U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 201619 
Storm Prediction Center ac 201618 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1118 am CDT Sat Oct 20 2018 

Valid 201630z - 211200z 

..there is a marginal risk of severe thunderstorms across parts of 
Southern Lower Michigan and far Northwest Ohio this afternoon... 

A few strong storms may produce gusty winds across parts of Southern 
Lower Michigan and Northwest Ohio this afternoon. 

..Great Lakes vicinity... 

A small marginal risk, mainly for isolated strong wind gusts, has 
been added to portions of south-central and south-east lower 
Michigan into far Northwest Ohio this update. A strong cold front 
continues to progress east/southeast across Michigan and line of 
low-topped, fast moving thunderstorms has developed ahead of the 
front. Poor dewpoints and weak buoyancy will generally limit the 
threat. However, surface temperatures have warmed into the low to 
mid 50s and some locations are a couple of degrees warmer than 
forecast. Modifying rap point forecast soundings with current 
surface data indicates very steep low level lapse rates around 8-9 
c/km. 0-3km flow remains somewhat marginal, generally less than 35 
kt, but the steep lapse rates coupled with fast storm motion and 
marginal low level flow could lead to a handful of stronger wind 
gusts in the 40-50 mph range as the line of convection shifts 
eastward today. For more details, reference mesoscale discussion 1588. 

Elsewhere across the lower Great Lakes, showers and low-topped 
thunderstorms also will be possible into parts of PA and New York. 
Conditions are even less favorable across this area for any sort of 
organized severe threat. An isolated waterspout may 
also occur over Lake Erie this afternoon given the steep low-level 
lapse rates and vertical vorticity along the reinforcing cold front. 
In fact, at least a couple of water spouts have already been 
reported near Lake City PA. 

..southwest deserts... 

Low level moistening is occurring ahead of midlevel low off the 
coast of California. Isolated surface-based thunderstorms may develop in 
conjunction with peak heating, though the bulk of activity is 
expected overnight as stronger forcing for ascent overspreads 
southern CA/NV/AZ. Any surface-based convection that can develop 
earlier in the day could produce some gusty winds with steep lapse 
rates and modest vertical shear profiles. However, as nocturnal 
cooling occurs, overnight convection will become elevated and severe 
threat is expected to remain minimal. 

.Leitman.. 10/20/2018 


Mesoscale Discussion

acus11 kwns 201608 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 201608 

Mesoscale discussion 1588 
National Weather Service Storm Prediction Center Norman OK 
1108 am CDT Sat Oct 20 2018 

Areas affected...Southern Lower Michigan 

Concerning...severe potential...watch unlikely 

Valid 201608z - 201815z 

Probability of watch issuance...5 percent 

Summary...strong damaging gusts (40-50 mph) are possible over the 
next several hours as a broken convective line with embedded cells 
moves east across Southern Lower Michigan. 

Discussion...visible satellite imagery shows a cumulus field 
developing over Southern Lower Michigan where surface temperatures 
are rising into the middle 50s degrees f. Radar imagery shows a 
low-topped convective line with embedded cells showing weak 
organizational characteristics located in a south-southwest to 
north-northeast line immediately east of Grand Rapids. As a potent 
mid-level shortwave trough continues to pivot southeast over the 
central Great Lakes this afternoon, large-scale forcing for ascent 
will maximize coincident with peak heating. 

Modifying rap soundings over Southern Lower Michigan for middle 50s 
surface temperatures yields a nearly 9 degree c/km 0-3 km lapse rate 
because of very cool mid-level temperatures (-25 degrees c at 500 
mb). Although winds in the lowest 3 km are 40 kt or lower per kgrr 
VAD data, it seems plausible as the convection slowly organizes, 
potential for strong gusts via horizontal momentum transport may 
occur. The main threat with the strongest storms is localized wind 
damage due to 40-50 mph gusts. 

.Smith/guyer.. 10/20/2018 

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


Latitude...Lon 42988486 43008346 42568268 41878309 41788398 42048531