Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181913

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
213 PM CDT Fri Aug 18 2017

209 PM CDT

Main concerns for the near term forecast is with the coverage and
timing of showers and thunderstorms tonight.

Surface low pressure continues to pull away to the northeast of
the Great Lakes region this afternoon. While weak surface high
pressure ridging was developing east across the forecast area this
afternoon, attention turns to an area of surface low pressure
over southern IA, and an associated upper trough across the
eastern Dakotas, MN and IA. This upper trough is progged to
amplify across the forecast area through early Saturday morning,
as fairly strong vort north of the FSD/SUX area digs southeast
into IL. In response, the weak surface low over IA will develop
eastward along a modest low-level baroclinic axis, and is expected
to support the development of showers and a few thunderstorms
this evening into the overnight hours. Most of the high-res
convective allowing model guidance is in good agreement in
developing convection across southeast IA and into western IL
through early this evening, then spreading it east across portions
of northern/central IL generally along/south of the I-80/88
corridors. Guidance generally downplays showers farther north,
with lighter more scattered precip expected across far northern

Short wave continues to dig into the southern Lakes and Ohio
Valley early Saturday, with mid-level drying noted in forecast
soundings across the forecast area during the early morning hours.
Nearly all guidance has precip out of the forecast area by
sunrise. Fairly strong subsidence then develops in the wake of
this wave during the day, with H5 rises of 70+ meters by
afternoon. Broad area of weak high pressure reflects this at the
surface, setting up light winds and decreasing cloud cover (save
for some flat cu) by midday. Light winds and high pre-dawn RH
could produce some shallow patchy fog in a few spots prior to
sunrise. Low level thermal progs from models support highs in the
low-mid 80`s in most spots, with a weak lake breeze keeping
readings a bit cooler along the Lake Michigan shore.



209 PM CDT

Saturday night through Friday...

Hot, humid, and at times active weather expected Sunday though
midweek with cooler and drier conditions in place to close out the
upcoming work week as pattern transitions from zonal early in the
period to highly meridional late.

Sunday and Monday, broad area of low pressure will be in place
across the central Great Plains while 850mb thermal ridge is in
place out ahead of the low across portions of the mid to upper
Mississippi Valley. This will allow for continued hot and humid
conditions Sunday and Monday with afternoon highs well into the 80s
and dew points creeping towards or into the 70s. Forecast soundings
show decent capping in place Sunday and there is no notable
source of forcing, so expect primarily dry conditions through the
day Sunday.

Nocturnal convection appears favored Sunday night as low level jet
intensifies well to our west, and now the 12Z GFS tries to bring a
convective complex across the local area Monday morning. NAM and
ECMWF paint a similar picture which raises concerns for PoPs and
cloud cover through the day Monday, including around the time of
the eclipse. Confidence in the details remains low at this
distance given the lack of any strong synoptic scale ascent and
models struggling with convective feedback generating low
amplitude waves over the area. Remnant showers and storms
somewhere in the region if not here locally seems plausible though
so have increased cloud cover and PoPs some through the day

A strong upper wave is expected to begin digging into the Upper
Midwest Tuesday and continue to carve out a deep trough across the
Great Lakes through the day Wednesday. Showers and thunderstorms are
more favored on Tuesday in response to this wave and attendant
surface cold front that will push across the region later in the
day Tuesday. An expansive area of high pressure will build south
from the Canadian Prairies Thursday and Friday resulting in drier
weather to close out the week, but also cooler with afternoon
highs only in the low to mid 70s Thursday and Friday.



For the 18Z TAFs...

Primary aviation forecast concerns are with timing and coverage of
showers/thunderstorms tonight.

Surface low pressure continues to pull away to the northeast of
the Great Lakes region early this afternoon, while weak surface
high pressure develops eastward across the lower/middle
Mississippi Valley. Morning MVFR stratocu continues to lift and
thin, with VFR conditions expected the remainder of the day. Winds
also continue to diminish with the approach of the surface ridge
and the weakening of the pressure gradient.

Attention turns to the west of the area this afternoon, on weak
low pressure over southwest Iowa. This low will track east into
central IL tonight, in association with a pair of mid-level
disturbances moving through the flow aloft. Warm advection ascent
across IA has been too dry to maintain showers for the most part,
though high-res guidance is in very good agreement in developing
convection across southeast IA late this afternoon as well as in
spreading it east into parts of central/northern IL this evening
and overnight. The trend in most of this guidance has been to
focus the best potential for stronger showers, and thunderstorms,
south of the terminals tonight. Weaker scattered showers are
possible however, in association with the mid-level trough.

Rain and lower clouds should clear the terminals by Saturday
morning. Broad high pressure spreads into the area from the west
during the day, with the weak gradient providing light, generally
north-northwesterly surface winds. While the weak gradient lowers
confidence in specific direction, speeds should be less than 10
kts. Weak lake breeze will also likely push into ORD/MDW by late



209 PM CDT

Low pressure over the northern Great Lakes will lift north away
from the region while high pressure builds in from the west
tonight and tomorrow. Moderate winds in place this afternoon will
diminish overnight as the high builds over the region. Modest
west to southwest flow will persist across the northern half of
Lake Michigan Saturday peaking around 20 kt at times, while
lighter flow across the south end will allow for afternoon lake
breezes and variable/onshore flow. South to southwest flow will
increase again Sunday as another low advances east across the
Canadian Prairies. A trailing cold front is expected to push south
across the Lake through the day Tuesday turning winds northerly
in its wake.






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