U.S. Severe Weather Forecast (Sääkatsaus)

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Day Three

000 
acus01 kwns 210542 
swody1 
Storm Prediction Center ac 210541 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1241 am CDT Fri Sep 21 2018 


Valid 211200z - 221200z 


..there is an enhanced risk of severe thunderstorms late this 
afternoon and evening...south and east of lakes Erie and Ontario... 


... 
Severe thunderstorms are possible across parts of the lower Great 
Lakes region late this afternoon and evening, accompanied by the 
risk for potentially damaging wind gusts. 


... 
It appears that the influence of mid/upper subtropical ridging will 
become increasingly confined to the southern tier of the U.S. During 
this period, with generally zonal belts of mid-latitude westerlies 
prevailing across the northern tier and Canada. Within the latter 
regime, one significant short wave trough is forecast to accelerate 
northeast of the upper Great Lakes region through Quebec and 
adjacent portions of the northeast. It appears that fairly strong 
cyclogenesis will proceed in association with this feature, with the 
center of the broad surface low forecast to migrate northeast of 
Lake Superior through Newfoundland and Labrador by 12z Saturday. A 
cold front trailing from the cyclone may advance southeastward 
through all (to the east of the rockies) but the Gulf and South 
Atlantic coast states. 


..lower Great Lakes region into northeast and Ohio Valley... 
A relatively cool low-level air mass is expected to linger across 
the northern mid Atlantic coast region through much of New England, 
beneath relatively warm and warming mid/upper levels. Guidance 
suggests that this stable stratification will linger through at 
least late this evening, with pre-frontal destabilization limited to 
a corridor of stronger boundary layer heating and low-level moisture 
return from the lower Ohio Valley through the lower Great Lakes 
region, southern Ontario and western Quebec. Colder air aloft 
associated with the short wave appears likely to lag to the west of 
the cold front even as it approaches this region, but it appears 
that peak afternoon mixed layer cape will still exceed 1000 j/kg, 
and could approach 2000 j/kg. 


In the presence of 40-50 kt flow in the 850-500 mb layer, at least 
as far south as the Lake Erie vicinity, the environment appears 
likely to become conducive to organized severe storm development. 
This may include low-level hodographs initially large and clockwise 
curved, beneath strengthening southwesterly high level flow, across 
parts of northwestern Pennsylvania into western New York state. 
These profiles would be favorable for supercells with a risk for 
tornadoes, given storm initiation. However, forcing to support 
thunderstorm development remains unclear until late afternoon, when 
it appears that a perturbation near/just southeast of the short wave 
within the westerlies will approach the lower Great Lakes region. 
As this occurs, thunderstorm development along or just ahead of the 
cold front may generally coincide with a transition to more 
unidirectional lower/mid tropospheric flow. But an evolving line of 
storms probably will be accompanied by the risk for severe wind 
gusts. Thunderstorms may develop southwestward into progressively 
weaker deep layer mean flow (and corresponding diminishing severe 
wind potential), toward the lower Ohio Valley through early evening. 


Farther east, thunderstorm activity is expected to weaken after dark 
as it advances into the Hudson/Champlain valleys and northern New 
England. Model forecast soundings do suggest some pre-frontal 
boundary-layer based destabilization is possible across this region 
overnight. However, beneath lingering warm mid/upper levels, this 
does not appear likely to be deep enough to support thunderstorms, 
though showers accompanied by locally strong surface gusts may not 
be out of the question. 


.Kerr/squitieri.. 09/21/2018 


$$ 

Mesoscale Discussion


000 
acus11 kwns 210629 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 210629 
miz000-210830- 


Mesoscale discussion 1489 
National Weather Service Storm Prediction Center Norman OK 
0129 am CDT Fri Sep 21 2018 


Areas affected...coastal areas of northwest lower Michigan and southeast 
Upper Michigan 


Concerning...severe potential...watch unlikely 


Valid 210629z - 210830z 


Probability of watch issuance...5 percent 


Summary...isolated strong to locally damaging wind gusts will be 
possible during the overnight (between 3-4:30 am edt), as a line of 
showers and thunderstorms moves across the coastal areas of 
northwest lower Michigan to southeast Upper Michigan. Limited 
duration and coverage of this threat precludes the need for a watch. 


Discussion...trends in mosaic radar imagery showed a line of showers 
and embedded thunderstorms extending from eastern Upper Michigan to 
Northwest Lake Michigan and into extreme northeast WI, with a 
movement to the east-northeast at 40-45 kt. Despite time of day, 
surface temperatures and dew points, east of this convective line, 
including across the discussion area, have been rising. This is 
likely a result of a poleward advection of Theta-E, given warmer 
lake temperatures on Lake Michigan at this time of year. Objective 
analyses have indicated some steepening of surface-3 km lapse rates 
and weakening of surface-based inhibition. This suggests that any 
stronger downdrafts accompanying the line of convection and further 
enhanced by a very strong southwesterly low-level jet per vads at 
mke/grb/apx could penetrate the destabilizing boundary layer to 
produce a damaging wind gust. 


.Peters/Edwards.. 09/21/2018 


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


Attention...WFO...apx...GRR...MQT... 


Latitude...Lon 45418491 44788537 44658579 44548605 44168626 44178649 
44728633 45658596 46068559 46268544 46198476 45978444 
45768448 45418491