Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180725

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
225 AM CDT Mon Sep 18 2017

219 AM CDT

Through tonight...

High pressure will build across the western Great Lakes today
while the cold front that passed over the region yesterday
settles over the Ohio Valley. East to northeast flow will help
temper afternoon highs today, especially near Lake Michigan where
onshore flow will keep temperatures only in the low 70s. Farther
inland expect afternoon highs in the mid to upper 70s.

Meanwhile, a shortwave trough lifting across the central Great
Plains this morning will lift across the mid Mississippi Valley
this evening, crossing the river around 00Z. Expect increasing
cloud cover out ahead of the wave late this afternoon and evening,
and there is a chance for showers and thunderstorms this evening
and tonight. Have backed off on PoPs before 00Z given the later
forcing and initially dry low/mid levels that will be hard to
overcome by the weak forcing. Also lowered PoPs some the evening
and overnight, especially north of the I-88 corridor, given the
less favorable diurnal timing. Much better chances will be well to
our west on the nose of the low level jet over the mid Missouri
Valley overnight.



219 AM CDT

Tuesday through Sunday...

Expect a warm pattern throughout much of the extended time frame
as an increasingly amplified ridge takes hold over the Great
Lakes and eastern CONUS. There could be a few lingering showers
with the shortwave trough that starts to move across the area
tonight primarily early in the day Tuesday with dry conditions
favored in the afternoon. Otherwise not much going on Tuesday
which will likely be the last near normal day temperature-wise
before a big warmup takes hold Wednesday. A strong jet digging
into the Pacific NW through the early part of the week will carve
out a deep trough over the West while downstream amplification of
the ridge is expected over the Upper Midwest. Very warm 850mb
temps to around 20C will be transported all the way into the
Dakotas Tuesday and the thermal ridge will inch east over the
local area by Wednesday resulting in well above normal
temperatures. Afternoon highs Wednesday are expected to be into
the upper 80s with some locations tagging 90.

Thursday through the weekend, strong upper ridge becomes
increasingly blocky which should lock the area into a continued
warm pattern. Guidance suggests a cold front coming up against
the ridge Wednesday night into early Thursday could bring a
decaying line of showers and thunderstorms to or at least close
to the local area, but beyond that it appears the strong ridge
will keep any precipitation off to our west with primarily dry
conditions locally.



For the 06Z TAFs...

MVFR cigs and gusty northerly winds at GYY will dissipate as the
gradient relaxes. High pressure builds over the upper midwest, and
winds become northeast to east at less than 10 kt across the
board. Guidance slows the arrival of the next upper level wave,
and have low confidence in precip timing, coverage, and occurrence
of thunder. Decided to push back the PROB30 for SHRA to 04/05Z,
but guidance is hinting that precip may focus south of I-80
leaving the terminals dry. Thinking stray light showers may pass
over the terminals late this evening while more substantial rain
showers will likely stay well south of the terminals. Winds become
southeast this evening and low end VFR cigs are expected with the



219 AM CDT

High pressure over the upper midwest will dominate the pattern today
and shift into southeast Canada tonight.  Northeast winds diminish
as the high builds overhead, and then become southeast behind the
high.  The high pressure ridge remains over the eastern third of the
CONUS while a low forms over the Dakotas Tuesday. The low deepens
over Saskatchewan, and south winds increase as the pressure gradient
tightens over the lake.  The low`s cold front diminishes just west
of the Mississippi River Wednesday, and it looks like southerly
winds will persist into the weekend.






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