Scientific Forecaster Discussion
fxus63 klot 192348
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
548 PM CST Tue Nov 19 2019
304 PM CST
Through Thursday night...
Cloudy skies, cool temperatures and some periods of light rain and
drizzle continue to be the weather story across the area this
afternoon. The precipitation is expected to end across the area by
early this evening, but cloud cover will likely continue over the
area tonight. It also appears that some of this cloud cover will
build down into some fog over, at least, southern portions of the
area tonight. This as a surface ridge of high pressure shifts over
Illinois overnight with light surface winds. Temperatures should only
drop off into the lower 30s as a result of the extensive cloud
cover and fog.
Quiet and slight warmer weather is expected during the day
Wednesday. However, another period of more active weather quickly
returns to the area late Wednesday night into Thursday. During
this period a mid/upper level impulse is expected to eject east-
northward out of the southwestern Continental U.S. Trough. As this occurs,
surface low pressure will consolidate over the Central Plains by
Wednesday evening, then shift northeastward over the upper
Midwest and the Great Lakes Wednesday night and Thursday. The
track of this area of low pressure will place all of northern Illinois
and northwestern in in the warm sector on Thursday. For this
reason, we will be in for a very mild (highs well into the 50s),
but windy day Thursday.
The best chances for rain look to be late Wednesday night into
early Thursday morning as a strong influx of moisture occurs along
a strengthening (50+ kt) southwesterly low-level jet. While
instability will be lacking, due to the strong lower level warm
air advection along the low- level jet, I would not be surprised
to see a few embedded thunderstorms with this activity into early
Thursday. In spite of this possibility, I have left the mention
out of the latest forecast, but it may need to be added at a later
time. Any lingering showers during the morning may end up moving
out of the area for a few hour period during the late morning and
early afternoon on Thursday as some drier mid-level air works over
the area. However, some additional showers may shift across the
area again in the afternoon/early evening in association with the
approaching cold front.
Outside of the rain, expect some rather gusty southerly winds and
mild conditions across the area on Thursday ahead of this cold
front. In fact, winds could reach up close to 40 mph for a period
on Thursday as the surface low deepens to a sub 1000 mb low over
southeastern WI. These gusty southerly winds should help push
temperatures into the mid to upper 50s area-wide ahead of an
approaching cold front. Expect the winds to turn west-
northwesterly late Thursday afternoon and evening following the
cold frontal passage.
312 PM CST
Friday through Monday...
Another vigorous trough currently off the Pacific northwest coast is
forecast to close off a mid-upper level circulation as it digs
south to Southern California by midweek. Medium range models all show this
upper low opening up and shearing eastward across the plains by
the end of the week. Overall pattern is quite complex as this
system will be affected by or perhaps interact with digging
northern stream trough. Therefore, there are several moving parts
and lots of room for model error, evidenced by the large model
differences and run-to-run variability that we've already seen.
There have been, and continue to be some model runs that could
threaten parts of the area with some snow on Saturday. While the
threat of significant snow looks quite low for our area, I
continued to follow close to the small nbm pops for Saturday given
the model volatility.
We look to get into a zonal flow pattern over the weekend into
early next week which would tend to favor near or perhaps a bit
above average temps and below average precipitation.
for the 00z tafs...
Lingering -ra/dz associated with a weak surface/mid-level trough
exiting to the southeast will end over the next hour or two at all
sites. Ceiling trends during the afternoon into taf issuance time
have been rather challenging to follow, especially for Ord/mdw. IFR
ceilings just to the west of both sites failed to reach the
terminals, and that should remain the case through this evening.
However, a mix of low and high-end MVFR ceilings should persist into
Wednesday morning as an approaching surface ridge results in a
rather stagnant airmass across the area. As an inversion lowers and
some nocturnal cooling occurs, ceilings will likely reach IFR levels
at times late tonight into Thursday morning. Confidence on how long
the IFR ceilings last or if they end up as prevailing is low, so
have limited the mention to a tempo group. Additionally, while it is
anticipated that br visibility should remain VFR, some periods of
MVFR visibility are possible through the night.
Ceilings are expected to rise, then sct, by late Thursday morning,
with VFR conditions during the afternoon. Meanwhile, winds will
shift from south-southwest to south-southeast and gradually increase at Ord/mdw around 21z
in response to an approaching strong low pressure system.
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