Scientific Forecaster Discussion
fxus63 klot 140902
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
302 am CST Thu Nov 14 2019
302 am CST
Today through Friday night...
Radar depicts the last of the light snow was just about finished
across far northeast Illinois/northwest in early as of 230 am the
morning. What lingers is very light, with visibility generally
6-10+ miles per surface obs, and should end between 3-4 am. Had
noted some reports of freezing drizzle earlier in the night where
mid-level saturation had decreased across parts of eastern Iowa and
southwestern WI, with some weak returns also showing up in the arx
88d reflectivity imagery. However, low level convergence along
the surface cold frontal boundary, currently moving into western
parts of the lot cwa, appears to have weakened since last evening
as wind veered more southwest ahead of it. Forecast soundings
indicate less favorable low level moisture profiles locally, and
dvn/mkx and lot 88ds are not detecting similar weak reflectivity
returns, with any fzdz risk appearing quite minimal.
For the remainder of the day, skies will likely remain mostly
cloudy this morning, thanks to extensive stratus noted upstream
across the upper Mississippi Valley per GOES infrared channel, and the
presence of a shallow inversion noted in forecast soundings around
875 mb. Will likely see some holes develop this afternoon,
allowing for some partly cloudy hours. Stronger mid-level height
rises, indicative of deeper subsidence, develops tonight behind
our departing positive-tilt short wave, which should allow clouds
to largely clear out. While temps aloft will moderate some as the
cold pool associated with the upper trough moves away, surface
temps will likely be slow to respond beneath the aforementioned
inversion. It will be slightly milder however, with afternoon
highs reaching the low-mid 30s. Clearing tonight and our early
snow cover will keep temps in the teens tonight outside of the
Chicago urban core.
A deep short wave passing well to our north across Ontario will
push a weak cold front south into our area on Friday. The front
will be losing its push as it arrives, but looks to sag into the
area midday, with an attendant increase in low cloud cover
developing late. As the front will be washing out across the area
by Friday night, it won't have a significant impact on our already
well-below normal temps, with highs again in the mid-30s Friday.
302 am CST
Saturday through Wednesday...
The extended period features the development and amplification of
a long wave trough across the eastern Continental U.S.. this would maintain
cooler than average temperatures across the region into next week,
with periodic chances of light precipitation as global guidance
depicts a series of weak clipper-type disturbances reinforcing the
trough early next week.
Saturday starts out with relatively flat flow aloft across the
forecast area. Guidance is in good agreement in digging an
amplifying upper trough southeast across the plains into Saturday
night, and across the Midwest by Sunday. Southeast low level winds
on the back side of retreating high pressure veer more southerly
in advance of the approaching surface reflection of the upper
trough. Moisture return looks limited initially, with
precipitation chances holding off until later Saturday night and
Sunday. Low level thickness progs would support a light rain/snow
mix, diminishing or ending as the mid-level trough axis moves
through Sunday night. 800 mb and h9 thermal fields indicate daytime
temps in the mid-upper 30s, and nighttime lows in the 20s.
Difficult to time minor precip events into early next week, as
guidance shows some differences in the details of individual low-
amplitude short waves entering the larger scale upper trough
across the region. Some broad consensus is apparent to highlight
the Monday night into Tuesday period for some low chance pops,
though overall confidence is fairly low at this distance. In
general, quantitative precipitation forecast amounts are low and rain/snow mixes supported by
temps during daylight hours.
for the 06z tafs...
Light snow continues across the region late this evening, but the
back edge is presently moving into far northwestern Illinois and
south central Wisconsin. Snow will continue through about 07z at
rfd and about 09z across the Chicago-area terminals before coming
to an end. Before snowfall ends, vsbys may periodically dip to
around 2 sm at times. Upstream, there have been a few reports of
some light fzdz across parts of far western Wisconsin where a
cold front is squeezing out what little moisture there is in the
lowest few hundred feet as cloud ice is lost with the airmass
quickly drying out aloft. At this time, it looks like the
greatest low-level moisture content will remain immediately to
the north of the Illinois/Wisconsin state line. Some lower cigs
may attempt to build in towards the rfd area towards daybreak, and
these have been added into a tempo group. In the vicinity of
Ord/mdw, the low-levels continue to look unsupportive of very low
cigs and/or drizzle potential, and a mention has been left out of
the tafs as a result.
Surface winds will incrementally shift to favor a west or west-
northwest direction as a frontal boundary pushes through the
region, but wind speeds are expected to remain under 11 kts or so.
Still some questions regarding the cessation of MVFR cigs
Thursday afternoon with some guidance looking perhaps a bit
aggressive driving VFR conditions back across the area terminals.
Favored a somewhat later arrival through the afternoon hours. Some
potential for MVFR to re-develop Thursday evening, but confidence
is too low to justify an explicit mention in the 06z taf cycle.
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