Scientific Forecaster Discussion
fxus63 klot 142337
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
537 PM CST Thu Nov 14 2019
227 PM CST
Through Friday night...
For the remainder of the afternoon and into the evening, cloudy
skies will persist as a stratus deck remains trapped under a strong
inversion. While latest satellite imagery shows some erosion of the
deck over ern Iowa, there is some concern that there will not be
enough low level dry air intrusion and that the inversion will
remain strong enough to inhibit any mixing. So, while the latest
guidance indicates that skies should scatter out and clear from
north to south through the evening and overnight, there is also a
chance that the stratus deck may remain over the region through the
night. This could lead to forecast bust potential for temperatures
and some light fog. Should skies clear out as initially though and
with winds forecast to veer from westerly to northwesterly
overnight, while remaining relatively light, then temperatures
should drop into the lower teens over the northwestern portions of
the County Warning Area. It would not be out of the realm of possibility for some
low-lying, sheltered areas to see temps approach the single digits
and some patchy light fog with 3 to 5 mile visibility could develop.
On the other hand, if the overcast stratus hangs in overnight then
temps will not be able to radiate down as much as the going
forecast. Currently, expect lows in the 12-15 f range west of the
Fox Valley and north of I-80.And upper teens to low 20s over the
rest of the County Warning Area. Friday should be dry with ample sunshine as high
pressure builds across the region. Temperatures should continue the
slow warming trend with highs expected in the mid to upper 30s.
Relatively quiet weather will persist Friday night with partly to
mostly cloudy skies and lows in the upper teens to mid 20s.
121 PM CST
Saturday through Thursday...
Relatively quiet weather persists at the start of the period,
with zonal flow and surface ridge in place. Some moderation to the
air mass expected during the day on Saturday out ahead of the
next system to move through the region, with this supporting some
slight increase to Max temps, but still well below normal. Upper
level trough expected to dig across the central Continental U.S. Saturday
into Saturday night, and then likely swing across the area on
Sunday. Some slight variability among guidance as to the exact
evolution of this system, but it is appearing that a period of at
least some light precip will occur on Sunday. This looks to be
supported by decent mid/upper height falls, fairly active mid
levels, and then with the upper level jet swinging through. Due to
some timing differences, have kept pops somewhat lower, but once
again think there will be a window of at least some light precip.
Currently, a rain snow mix messaging for this period remains
valid, but will need to monitor for the possibility for all snow
as it's possible for the anticipated forcing to keep the thermal
profile and surface temps cooler, if this forcing is strong
enough. Additionally, some freezing rain remains possible given
the low Saturday night into Sunday morning temps. Confidence still
too low on exact trends at this time though.
No big change to temp trends at the start of next week, with
below normal temps continuing. With the previously mentioned upper
level trough lingering across the region through midweek, some
low chance pops are warranted with the potentially active
northwest flow in place.
for the 00z tafs...
Concerns: none major. Timing the west-to-east cessation of MVFR
cigs this evening, low potential for some light br late tonight,
and a east wind push off the lake Friday evening with another
round of low cigs.
The back edge of the earlier expansive MVFR deck is quickly
approaching the c90 tracon, and should be rolling through rfd
shortly after 01z, and into the Chicago-area terminals during the
03-05z timeframe. While some occasional dips under fl020 will be
possible before cigs erode, expect cigs to mostly hang out above
020. With the return of sky clear conditions overnight, there is some
potential for br formation as temperatures cool quickly. Overall,
conditions are not overly conducive to the development of
operationally-impactful vsby reductions, but we'll continue to
monitor observations and guidance trends this evening.
A weak boundary will slip into the region late Friday morning with
a very gradual veering of the surface winds expected. There is
some signal for this boundary to bring with it another (brief)
round of MVFR cigs, but confidence in this is too low to warrant
an introduction at this juncture. Winds will favor a northwesterly
direction into Friday afternoon before a more notable lake-
enhanced boundary pushes across the Chicago-area terminals either
late Friday afternoon or early in the evening. Some indications
that cigs could come in below what is currently advertised in the
00z tafs, but this and timing will be refined in future taf
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