Scientific Forecaster Discussion
fxus63 klot 212017
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
317 PM CDT Mon Oct 21 2019
312 PM CDT
Through Tuesday night...
Main forecast concerns/challenges are with lingering shower and
thunderstorm chances this afternoon, and with strong winds/gusts
possible again tonight into Tuesday.
Deepening surface low continues to lift across the upper Midwest,
while associated front is swinging through the area. This setup
still supporting precip development across parts of northeast Illinois
and northwest in. With greater large scale ascent departing to the
north, have noted the coverage of this precip lower over the last
couple of hours. Think the current scattered development will
likely remain before this precip exits over the next couple of
hours, with support still likely stemming from the front. As
previously thought, thunder has remained really limited with no
change to this trend expected. However, before this precip exits
over the next couple of hours, an isolated storm producing brief
heavy downpours will be possible. Mainly dry conditions then
expected for most locations tonight through Tuesday. With a large
upper level trough and cyclonic flow in place, isolated to
scattered light showers will be possible across parts of northern
Illinois Tuesday morning.
Winds continue to veer to a south southwest direction this
afternoon in the wake of the surface trough/front, with gusts in
the 30-40 mph range. For the remainder of the afternoon, should
see these similar winds/gusts, and possibly into part of the
evening. However, still monitoring the potential for stronger
winds/gusts returning later tonight into Tuesday morning. Cold air advection will
be the trend during this time, however, there is still some
uncertainty on the extent of the mixing tonight. If we mix more
than current guidance is showing, than gusts approaching 45 mph or
higher will be a concern. If this were to occur, locations across
north central would be the areas to monitor. The threat of these
stronger winds/gusts is also possible on Tuesday especially as
daytime mixing occurs. The frequency of these gusts may not be too
high though, as guidance is showing the flow weakening during
300 PM CDT
Wednesday through Monday...
A warm front will lift back into the area ahead of another
Pacific disturbance in northwest flow aloft shifting out of the Canadian
provinces. This disturbance is slated to translate down the left
over front late Wednesday, bringing the next bout of precipitation
chances to the area. Even though warm advection looks fairly
Stout with this next wave, it will have to work hard against a
very dry atmospheric column. Thus, we've continued to cap pops in
the slight to low-end chance category Wednesday afternoon and
evening from I-80 and north, highest chances far north. High
temperatures will be just on the cool side of normal for most,
ranging from the mid to upper 50s I-80 and north to upper 50s to
low 60s south of I-80.
This system will drive a cold front through the region Wednesday
night into Thursday, with dry conditions in its wake for most
areas. The NAM is a bit more aggressive with precip potential with
the frontal passage, and appears to be an outlier at this time. In
addition, behind the front, tightening thermal gradient aloft
could support f-gen driven banded light rain into portions of
central Illinois and Indiana Thursday afternoon and evening. Best
chances of this appear to be mostly south of the cwa, but do have
some slight chance pops for areas south of I-80. Clouds and cool
advection will keep temps Thursday only in the upper 40s to lower
50s north of I-80 and lower to mid 50s south.
Temperatures by Friday morning may drop to near or below freezing
across a good chunk of northern Illinois outside of Chicago,
especially if clouds can clear out. Friday will be another cool
day in the upper 40s to lower 50s but likely with light winds and
a good deal of sun during daylight hours. After a chilly night
Friday night and start to Saturday, it now appears return
southerly flow will be quicker to return on Saturday, yielding
close to seasonable highs for the last weekend of October. Models
diverge into Sunday, with recent runs of the European model (ecmwf) lifting a
southern branch system northeastward across the region. Given
large model variance and thus lower confidence, have only slight
chance to lower chance pops for Saturday night into Sunday.
Temperature departures will be driven by whether there is precip
in the area. Whatever happens Sunday, the last work week of
October should get off to a dry and near seasonable start Monday.
for the 18z tafs...
Concerns through tuesday:
* strong/gusty southerly winds becoming southwest this afternoon
* chance of ts in the area through 19-20z this afternoon
* MVFR cigs early this afternoon returning tonight and continuing
through most/all of Tuesday
* strong southwest to west-southwest winds Tuesday
* chance of light rain or dz Tuesday morning
A strong low pressure system will impact the area through Tuesday.
Main issue initially is wind direction close to due south (170-180
deg) with gusts to 30+ kt. Also threat for ts in the Chicago area
through 19-20z, for which a thunderstorms in the vicinity was included for at least air
space impacts. Can't rule out direct terminal impacts but would
likely be brief. Visibility could drop as low as ~2sm in heavier rain showers
activity but will mostly be in 3-5sm range. Cold front passage
just prior to 20z will shift winds to southwest (210-230 deg), and
have higher confidence in this trend. Gusts will likely be at
least occasional in the 30-35 kt range. It's possible there's a
lull for a time this evening and earlier in the overnight where
gusts over 30 kt are less frequent. Late tonight through Tuesday,
forecast soundings suggest a high likelihood of frequent gusts of
about 35 kt from a southwest to west-southwest (230-250 deg)
direction. Occasional gusts approaching 40 kt are possible Tuesday
morning. Speed and gust magnitude may come down some after 21z
MVFR cigs in place ahead of and just behind the cold front this
afternoon will become VFR, with MVFR returning tonight, though
uncertain on exact timing. There could be a period of -ra or dz
Tuesday morning, which may be capable of temporary visibility
reductions. Included a prob30 in the tafs for now. At rfd, cigs
may drop to 1000 ft above ground level or just under for a time Tuesday morning.
The stronger gales which were observed across the nearshore waters
earlier today have since diminished, however, do think winds/gusts
will increase once again into this evening and the overnight
hours. More prevailing gales are still likely tonight into
Tuesday, with no big changes to the gale headline. Did move up the
start time of the warning though, to account for a slightly
earlier arrival of the stronger gusts.
Illinois...Lakeshore flood advisory...ilz014 until 9 PM Monday.
Lm...Gale Warning...nearshore waters until 4
Gale Warning...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...3 am
Tuesday to 9 PM Tuesday.
Small Craft Advisory...nearshore waters
until 3 am Tuesday.
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