Scientific Forecaster Discussion
fxus63 klot 170210
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
910 PM CDT Mon Oct 16 2017
909 PM CDT
No impactful weather to speak of for the rest of the work week,
but with respect to tonight and tomorrow's forecast, made a few
small changes. The envelope of the expansive surface high over the
Ohio Valley is close enough for much of tonight to keep winds calm
to light south-southwest. With widespread dew points in the upper
30s late this afternoon and this evening, am anticipating
favorable radiational cooling to enable lows to dip to just below
40 in spots, with around 40/low 40s elsewhere outside Chicago and
upper 40s to around 50 downtown. As some patchy cirrus passes by
overnight and winds pick up a bit toward daybreak, the temperature
drop should flat-line in the pre-dawn hours. On Tuesday, continued
dry air mass on the northwestern periphery of the high, gusty
south-southwest winds (up to 25-30 mph) and low level warm advection
will be very favorable for a sharp temperature rise of 25-30 degrees
by afternoon, with highs in the upper 60s to around 70. Bumped up
wind gusts across the area and also increased high temperatures
slightly in some areas, including Chicago.
136 PM...no forecast concerns this period. High pressure centered
over MO/Arkansas will slide east to Tennessee/Kentucky by morning and this will
likely allow some light southwesterly winds overnight...possibly
increasing some toward morning. So while temps will quickly fall
into the 40s this evening...they may end up holding steady in the
lower 40s overnight and then rise quickly Tuesday morning. Its
possible the usual cool spots and perhaps the southeast cwa may
dip into the upper 30s but confidence is low. Thus...no mention of
frost anywhere in the area tonight/Tuesday morning. Southwesterly
winds will steadily increase Tuesday morning and expect winds in
the 15-25 mph range with some higher gusts. Guidance highs in the
upper 60s look on track but with full sun and if low levels mix a
bit deeper...possible some areas will tag 70. Cms
136 PM CDT
Tuesday night through Monday...
Quiet and seasonably mild conditions are expected for the remainder
of the week, with the next chance of precipitation coming Saturday
night into Sunday.
The broad ridge across the eastern Continental U.S. That stretches from
Texas through the Great Lakes today will pivot slightly by early
Wednesday, aligning from East Texas into New England. This
orientation will continue to block any return of Gulf moisture into
the Midwest. At the same time, a deep low will be moving across
Canada and creating a considerable pressure gradient against the
ridge to its southeast. This gradient appears to peak Wednesday
evening, though the strongest winds would likely be Wednesday
afternoon around the time of peak mixing. Decoupling with the loss
of diurnal heating appears likely to limit speeds later in the
evening. If timing of the strongest gradient was a bit earlier in
the day, a Wind Advisory might have been Worth considering.
The gradient relaxes Thursday and Friday with the passage of the
Canadian low into the eastern provinces, but the ridge to our
southeast remains in place and continues to block Gulf moisture. By
late Friday the situation begins to change as the center of the
ridge moves into the Carolinas and starts to open the western Gulf.
It will take a day or so for moisture to work its way into the upper
Midwest, but the anticipated approach of a cold front Saturday night
into early Sunday would provide a focus for this moisture advection
and the support for a wet Sunday.
for the 00z tafs...
Winds are the only weather element of interest through Tuesday. As
the pressure gradient increases late tonight, sustained south-
southwest winds will start to inch up. This will likely limit
concerns for low-level wind shear, though southwest winds will be
35-40 kt at 1000-1500 ft, and possibly a tad higher over rfd.
Occasional surface gusts are likely by shortly after daybreak,
with southwest gusts increasing in magnitude and frequency by
mid-morning. There could be sporadic southwest gusts in excess of
25 kt during the late morning through mid-afternoon. Confidence
in wind elements is medium-high.
224 PM CDT
Despite the quiet weather this week, the persistent and occasionally
strong south to southwesterly flow could produce some challenging
conditions for portions of the lake at times.
The primary window of concern comes late Wednesday as a very tight
pressure gradient and mild lake waters will support a mix down of
winds to around 40 knots and considerable wave growth at the north
end. A gale watch will be posted for this period, and while the
south end also could see some gale force gusts, winds are not
expected to be quite as strong for as long in this area. Even so,
it is possible the watch would need to be extended farther south if
Even before the primary period late Wednesday, waves of 6 ft or
greater will be likely across the northern and eastern portions of
the lake. Conditions then relax toward the end of the week.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...7 am Tuesday to 7 PM Tuesday.
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