Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/06/21

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 846 PM CST Tue Jan 5 2021

.UPDATE...
845 PM CST

The main update this evening was to expand the Dense Fog Advisory
eastward a bit based on observation trends. Otherwise, made a few
minor adjustments to raise overnight lows a degree or two in spots
with temperatures not going anywhere with the widespread cloud
cover in place.

Widespread dense (freezing) fog continues west of a Woodstock to
Pontiac line, with a slow eastward march noted over the last few
hours as the stratus deck is gradually building down to the
surface. Fairly confident that this dense fog will continue too
ooze eastward this evening into the expanded advisory area, but
confidence drops off over Cook and Lake (IL) counties where
webcams and observations continue to show visibilities are fine.
Recent AMDAR soundings near MDW do show some lingering dry air
immediately off the surface, and as 925-975 mb winds turn more
easterly with time, it`s possible additional drier air gets
injected into the near-surface layer to limit the dense fog
potential with eastward extent. We`ll continue to monitor trends
this evening, however. Slick spots remain a possibility with air
temperatures in the mid 20s. Finally, have seen some sporadic
observations of snow or unknown precipitation out west. While a
few flurries are possible, saturated depths appear too shallow to
support meaningful precipitation productions at this time. Updated
products have been transmitted.

Carlaw

&&

.SHORT TERM...
319 PM CST

Through Wednesday night...

We have high pressure moving in from the west and ample low-level
moisture from the snow pack, continued low cloud ceilings, and
restricted visibilities from freezing fog / fog throughout the
forecast area. Clouds have eroded in our extreme west, pushing
right up to Rockford.

Forecast challenge continues to be visibilities. With continued low
visibilities to the west, light winds, and high pressure ridging
over our area, dense fog is expected to develop in the western CWA,
and with light northwesterly winds, move slowly to the east, with
1/2 to 1/4 mile visibilities as far east as O`Hare possible into
Wednesday morning. Winds will shift more easterly, and slowly
push the limited visibilities back west, but with extensive low
cloud ceilings to our east, I believe we will continue to have the
cloud cover, moving once again over any cloud-free areas
presently out west.

With strong high pressure to our north, the next weather system will
be pushed south of our region towards the gulf coast states. This
strong high pressure will also increase our gradient slightly,
reducing the chances for dense fog formation Wednesday night into
Thursday, although presently low clouds are expected to remain.

Temperatures will not fluctuate much due to cloud cover except in
the extreme western regions, allowing for slightly lower
temperatures in western areas due to cloud cover loss. With
continued cloud cover tomorrow, plus clouds moving back over cloud-
free areas, highs on Wednesday will also be moderated.

BKL

&&

.LONG TERM...
250 PM CST

Thursday through Tuesday...

The main story in the long range is that quiet and mostly dry
conditions are expected into early next week. Temperatures will be
rather seasonable in the upper 20s to mid 30s for highs and upper
teens and lower 20s for lows.

Models continue to show a developing Central Plains low diverting
down into the southern and southeastern CONUS late Wednesday into
Thursday in response to an enhancement of the southern stream of
the upper level jet. This will assist in keeping the majority of
weather systems to our south over the next several days, thus
leaving the area mostly dry. The one caveat is east northeast flow
will develop off the lake into Thursday morning which could
result in some lake effect flurries/drizzle for areas near the
lake. Have opted to maintain a dry forecast for now until
confidence increases. High pressure will prevail into the weekend
before a weak system moves across the upper Great Lakes on Sunday,
with an associated cold front dropping south out of Wisconsin in
the afternoon. There is a low chance that some light precip moves
into the area along the front, but given the dry pattern we will
have been in, not expecting there to be a lot of moisture
available for it to maintain things all the way into Illinois. As
is often the case this time of year, there may be some localized
weakly forced snow potential not easily identified in the longer
term model guidance, but will maintain a dry forecast beyond the
weekend given the ragged split upper jet pattern in place.

Petr

&&

.AVIATION...
For the 00Z TAFs...

547 PM...Primary forecast concern is dense fog through Wednesday
morning.

Dense fog is widespread across much of north central IL early this
evening and is expected to continue through mid morning and expand
in coverage. Confidence is medium that at least 1/2sm vis will
reach ord tonight but confidence is low for mdw and gyy. It is
possible 1/4sm reaches ord and trends will need to be monitored.
The fog will lift during the mid morning hours and should lift
completely for the Chicago terminals allowing cigs to become mvfr
by early Wednesday afternoon. Light westerly will become light and
variable or calm tonight and shift light easterly Wednesday. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-
     ILZ039-ILZ106-ILZ107 until 10 AM Wednesday.

IN...None.
LM...None.
&&

$$

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