will help focus 
thunderstorm development along the front through tonight. Poor 
midlevel lapse rates within the moisture plume, as well as 
relatively weak vertical shear, will preclude severe storms along 
this corridor. 

.Thompson/Dean.. 09/22/2018 


Mesoscale Discussion

acus11 kwns 212304 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 212304 

Mesoscale discussion 1494 
National Weather Service Storm Prediction Center Norman OK 
0604 PM CDT Fri Sep 21 2018 

Areas affected...eastern Ohio...western and central 
Pennsylvania...western and central New York 

Concerning...Severe Thunderstorm Watch 386...387... 

Valid 212304z - 220100z 

The severe weather threat for Severe Thunderstorm Watch 386, 387 

Summary...mainly a damaging wind threat continues with storms along 
a cold front. Gradual weakening is expected as the boundary layer 
cools, and storms begin moving into a less heated/more stable 
airmass. A wider warm sector in western/central Pennsylvania may 
allow for storms with isolated damaging wind gusts to continue east 
of Severe Thunderstorm Watch 0386, and a Severe Thunderstorm Watch 
issuance downstream may be necessary. 

Discussion...earlier semi-discrete storms have recently congealed 
into a squall line, from roc, down to lck, with the corridor of 
strongest storms (with a history damaging wind gusts) located from 
just west of bfd to phd. These storms have organized in a kinematic 
environment characterized by 50-60 knots of bulk effective shear, 
with up to 250-300 m2/s2 effective srh noted in northwest PA into 
western New York. Despite the favorable shear environment, the airmass 
downstream of the squall line (central PA to central ny) is more 
stable, with surface temperatures mainly in the low 70s, owing to a 
stratus deck which has limited insolation. 

As such, the squall line is expected to traverse a more buoyant 
airmass for the next few hours, characterized by surface 
temperatures in the lower 80s and MLCAPE up to 1000 j/kg. During 
this time, storms may continue to produce damaging wind gusts and 
perhaps exhibit transient low-level rotation (given the strong 
low-level speed shear in place). Thereafter, a weakening trend is 
expected given the cooler temperatures to the east and the 
cooling/decoupling of the boundary layer overall. 

The buoyant warm sector between the ongoing squall line and a more 
stable airmass to the east is narrower in New York as opposed to 
western and central Pennsylvania. Storms may continue past the 
eastern bounds of Severe Thunderstorm Watch 0386 with at least an 
isolated damaging wind threat and an additional Severe Thunderstorm 
Watch issuance may become necessary in a few more hours. 

.Squitieri/grams.. 09/21/2018 

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


Latitude...Lon 40098211 40668190 41428057 42487904 43337784 43817645 
44247538 43967495 43417492 42097615 41227696 40377816 
39947927 39978099 40098211