Scientific Forecaster Discussion
fxus63 klot 061127
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
527 am CST Fri Dec 6 2019
330 am CST
After a mild afternoon with temps in the upper 40s to low 50s, a
cold front has made its trek across the forecast area early this
morning ushering in a period of cooler temps for today. Winds have
become gusty at times behind the front out of the north, with
gusts to 25-30mph and occasionally higher gusts possible through
midday. Rockford has already gusted to 30 mph so far this morning.
Expect temps to continue their gradual drop to near 30, with
morning lows likely occurring even a few hours after sunset with
continued cold air advecting into the region. A fairly expansive,
albeit patchy stratus deck has moved in behind the front this
morning as well and should linger through late morning before
gradually lifting and moving off to the southwest. Even if skies
manage to clear out more quickly today, still expect highs to
remain in the 30s.
The stronger north winds this morning will increase wave action
on Lake Michigan through the afternoon, with waves building into
the 6-9 ft range for the Northwest Indiana nearshore, which is
approaching Lakeshore flood advisory criteria.
By tonight surface high pressure moves in overhead with clear
skies and light winds suggesting lows drop quite quickly into the
low to mid 20s. Some areas could see lows in the teens by early
Saturday morning, especially areas west of the Metro. Winds turn
southerly on Saturday as the high shifts off to the east. This
should help bring in some warmer air, with highs generally around
320 am CST
Saturday night through Thursday...
The long term forecast period will start out relatively mild as the
region will be under broad warm advection in swly flow between sfc
high pressure over the mid-Atlantic region and a sfc trough/cold
front across the Central Plains. Lows Sunday morning should be in
the lower 30s. Prevailing southwest flow will continue through the
day on Sunday as the sfc trough/cold front progression slows,
blocked by sfc ridging through the sern Continental U.S./Ern glfmx. Highs
Sunday should be in the middle to upper 40s, with the chance for
temps to reach 50 f, particularly for the far southern portions of
the County Warning Area. Pcpn associated with the approaching system may begin as
early as late Sunday afternoon over the far nwrn portions of the County Warning Area
as the front approaches, but better chances for pcpn, mainly in the
form of rain, will begin Sunday night and continue into Monday
night. As the cold front pushes across the region on Monday, winds
will shift from swly to nwly through the afternoon, with strong cold
advection setting up in the wake of the fropa. Temperatures should
drop off quickly Monday night while nwly winds strengthen, with
gusts of 25-30 mph likely. By Tuesday morning, temperatures will
range from the middle teens northwest to the middle 20s southeast.
Pcpn will transition from rain to snow as the colder air filters in
behind the cold front, with some light accumulation possible.
Following the passage of this system, the main forecast concern will
be temperatures for the middle of next week. Latest long range
guidance continues to suggest lows in the single digits across
portions of the area north and west of a line from Chicago to
Pontiac, with 10 to 12 f south and east of this line. There will
likely be little moderation to temperatures as Arctic high pressure
slides across the area Wednesday and then settles into the Ohio
Valley Thursday. With winds expected to remain arnd 10-15 mph, wind
chill readings should be in the should may drop to -5 to -10 f for
Tuesday and Wednesday morning. Conditions should begin to moderate
by Thursday as the Arctic high pushes into the sern Continental U.S. And flow
shifts to swly.
for the 12z tafs...
The concerns with the Chicago area tafs are this morning's gusty
In the wake of a cold front that passed very early this morning,
gusty northerly winds will continue through the morning.
Confidence is high in gusts exceeding 20 kt being frequent through
this morning, with some exceeding 25 kt, especially through 14z.
After 16z, gusts should start to diminish, but probably only
gradually through 18z-19z, and then abate altogether. Confidence
in wind speeds through the taf is medium-high.
As for forecast wind direction, 350-360 degrees this morning is
with high confidence, and 360-010 this afternoon is with medium
As for clouds, a broken layer of MVFR will be passing over the
region this morning before strengthening subsidence and weakening
cold advection result in the clouds scattering.
Lm...Small Craft Advisory...Illinois nearshore waters until 6 PM Friday.
Small Craft Advisory...in nearshore waters until midnight
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