Scientific Forecaster Discussion
fxus63 klot 221720
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1120 am CST Fri Nov 22 2019
202 am CST
Today: a fairly quiet day is in store as surface and mid-level
ridging begin to build into the region. A widespread stratus deck
extending northwest into Minnesota is showing some pockets of
clearing early this morning, with the most pronounced one over the
west half of the County Warning Area. Continued dry air advection above the cloud
deck, which guidance suggests is around 2ft thick, will likely
result in some additional subdued erosion through daybreak, but
current thinking is a little assistance from the sun is needed to
induce enough mixing to fully erode the cloud deck. Expectation is
for a scattering trend to begin mid to late morning, leading to
mostly clear skies by sunset.
Tonight: remaining low-clouds should have cleared the County Warning Area by
early this evening. A west to east surface ridge will continue to
edge into the area, splitting the County Warning Area overnight. A decent surface
set-up for good radiational cooling will be mitigated for much of
the night as high clouds filter in from the southwest, especially
overnight. Lows should generally be in the mid 20s, with some
pockets near 20 if high-clouds end up being rather sparse.
Saturday: a fairly stacked low in the mid-levels currently over Utah
and Colorado will drift eastward across the Ohio River valley on
Saturday. Overall best forcing will remain well southeast of the
County Warning Area. Meanwhile, an exceptionally deep dry layer with relative humidity generally
less than 10% between 2-10kft will limit precip potential, even
with some modest mid-level forcing directly with the low. With the
noted dry air, wet-bulb temps would be supportive of mainly snow,
even with temps pushing 40f. Overall expectation is for a small
chance of a light precip across mainly the southern tier of
counties in the cwa, roughly south of a line from Pontiac to
221 am CST
Saturday night through Friday...
Following the passage of the upper trough on Saturday, we remain in
a northwest to eventually west flow aloft pattern for several days.
This will shift US to a weakly warm advective regime in the west and
eventually southwest low level flow as several weaker systems pass
by generally to our north. We cannot rule out some isolated precip
chances in this pattern Sunday and Monday, but for most areas it
will head US back near seasonal temperatures with intervals of sun
Models are in fair agreement that a somewhat lower amplitude though
fairly potent wave will eject out of The Rockies Tuesday into
Tuesday evening, which appears to be the period of impact for the
local area. There are some subtle differences with respect to the
surface low position and amount of cold air that feeds into the
system, with the GFS the more south/easterly placement supporting a
bit more of a snow profile. The ec/Gem are a bit more amplified and
stronger with the surface low, advertising more of a hydrologic
and wind concern as these sources track the surface low SW-NE and
right through the Chicago area. This would place the higher
confidence of a snow profile toward north central Illinois and into WI.
After a quick shot of colder air a passing surface ridge looks to
bring cool but mostly dry conditions for the busy travel day
The flow becomes a bit more complicated later in the week as a deep
upper trough will dig south across the West Coast. This will place
the local area in a somewhat milder flow pattern. Blended model
precipitation solution keeps some lower chances in place in the warm
advection regime, with chances likely creeping back up once this
wound up system of the West Coast approaches the Great Lakes, but
still considerable spread in how that will pan out.
for the 18z tafs...
Overall, there are no major weather concerns through the period
for the area terminals.
Surface high pressure will move across the area this afternoon. As
it does so, expect the northerly winds over the area to become a
bit more variable, possibly evening shifting east-northeasterly
for a period late this afternoon. The good news is that wind
speeds should remain light, generally under 7 knots. Winds will then
turn south-southwesterly tonight and continue on Saturday.
While VFR conditions are expected through the period some higher
level cloudiness is likely to move over the area on Saturday as
as storm system passes just south of the area. It appears that any
precipitation with this system will also remain to the south of
Lm...Small Craft Advisory...Gary to Michigan City in until 3 PM Friday.
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