Scientific Forecaster Discussion
fxus63 klot 240459
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1059 PM CST Thu Nov 23 2017
807 PM CST
For evening update...
Clear skies and light winds have allowed a quick fall off in temps
this evening, with readings already dipping to around 30 degrees
in some of the typical cooler spots across northern Illinois. Have
therefore lowered mins into the upper 20's across parts of the Fox
Valley and west into the Rochelle area for tonight. Surface
pressure falls developing across the upper Midwest in response to
low pressure deepening north of the Canadian border may help slow
or halt the temp fall in locations where a light south wind will
persist overnight, so many locations are already at/near their
Otherwise, windy and much warmer weather in going forecast appears
on track for Friday, with some 60 degree temperatures likely
especially across the Illinois portion of the forecast area. Models
typically under-forecast the depth of the mixed boundary layer in
these situations, so would not be too surprised to see 60 degree
weather spread all the way into the Chicago area. Of course winds
gusting to 30-35 mph may make if feel cooler.
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.
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.
for the 06z tafs...
Low pressure is over southern Manitoba late this evening and will
move to James Bay Friday evening while a trailing cold front
pushes across northern Illinois and Northwest Indiana. Through the
day Friday, strong south-southwest winds will be in place ahead
of the approaching cold front. Warm air advection into the region
will result in poor low level lapse rates which will inhibit
effective mixing of 50-60kt winds between 015-020. Still,
anticipate being able to mix down frequent gusts into the mid to
high 20 kt range and would expect at least a few sporadic 30+ kt
wind gusts. Cold front will move across the terminals late in the
evening and overnight with winds turning northwest. There may be a
brief lull in gustiness immediately ahead of the front within the
frontal trough, but expect winds to become gusty again behind the
cold front as steep low level lapse rates develop. No
precipitation is expected with the front. There is still an off
chance for MVFR stratocu to develop behind the front but
confidence remains higher for conditions remaining VFR at this
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
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
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