Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 212336

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
636 PM CDT Sat Oct 21 2017

243 PM CDT

Through Tonight...

Main forecast concerns/challenges are with precip trends later
tonight into early Sunday morning. In the near term, temperatures
have been able to rise well into the 70s under sunny to partly
cloudy skies. Somewhat lower temps are in place over the eastern
CWA, as a steady stream of cloud cover continues to push overhead.
This is due to some weak energy traversing the region and with
WAA continuing across the region. Conditions have remained dry for
most locations, but it is in this area where isolated sprinkles
have been probable. Will likely continue to see this same trend
continue into the evening, but with once again, most locations
staying dry.

Slowing trend with the upstream system has once again occurred
today, with these dry conditions expected well into the night.
Won`t really begin to see precip across the western CWA until
around midnight or soon after. This is as upstream energy over the
central Plains lifts across the region and helps to push this
surface trough/front east. A bigger push doesn`t arrive until
later tonight and more into Sunday with the surface features and
precip tied to it more than likely staying confined to the western
CWA, I-39 corridor and areas to the west. Have adjusted pops to
reflect latest trends, with most locations in northeast Illinois
and northwest Indiana staying on the dry side tonight. Thunder
chances remain mainly for the western CWA. However, given the time
frame, would think that thunder would be on the diminishing
trend. A steady south wind will persist tonight with sporadic
gusts likely, and in this pattern, would expect to see milder
temps. Have brought min temps up a couple of degrees.



232 PM CDT

Sunday through Saturday...

A notable change in the pattern and just feel of the weather is
in store for the coming week. This will come in waves, with the
first being a cold front on Sunday and the second a cold front
Monday night into a blustery Tuesday. Each of these shots along
with the ones later in the week will come with clouds and rain
shower likelihood.

As for Sunday, the morning will begin with the area in a warm
sector ahead of a cold front. Guidance has become more centered on
a low lifting along this boundary from the Texarkana area early
in the morning to near Chicago by evening. This may mean for the
eastern half of the CWA a slower start to showers. Also
supporting a possible mainly dry morning east is that there is a
signal in guidance for ample convection in the Lower Mississippi
Valley region in the upstream trajectory of our moisture. By early
afternoon though, with the high moisture plume shifting over the
area, there should be a swath of showers easing their way east
encompassing a large part of the CWA. Confidence is not the
greatest on what shower coverage will be in the afternoon, but
feel it should still be enough to warrant high PoPs.

Highs on Sunday will likely be reached early, with the east seeing
mid 70s if it stays dry in the morning. Cooler air will only ease
in with highs still in the mid 60s Monday, before the true cold
advection develops. This will be in response to phasing of
northern and southern streams to the east of the area. This is a
good synoptic setup for not only cold advection and windy
conditions, but potential for a large precipitation shield to
wrap around. Depending on where and how this phasing occurs, this
rain probably will be more favored north and east in the CWA.

For highs on Tuesday, have leaned toward cooler raw guidance, with
upper 40s to around 50. Wind gusts presently are 25 to possibly 30
mph, but that could be underdone, especially considering the 12Z
EC solution has a 979 mb low in northern lower Michigan. Given
the continued winds and probably some clouds on Tuesday night, do
not have any frost forecast despite mid 30s for lows. It is
possible if enough surface ridging is in place Wednesday night
that there is frost or maybe even freeze concerns then. Low
confidence at this point.

Confidence beyond Tuesday/Wednesday in specifics becomes quite
low the rest of the forecast. The pattern remains active which
makes sense given the North American jet pattern and larger scale
teleconnections. There is a general sign of a deep low pressure
in the Upper Midwest/Great Lakes region late in the week, but
difficult to say. Could be a sharp temperature swing up and then
down depending on how things play out.



For the 00Z TAFs...

Aviation concerns are primarily timing arrival of rain/showers
Sunday morning, and development of MVFR cigs/vis in rain during
the day. In addition, gusty south winds will ease tonight though
winds 45-50 kts 1500-2000 ft agl will again present a LLWS issue.
Cold front brings sharp wind shift from south-southwest to north
Sunday afternoon.

Surface low pressure continues to deepen and track slowly
northeast across Manitoba, with a slow-moving cold front trailing
across the Plains. Low level moisture will continue to increase
tonight on breezy south flow, with rain chances increasing by
early Sunday morning as the cold front moves into the Mississippi
Valley. While scattered showers may develop as early as the pre
dawn hours, especially for RFD, the greatest potential for
persistent rain/showers looks to be mid-morning through mid-
afternoon immediately ahead of the surface cold front and
approaching mid-level trough. Weak elevated instability will also
present the potential for some embedded thunder, though better
chances are expected mainly west and south of the terminals. VFR
cigs will thicken and lower into early Sunday as moisture
increases, with MVFR cigs expected to develop with precip Sunday.
Patchy IFR cigs not out of the question by early afternoon.
Gradual drying develops behind the front toward evening at RFD and
toward midnight for Chicago TAF sites.

South winds around 15 kts still showing some gusts above 20 kts at
ORD/MDW in 1-minute ASOS obs. Slower temp drop this evening along
with tight gradient will help keep sporadic gusts early in the
evening. Once gusts diminish though, 1500-2000 ft winds of 45-50
kts will support LLWS overnight. South-southwest winds expected to
shift north-northwest with cold frontal passage Sunday, early
afternoon at RFD and close to 21Z/4 pm for Chicago. Gusts around
20 kts expected initially behind front, with a gradual decrease
into Sunday evening.



257 PM CDT

A more classic mid-late autumn pattern will establish itself over
the lake during this coming week. First for tonight, south winds
to 30 kt will show some potential for occasional gales late
tonight and Sunday morning, mainly over the north and central.
A cold front will ease its way over the lake during mid to late
afternoon Sunday. The west to northwest winds behind this front
will actually be less than ahead of the front.

A second cold front on Monday night will be a more potent one,
especially as there is increasing signal of phasing lows near the
Lake Huron area. Given the conceptual model of the pattern, this
could easily mean high gales and possibly storm force winds over
at least the northern part of the lake Tuesday and Tuesday night.
Accordingly, waves could be underdone too in the forecast, with
southern and eastern parts of the lake having potential for 12+
foot waves through much of that time. That will be a period to
keep a close eye on.

The active pattern continues through the rest of the week, with
possible gales again late in the week.



LM...Small Craft Advisory...nearshore waters
     until 4 PM Sunday.




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