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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1145 am CST Thu Nov 21 2019

Short term...
256 am CST

Through Friday...

No real changes in forecast for the today which is in good shape.
Surface low over Iowa early this morning will quickly move to Sault
Ste. Marie by early this evening. Large area of warm air advection
driven showers has just about overspread the cwa, running a bit
ahead of schedule. Very strong low level Theta-E advection on the
nose of 60-70kt low level jet has lead to some heavier showers on
the western flank of the incoming slug of rain, and those heavier
showers should reach the Chicago area in time for the first part
of morning rush hour.

In the wake of this first batch of warm air advection driven showers, look for
decrease in coverage of precip, outside of perhaps a bit of
drizzle or spotty showers. It is during this drier time that
strong & gusty southerly winds will draw the mildest and most
"humid" air mass north into the area that we've seen in weeks with
temps climbing well into the 50s and perhaps making a run at 60f
south of I-80.

Next, a possibly broken band of showers and perhaps an isolated
thunderstorm will move across the area during the mid-late morning
hours out ahead of a cold front which will sweep across the area
this afternoon. The passage of the cold front will Herald the end
to the rain and result in a wind shift to gusty westerly with
temps falling in the wake of the front.

Seasonably chilly air mass will be in place tonight into Friday in
the wake of this system. Gusty winds, which could gust to 40 mph
in spots this morning, will gradually ease starting late this
afternoon abut especially tonight. Low clouds will likely linger a
good portion of tonight, though guidance is pretty insistent in
showing clearing late tonight from west to east. Assuming skies
clear as expected, Friday looks to be a sunny, seasonably cold
day with light winds and highs in the 30s.

- Izzi

&&

Long term...
325 am CST

Friday night through Wednesday...

The main concern is the potential for some wet snow on Saturday,
particularly from I-55 and southeast. Most of Friday night will be
quiet and fairly chilly under weak high pressure ridging. Clouds
will be on the increase overnight in advance of saturday's system.
Models have been struggling with handling of the mid and upper
level features, likely owing to it evolving from a closed low
currently over the central and southern rockies. The closed mid-
upper low will migrate eastward through Friday, with surface
cyclogenesis on its southeast flank over the lower Mississippi
Valley by Friday evening. The surface low will then track
northeast across the Ohio Valley into Saturday.

While the surface low track will be fairly far off to our
southeast, the northwest fringe of the developing deformation
precip should get into at least the southeast 1/2 or 1/3 of the
County Warning Area. Have leaned toward the global models for this portion of the
forecast, which also have the support of a sizable number of
ensemble members, and discounted NAM as a southern outlier for
now. With this in mind, bumped up pops near and southeast of I-55
and did include some likely mention for east central Illinois counties
into portions of Northwest Indiana.

The air mass aloft will be fairly marginal and possibly above 0c
prior to wet bulb cooling, while surface temps should get down to
below freezing before precip onset. Can't rule out some mixed
precip for a time, but it appears the column will cool to support
wet snow as p-type for a time Saturday morning. Other question is
how long temps will get held down by precip processes as to
duration of snow as primary p-type. This in addition to
uncertainty with how far northwest the precip shield gets given
how much models have struggled handling system track and evolution
overall. In the current official gridded forecast, have only a
small area of snow accums under 1 inch southeast of I-55, but this
is certainly lower than the deterministic GFS and European model (ecmwf). Thus
expect additional changes to this part of the forecast should
subsequent trends support the global guidance. It's possible some
light precip could linger into Saturday evening, though have not
yet added any pops.

Later Saturday night and Sunday through Sunday night will be dry
with near seasonable temperatures. Looking ahead to the abbreviated
Thanksgiving work week and busy Holiday travel period, forecast
confidence is unfortunately very low. With southwest flow and
warm advection, Monday has a decent chance to have above normal
temps into the upper 40s-low 50s (mid 40s normals). The jet stream
pattern across North America will become quite active next week,
so ingredients could come together to support a stronger storm
system at some point, as hinted at by recent operational runs and
several ensemble members. However, uncertainty is far too high to
be confident in any timing specifics. Have some mention of precip
from Monday evening and Onward, but in reality, we won't have
precip throughout this time. Temperature trends are also low
confidence due to the active flow pattern and high uncertainty.

Castro

&&

Aviation...
for the 18z tafs...

The main band of pcpn has moved off to the east, through a few
isold showers or patchy drizzle may persist into the early
afternoon hours. Following the period of rain, cigs have lowered
to IFR at some locations. Expect that the window of opportunity
for the IFR cigs to be relatively brief. Expect that cigs should
improve to MVFR levels following the passage of a strong cold
front. At issuance time, the front was moving into far nwrn Illinois and
is expected to sweep relatively quickly across the region. In
addition to the IFR cigs and patchy ra/dz, sswly winds arnd
20g30kt will persist. Following the passage of the front, winds
will veer through wly through the afternoon and then nwly this
evening and overnight. Gustiness should persist through much of
the night as strong cold advection sets up behind the front as
well as a strong pressure rises on the order of 5-8 mb/3 hr.
Latest forecast soundings suggest that the MVFR cigs may persist
into the late night hours before the low level dry air overspreads
the region as a large area of high pressure with a cold, dry air
mass spreads across the region by early tomorrow morning, bringing
clearing skies and diminishing winds.

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...Gary to Michigan City in until 3 PM Friday.

Small Craft Advisory...Winthrop Harbor to Gary in until 10 am
Friday.

&&

$$

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