Area Forecast Discussion
Issued by NWS Chicago, IL

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443
FXUS63 KLOT 281921
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
221 PM CDT Tue Mar 28 2017

.SHORT TERM...
157 PM CDT

Through Wednesday...

Main concern for the short term forecast period will be timing of
pcpn spreading from west to east across the region tomorrow.

Conditions will remain quiet through the remainder of the
afternoon and overnight as a ridge of high pressure builds
across the upper Mississippi Valley to the upper Great Lakes,
while low pressure begins to develop over the south high plains.
Prevailing enely-nly will keep wind flow off of Lake Michigan,
with lakefront locations remaining cooler than inland locations.
Highs today should range from around 40 F near the lake to around
50 F well inland. The only concern of note through the night will
be the amount of cloud cover across the region. Latest satellite
imagery shows some drier air moving swd through srn WI as the
ridge builds ewd, far IL could see at least some partial clearing
overnight, which, in turn, would allow for temps to drop off a
little faster than the remainder of the area. Lows tonight should
range from the middle 30s along the WI border to the upper 30s to
low 40s south of I-80. Any partial clearing of cloud cover should
be short lived as low pressure developing over the south high
plains deepens and tracks to the northeast as the parent upper low
lifts into the central plains through the day tomorrow. While sfc
flow should trend to more sely, mid level toughing should help
begin to draw some Gulf moisture northward. Weak short wave energy
lifting out ahead of the upper trough will help to initial some
pcpn across the area, with pcpn chances generally spreading from
west to east through the day, possibly beginning over the RFD area
by early afternoon and spreading to the Chicago area by late
afternoon or early evening. While the upper low begins to lift out
of the desert southwest tomorrow, upper ridging will build over
the upper midwest, allowing for temperatures a few degrees higher
Tuesday than were seen on Monday. While lakefront locations will
still remain relatively cool, with highs in the lower 40s, inland
locations should see highs in the lower to middle 50s.

&&

.LONG TERM...
221 PM CDT

Wednesday night through Tuesday...

The upper level split flow pattern will continue this forecast
period with series of upper level systems progressing west to east
across the CONUS. Hence main forecast concerns will be rain with
scattered thunder Wednesday night through Thursday night with the
next approaching upper low... followed by rain chances returning
with the next upper low Monday-Tuesday.

Upper level low currently near the 4 corners area will work slowly
east into the southern plans Wednesday. Both rising heights aloft
ahead of the low combined with relatively dry anticyclonic boundary
layer flow around slowly departing Hudson Bay surface high may
help to slow increase of precip coverage/intensity into northern
IL initially. Weak upper shortwave riding a top the shortwave
riding combined with modest isentropic ascent may help squeeze out
sprinkles/patchy light rain as early as afternoon over the
RFD/I-39 corridor and toward the evening rush across the Chicago
metro.

Better rain to get going overnight Wednesday into Thursday as the
column saturates and given broader ascent ahead of the northward
moving warm front. 12z GFS/NAM QPF amounts trending upward from
previous runs with 48 hr rains of 1.0 to 1.25 inches Wed-Fri
seeming reasonable now. While mid level lapse rates remain more
impressive to our south, can`t rule out some rumbles of thunder,
particularly southern sections of the forecast area late Wed night
and Thursday... and the entire area by Thursday afternoon/evening
as lapse rates steepen slightly as the surface low reaches to
just south of Kankakee by Thursday evening per GFS/Canadian GEM
solutions.

Given such synoptic set up by Thursday afternoon/evening also would
expect fairly tight NNE gradient across northern IL and hence a
brisk NNE flow with gusts 25-30 mph... perhaps as high 35 mph at
the lakefront.

Low slides slowly off to the east Friday. Weak surface ridging
allows for residual low level moisture to persist much of the day
Friday however before rising heights aloft and associated
subsidence helps scour out low level moisture Friday night into
Saturday. Weekend shaping up generally dry with next chance of
rain arriving later Monday as next in the series of upper troughs
tracks well south across the southern plains but then lifts
northeast across the Ohio Valley into Tuesday.

Ed F

&&

.AVIATION...
For the 18Z TAFs...

Ceilings remain the primary forecast challenge and even the short
range guidance continues to have a poor handle on the details of
the trends. The observed trends this morning have been for a very
slow lifting/improvement of cigs, and anticipate that this should
be the trend for the remainder of the afternoon and through the
night. Looking at sounding information, an inversion should remain
in place through the period, which would inhibit vertical mixing
down of drier air aloft, so have maintained the mvfr cigs through
the night for the Chicago area terminals. With high pressure
building across the upper Mississippi Valley and upper Great
Lakes, winds should generally remain nely, so flow will remain off
the lake for the Chicago area terminals, helping to keep the lower
cigs in place. Latest satellite trends indicate some upstream
scattering of cloud cover over srn WI which coincides with the
drier air. So, have improved cigs at RFD to vfr, given the drier
advection. The greatest uncertainty to the forecast will be for
cigs tomorrow morning. With an inversion becoming stronger
overnight, and winds remaining off of the lake, there is a chance
that cigs will not have an opportunity to improve to vfr tomorrow
morning, but confidence is relatively low at this time to keep the
mvfr cigs through the morning hours tomorrow. So, have maintained
an improving trend for cigs for tomorrow, but there is a chance
that cigs will need to be lowered in later updates. Otherwise,
have relatively high confidence in wind trends given the
persistence of the ridge of high pressure moving across the upper
Mississippi Valley and upper Great Lakes while low pressure
deepens over the central plains tomorrow. This pattern would
suggest a trend to more ely winds tomorrow.

&&

.MARINE...
258 AM CDT

Moderate north flow will be in place across Lake Michigan today
behind low pressure departing from the Ohio Valley to the Mid
Atlantic Coast and high pressure in place across the Canadian
Prairies and Upper Midwest. Low pressure will develop over the
southern Plains Tuesday and then lift to Missouri midday Thursday.
Winds over Lake Michigan will veer to the northeast Wednesday and
to the east on Thursday as the low approaches. East winds on
Thursday are expected to be around 30 kt across the south half of
the lake, and there is some potential for a short window of
gales. Winds will back to the northeast as the low continues east
to the Mid Atlantic coast late in the week, but will remain
moderately strong gusting to 30 kt at times into Friday. Winds
should diminish some over the weekend as high pressure builds into
the western Great Lakes.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IN nearshore waters until 9 PM Tuesday.

     Small Craft Advisory...IL nearshore waters until 3 AM Wednesday.

&&

$$

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