Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211600

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1100 AM CDT Sat Apr 21 2018

1100 AM CDT

Minor tweaks to going forecast, mainly to adjust sprinkle coverage
through midday per recent radar and high-res guidance trends.

Regional radar mosaic depicts an area of weak returns from
northern Iowa to northwest/north central IL. Dry easterly low
level flow was preventing much of this from reaching the surface,
though 925-850 mb frontogenetic forcing juxtaposed beneath right
entrance to upper jet core over the northern/eastern Lakes has
managed to produce some precip to the surface from DBQ to RFD over
the past couple of hours. Guidance depicts overall weakening of
this forcing across the area into the afternoon, though expect
that some sprinkles will linger through at least midday across
portions of the forecast area from RFD to CHI area. Thus expanded
low pops a bit through midday/early afternoon to better match
current radar and high res guidance depiction of very light
precip. Otherwise, no significant changes at this time.



224 AM CDT

Through tonight...

High pressure will remain over the central and eastern Great
Lakes today and tonight resulting in fair weather and a light
easterly breeze across northern Illinois and northwest Indiana. To
our west, baroclinic leaf is noted on satellite imagery with
cloud cover increasing across the local area as this feature
slowly shifts east across the Great Plains today. GFS continues to
develop a narrow corridor of light precipitation today, and now
the NAM and several high-res models have joined the GFS camp.
Forcing is not overly impressive, mainly in the form of broad
diffluence aloft in advance of the upper low to our west and
modest compact shortwave. Meanwhile, very dry antecedent
conditions are in place. 00Z KDVN Raob recorded 0.21 inches PWat
and shows very dry conditions below 500mb. No low/mid level
moisture advection into the region is expected under the influence
of the 850mb high pretty much overhead through the morning, so
would anticipate the dry layer will be very hard to overcome. May
see some echoes on radar but think most precip would evaporate
before reaching the ground. Mid-high level lapse rate are poor,
which should prevent any convection. At most, perhaps a few high
based sprinkles will be possible, but likely not enough to fully
wet the ground. Will carry silent 10-14 PoPs today and introduce a
slight chance for some sprinkles. Cloud cover will limit mixing
this afternoon and likely trim a couple degrees from our high
temps compared to yesterday, but still expect near seasonable
warmth this afternoon with highs in the mid to upper 50s. Onshore
flow will keep temperatures in the 40s near the Lake Michigan



224 AM CDT

Sunday through Saturday...

Quiet weather is expected to continue Sunday into Monday as
mid/upper closed low moves from the central Great Plains to the
Tennessee Valley. Expect the local area to fall on the northern
periphery of the clouds from this system, but dry weather will
prevail with temperatures expected to be near the seasonal norms.

Upper low will meander north towards the Ohio Valley Tuesday and
some models show areas east of the I-57 corridor getting some
light rain on Tuesday. Meanwhile, another disturbance is progged
to dig from the Northern Plains Tuesday across the Mid Mississippi
Valley late Wednesday. More widespread showers appear possible
somewhere in the region, but there are large discrepancies in
location, so for now will maintain slight chance/low chance PoPs
Tuesday night into Wednesday.

Yet another wave takes aim on the area late in the week with a
more organized cold front expected to sweep across the region
Thursday night bringing another chance for showers. Looks like
only a glancing blow of cooler temps will occur Friday then
temperatures moderate again by Saturday. Broad upper troughing
sets up behind the front over the Great Lakes next weekend so the
cold air won`t be too far away next weekend.



For the 12Z TAFs...

Surface high pressure will continue to prevail over the local
airports today, keeping winds mainly light apart from a lake
influence into the Chicago sites. This should push northeast
winds this afternoon to 7-8 kt with sporadically higher.

As mid clouds continue to thicken and lower, virga and patchy
sprinkles will likely develop over far northern Illinois this
morning. The very dry low-levels should prevent any visibility
restrictions or significantly lowering ceilings.



101 AM CDT

Continued quiet weather over Lake Michigan through Monday night
with high pressure prevailing. This will present a variable wind
direction at times but with minimal speeds. A cold front will move
southward across the lake during or around Tuesday night bringing
northerly winds in its wake. This may present Small Craft
Advisory criteria conditions in the Illinois and Indiana
nearshores into Wednesday.






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