U.S. Severe Weather Forecast (Ionchas Comhiomprach)

An Tríú Lá

acus01 kwns 181601 
Storm Prediction Center ac 181600 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1100 am CDT Tue Sep 18 2018 

Valid 181630z - 191200z 

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

Isolated hail and damaging wind gusts are possible today over parts 
of the upper Midwest. 

A persistent cluster of thunderstorms is affecting much of northern 
Iowa and adjacent regions this morning, with the leading-edge outflow 
surging southward across eastern Iowa. Much of this region has been 
stabilized by the convection, leaving a corridor from eastern NE 
into northwest Illinois where sufficient heating is possible later today. 
This axis may see storm intensification by mid/late afternoon, with 
the strongest cells capable of hail or locally damaging wind gusts. 
Present indications are that the severe risk will remain isolated 
and marginal in nature. 

.Hart/Darrow.. 09/18/2018 


Mesoscale Discussion

acus11 kwns 181049 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 181048 

Mesoscale discussion 1475 
National Weather Service Storm Prediction Center Norman OK 
0548 am CDT Tue Sep 18 2018 

Areas affected...extreme eastern South Dakota...southwest Minnesota...and adjacent 
northwest Iowa 

Concerning...severe potential...watch unlikely 

Valid 181048z - 181315z 

Probability of watch issuance...20 percent 

Summary...a locally strong/damaging wind gust is possible early this 
morning across extreme east-central and southeast South Dakota into 
southwest Minnesota and extreme northwest Iowa, as a line of storms 
moves east through this region. Isolated hail will remain a threat 
as well. 

Discussion...trends in mosaic radar imagery showed a compact 
forward-propagating mesoscale convective system had accelerated since 0830z across eastern 
SD, with current track toward the east-northeast at close to 50 kt. 
At 907z, Hon recorded a wind gust of 46 kt and at 912z, mhe recorded 
a wind gust of 53 kt with this line of storms. This mesoscale convective system is rooted 
above a stable boundary layer, given the surface analysis showed a 
synoptic boundary extended from southern Iowa northwest into northeast 
NE and then westward, with temperatures dew points in the 50s across 
much of eastern South Dakota and southern Minnesota. However, 00z soundings at 
rap/abr indicated and rap analysis/forecast soundings suggested dry 
air was located beneath 700 mb (sub-cloud layer). Despite the 
presence of a stable boundary layer, higher-momentum air within 
downdrafts could be enhanced as they are flung through the 
aforementioned sub-cloud dry air, reaching the ground as a severe 
wind gust. This scenario appears possible into southwest Minnesota and 
perhaps a row of counties in northwest Iowa. There is uncertainty 
with how far east this potential wind threat will extend into 
southern Minnesota, as 1) forecast soundings suggest less of a sub-cloud 
dry air layer that far east, and 2) recent surface observations at 
fsd (45 kt at 1030z) and bkx (43 kt at 1020z) indicated sub-severe 
wind gusts. 

.Peters/Edwards.. 09/18/2018 

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


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