Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202325

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
625 PM CDT Wed Sep 20 2017

1246 PM CDT

Through Thursday afternoon...

Concern is on convective development late this afternoon/early
evening and how far east/south it will make it, along with hot
and humid conditions that look to persist for the coming days.

It is indeed hot and humid for mid September, and ORD/MDW are
already almost at 90 degrees today. Warm south winds and little
cloud cover will allow most locations to get close to 90 today.
Convective temperatures on the RAP are in the 90s across IA,
which looks to delay any initial convective development near the
cold front until late this afternoon. There is pretty good
agreement though that thunderstorms will develop between 5 and 6
pm near the Mississippi River. The question will be with the upper
forcing being a bit weaker farther east, and with the convection
arriving around sunset or so, that should limit the eastward
thunder concerns with this initial development. There could be
some sprinkles that get closer to Chicago but the forcing does not
appear to overcome dry mid levels.

While west/northwest IL is favored at this time, the low level
jet will ramp back up in this high PWAT environment, which could
lead to an uptick in showers and storms late this evening/early
overnight hours. PWATs near the front remain elevated and do
increase, so if showers and storms do get a bit closer, some
modest rain rates could occur in the far northwest.

Upper level heights rebuild tomorrow, and continued deep layer
southerly flow will lift the effective frontal boundary back
northward. The lower level airmass will be on par with today`s if
not a bit warmer, and thus many areas will likely tag 90 or even
the lower 90s. These are near record levels for mid to late
September. With continued upper 60s to around 70 dewpoint, this
places heat indices in the middle or even upper 90s.




Thursday night through Wednesday...

155 pm...A warm frontal boundary will lift back north of the area
Thursday night with hot and humid conditions continuing into this
weekend. Bumped up high temps both Friday/Saturday to near
guidance which is generally in the lower 90s Friday and around 90
Saturday. Given high temps today/currently are around 90...these
forecast high temps for Friday and Saturday may end up being a few
degrees too cool. Not expecting much change to dewpoints either...
in the mid/upper 60s thus heat index readings could reach the
mid/upper least for a few hours during peak heating in
the early/mid afternoons. Thermally...conditions cool slightly
Sunday/Monday but its possible that with little change to this
pattern the models may trend warmer into early next week. No
change to highs in the 80s Sunday/Monday...but these could also
end up too cool.

Dry conditions expected until a cold front arrives Tuesday night
next week which will bring the next best chance of rain. Much
cooler air will spread into the region behind this frontal
boundary by Wednesday afternoon/evening. There may also be an
isolated shower Sunday/Monday but chances are below mentionable
from this distance. cms


For the 00Z TAFs...

Primary aviation concern this TAF cycle is with SHRA/TSRA
chances. Storms have developed from eastern IA north into WI ahead
of a weak cold front. All indications are that this activity will
dissipate before reaching the immediate Chicago terminals, with
best chance of impacts being at RFD. Have maintained a short tempo
at RFD this evening to cover this threat. Otherwise, where skies
clear overnight, could see some light fog/MVFR vsby, especially in
the more rural areas like GYY/DPA. Otherwise, look for VFR
conditions and light to moderate south winds through the TAF

- Izzi



155 pm...Strong low pressure over western Manitoba will slowly
weaken tonight and Thursday as a weak trailing cold front moves
south across Lake Michigan tonight into Thursday morning. This
front is expected to stall over central portions of the lake then
move back north as a warm front by Thursday night. A large trough
of low pressure will remain nearly stationary across the plains
through early next week which will maintain southerly flow across
the lake. This trough/front will move across the Great Lakes
region Tuesday night next week shifting winds to the northwest
when a period of 15-25kts is expected. cms





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