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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1221 am CDT Wed Mar 29 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 Illinois 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 Continental U.S.. 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
Illinois 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 quantitative precipitation forecast 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 north-northeast gradient across northern Illinois and hence a
brisk north-northeast 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 06z tafs...

Another challenging forecast on tap with several concerns
throughout the period. First and most pressing is the developing
low stratus and fog moving in off of Lake Michigan. Areas
immediately adjacent to Lake Michigan have fallen to less than a
quarter mile in dense fog. While the fog has been lifting as it
moves inland, there have been rapid falls over the past hour in
ceilings and more recently visibility. Dense fog of 1/4sm or less
is a distinct possibility but right now there is higher confidence
in 1/2sm to 1sm visibility. Expect low conditions to persist
overnight with improvement expected mid to late Wednesday morning,
around 15z or so. Winds will become gusty out of the east-northeast tomorrow
afternoon.

Meanwhile, low pressure is progged to lift across the southern and
Central Plains through the period. Several models have become much
faster bringing precip to the terminals, however forecast
soundings indicate very dry air in place through most of the day
tomorrow. With that in mind, leaning towards slower arrival of
precip to the terminals, around or after midnight local time late
Wednesday. Weak instability is now progged to lift north into the
chicagoland area, so cannot completely rule out an isolated
thunderstorm or two around the terminals.

Bmd

&&

Marine...
256 PM CDT

A north-south oriented ridge of high pressure across the western
Great Lakes will slowly move off to the east tonight and
Wednesday. This will allow for low pressure over the Southern
Plains to lift northeast into the mid Mississippi Valley
Wednesday night into Thursday. This low will then slowly move
northeast across the eastern Great Lakes Friday while a ridge of
high pressure builds back into the Lake Michigan basin Saturday...
moving over the lake Saturday night into Sunday.

Strongest winds of the period will be felt late Wednesday night
through Friday (northeast winds to 30 kts) as the low passes to
the south of Lake Michigan.

Ed f

&&

Lot watches/warnings/advisories...
Illinois...dense fog advisory...ilz006-ilz014 until 9 am Wednesday.

In...dense fog advisory...inz001-inz002 until 9 am Wednesday.

Lm...dense fog advisory...Illinois nearshore waters until 9 am Wednesday.

Small Craft Advisory...Illinois nearshore waters until 3 am Wednesday.

Dense fog advisory...lmz743-lmz744-lmz745-lmz777-lmz779 until 9
am Wednesday.

&&

$$

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