Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 232037
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
237 PM CST Thu Nov 23 2017

.SHORT TERM...
235 PM CST

Through Friday Night...

A quiet Thanksgiving Day of weather with temperatures right around
normal for late November. High pressure will depart eastward
tonight with the area in turn under influence of the next deep
low pressure across southern Canada. South to southwest winds will
increase sharply during the morning as a tight pressure gradient
and added isallobaric component from strong pressure falls
envelop the region. Afternoon wind gusts of 25-35 mph are forecast
depending on just how much the near-surface layer warms and mixes.

This regime will steer a pronounced low-level thermal ridge into
the area, with 925mb temperatures of 11C-15C. These values in
late November correlate to surface temperatures in the low-mid
60s based on reanalysis data. While there will be a strong
inversion probably below 925mb, additional support for a warmer
Friday are also the facts that today over achieved with mid-upper
40s, as well as upstream observed temperatures in western Iowa
into Nebraska this afternoon in the 60s. So have bumped Friday
highs up quite a bit from guidance, as has been the trend for a
few days, with forecast highs around 60.

The system cold front is expected to move through early in the
evening. With some recent northwest flow cold fronts, model
guidance has been too dry with passage, and there are some signs
this may be the case with this fropa (increasing moisture,
700-925mb f-gen, additional jet support). So we have added a
slight chance of some light rain/sprinkles, mainly between 5-9 pm
on Friday. Behind the front, winds will turn northwest and gust
again to around 30 mph. Temperatures will be on the fall but not
overly sharp, especially considering what cold fronts can do this
time of year.

MTF

&&

.LONG TERM...
235 PM CST

Saturday through Thursday...

A quiet weekend will unfold with high pressure in control with
temperatures right around normal under ample sunshine. The upper
air pattern will remain similar through the first part of next
week with an active jet to the north. The next low will pass
north of the area on Monday night with again breezy mild
conditions in advance on Monday, and a cold front with possible
precipitation on Tuesday. The warmth on Monday could again be well
into the 50s or even 60+ given the forecast pattern, however will
depend on just how quickly the thermal ridge moves in. The 12Z GFS
is quite a bit more robust than the EC run. Global solutions also
try to show a closed upper low across the southern Plains
advancing into the Ohio River Valley during midweek. Confidence
is low on how/if this will impact the area.

MTF

&&

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

The one concern for aviation weather is wind on Friday, with
south-southwest gusts around 30 kt in the afternoon appearing more
likely.

Southwest winds today will show some fluctuation at times late
this afternoon into the evening but will be below 10 kt in that
time. As a deep weather system moves east across southern Canada
late tonight into Friday, south-southwest winds will be on the
increase. The pattern favors temperatures to warm considerably at
the surface on Friday and as such, the atmosphere will likely mix
down higher gusts than shown by models in the afternoon. Still,
there is decent uncertainty on just how high gusts will get and
especially how frequent gusts of around or even slightly higher
than 30 kt will be in the afternoon. Confidence is high though on
the peak time of winds being between 17Z-23Z, and in the direction
being between 190-220 degrees during that time.

Wind speeds around 2000 ft look to approach 50-55 kt from the
southwest, and there should be a fairly steep inversion between
1000-2000 ft. While not mentioning low-level wind shear because it
will likely be below criteria, 50 kt+ speeds are not often seen
below 2000 ft during the afternoon, so worth noting here.

MTF

&&

.MARINE...
235 PM CST

The active marine pattern continues with a gale event on Friday as
another deep low pressure moves eastward just north of the Great
Lakes. South-southwest winds will be on the increase tonight with
gales likely by sunrise over the open water. There is some
uncertainty over the nearshore areas, though gusts at and a
little above 30 kt are likely much of the afternoon. A cold front
will pass early Friday evening, and as winds turn northwest, there
could be a few hour period of occasional gale force gusts.
Otherwise 30 kt winds are likely.

The next system will influence the lake on Monday again as low
pressure passing north of the Lakes. Southwest winds on Monday
look to potentially reach gales later in the day or into Monday
night. The cold front is likely to pass sometime early Tuesday at
this point.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning...LMZ777-LMZ779...6 AM Friday to 9 PM Friday.

     Gale Warning...LMZ743-LMZ744-LMZ745...9 AM Friday to 6 PM Friday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...3 AM Friday to 6
     PM Friday.

&&

$$

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