Area Forecast Discussion
Issued by NWS Chicago, IL

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405
FXUS63 KLOT 180804
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
304 AM CDT Fri May 18 2018

.SHORT TERM...
254 AM CDT

Through tonight...

Early morning water vapor imagery shows a 500mb closed low
rotating across Missouri. To its east, pinwheeling vort lobes will
lift north into the forecast area and a broad region of
isentropic ascent will overspread much of the Midwest this
afternoon through most of tonight. 300-310K surfaces begin to
saturate from the south by early this afternoon, but a dry
easterly low level flow should help to keep precipitation isolated
to widely scattered with the best chances east of the I-55
corridor through early evening. Deeper moisture arrives late this
afternoon into the evening when PWats rise to around 1.50-1.75
inches. Precipitation should then become more widespread this
evening and tonight, especially across the eastern half of the
CWA. Mid level lapse rates are fairly poor resulting in weak, if
any, instability, peaking under 200 J/kg over east central
Illinois and northwest Indiana. Will not make any mention of
thunder for this evening and tonight. Meanwhile, the
aforementioned mid level low will be sliding east along the Ohio
Valley tonight. 700mb trough axis is progged to shift east across
the CWA during the predawn hours Saturday which should mark the
back edge of the precipitation.

At the surface, easterly flow will persist today on the periphery
of a broad high pressure centered over Quebec. Modest mixing
this afternoon should be able to tap into stronger winds aloft
resulting in gusts in the 20 to 25 mph range this afternoon, with
a few sporadic higher gusts possible. For much of the Chicago
metro, persistent onshore flow through the day will result in
temperatures similar to yesterday, topping out near 60 immediately
along the Lake Michigan shores, and low to mid 60s elsewhere near
the lake. Farther inland, temperatures will be cooler today,
topping out in low to mid 70s.

Deubelbeiss

&&

.LONG TERM...
254 AM CDT

Saturday through Thursday...

Precipitation chances wind down Saturday behind the departing
upper low, and temperatures should rebound some from today. Weaker
flow should limit the inland extent of the coldest temperatures
near the lake, but a more typical afternoon lake breeze will
result in falling afternoon temperatures for portions of the
Chicago metro.

Active weather will be likely over parts of the Midwest during the
remainder of the weekend through the first half of the upcoming
week. Upper flow pattern across the Great Lakes is progged to become
zonal Saturday night through midweek with fairly nebulous periods
of weak forcing making some of the details of the forecast
unclear. A strong MCS appears likely over portions of the Central
Plains Saturday night on the nose of a strong low level jet and
within a corridor of moderately strong instability. Models vary to
some degree to where exactly this will occur, but somewhere near
the mid Missouri Valley where another closed mid level low is
progged to set up is a good bet with resulting activity riding the
instability gradient ESE across the mid Mississippi Valley.
Decaying showers and/or thunderstorms may move into portions the
local forecast area early Sunday. The mid level wave is then
expected to very slowly shift east into the Upper Midwest Sunday
night and Monday and could serve as the impetus for additional
showers and thunderstorms.

Tuesday through Thursday, a large high pressure is expected to be
centered over Hudson Bay influencing much of the Great Lakes region
around midweek. Chances for showers and thunderstorms continue along
on the periphery of the high which will set up over the Midwest. GFS
and GEM show the greatest chance for showers and thunderstorms
locally Tuesday night into Wednesday, then shift the chances to
the Ohio Valley by Thursday. Meanwhile, the ECMWF keeps the local
area dry with the greatest chances near the Ohio Valley Tuesday
through Thursday.

Deubelbeiss

&&

.AVIATION...
For the 06Z TAFs...

Dry and VFR conditions expected for much of today, while northeast
winds continue. Expect these northeast winds to increase later
this morning into the afternoon, with gusts in the low 20kt range
still anticipated. Have continued the same precip timing/coverage
later today, with scattered showers expected to move overhead
this afternoon into the evening. Scattered showers will likely
continue for much of the evening, and possibly linger through the
overnight hours. Will need to monitor ceiling trends later this
evening into tonight, as lower ceilings than currently forecast
are possible. Confidence is still low, but with a period of low
end MVFR and even IFR possible at times tonight.

Rodriguez

&&

.MARINE...
216 AM CDT

Pattern over the lake will remain slightly similar today into
tonight, as high pressure slowly moves east through Quebec and
while a trough of low pressure develops over the central part of
the U.S. In this setup, east northeast winds will continue with
speeds of 15 to 25 kt likely to continue. Across the nearshore
waters, speeds will continue to be highest over the IL shore where
gusts around 20 kt and higher waves will be observed. Gradient
and winds weaken later tonight into Saturday but it does appear as
if speeds will increase out of the north once again Saturday
night, with speeds possibly higher than currently forecast.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IL nearshore waters until 10 PM Friday.

&&

$$

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