Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250932
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
332 AM CST Sat Feb 25 2017

.SHORT TERM...
256 AM CST

Through tonight...

Main forecast concerns are with snow showers across the entire
CWA early this morning, and then with a much colder day today.

Upper level low is lifting across the region this morning, with
stout vort lobe swinging around it. At the surface, low pressure
is well northeast of the CWA, but with trough axis on the backside
of this system continuing to push through the area. With these
features in place, have seen precip blossom overnight. Precip did
initially see a brief wintry mix despite a quickly cooling column,
as deeper moisture was lacking. Overall trends would suggest that
the bulk of any precip this morning is all snow. However, the far
southern tier of counties may still observe a brief rain snow mix
before all snow is then quickly observed. Do think this snow will
continue across the area for the next 2-3 hours, before exiting.
Minor accumulations still expected with the highest amounts likely
closer to the IL/WI border, where deformation snow has been more
persistent. However, have seen more showery type snow for
remaining areas in northern Illinois this morning, including the
southern tier of counties that could support some quick minor
accumulations as well. This is especially possible for those areas
from Pontiac east to Watseka and Fowler IN, where better low
level convergence is allowing for slightly stronger snow showers.
Given the recent warm temperatures, the bulk of any accumulation
will be on grassy surfaces. However, lesser used roads may become
snow covered.

Strong CAA still occurring this morning, with temps steadily
falling into the 20s. With temps staying in the 20s for most
locations today and with gusty northwest winds, it will feel
rather chilly today. Back edge of the stratus is expected to reach
the western CWA later this morning. However, pattern would
suggest additional stratocu development throughout the day, and so
have held onto cloud cover a little longer today. Clearing skies
then expected later this evening, but anticipate clouds to return
with the arrival of mid/upper level cloud cover associated with
approaching mid level trough.

Rodriguez

&&

.LONG TERM...
331 AM CST

Sunday through Friday...

The rest of the weekend into early next week does appear to
remain on the dry side. Mid level trough expected to push further
east through the region on Sunday, but it does appear that the
stronger forcing stays just to the south of the CWA. Some
moderating of the air mass will occur, and should see temps
rebound back into the 40s on Sunday. Continued dry conditions
likely to occur on Monday, with slightly warmer temps in place.
Westerlies usher in additional shortwave energy across the central
CONUS Monday night into Tuesday. Surface reflection expected to
lift northeast across the area during this time, with additional
warming likely. Still some model variability with the next system,
but precip chances do increase by Monday night. Precip type
should be all liquid Monday night into Tuesday and as additional
forcing and moisture increase Tuesday, could see some scattered
thunderstorm development. Guidance begins to really vary beyond
this point, Tuesday night into Wednesday. Additional precip
chances are still warranted though, but confidence is low with
trends/timing. Will need to keep an eye on this system, as it is
possible for much colder air to filter in across the area while
the stronger forcing is still in place. This would then support
more wintry type precip over much of the CWA.

Rodriguez

&&

.AVIATION...
For the 06Z TAFs...

Trends in cigs and vis will remain the primary aviation forecast
concerns through the night, but there will also be some concern
regarding the impacts of a period of light precipitation
overnight. Pcpn has already changed over from freezing drizzle to
snow at RFD and the Chicago area terminals should still see a
short period of light rain/drizzle/freezing drizzle before
changing over to all light snow as colder air filters across the
region. A secondary concern will be wnwly winds becoming gusty at
25-30 kts late tonight and through the day tomorrow..

Surface low pressure is passing across Lake Huron with a sharp
cold front extending southwest to the lower Mississippi Valley and
a sharp warm front extending east through the eastern Great Lakes
region. Winds are backing to nwly with cooler air filtering
across the region. Expect that cigs will gradually improve
through the night as the colder, drier air filters in at the lower
levels.

As the surface low continues to pull away from the area, the
upper trough axis will pass across the area late tonight into
early tomorrow morning. An area of light snow is overspreading
ncntrl IL, with some drizzle and freezing drizzle immediately
preceding the snow. The pcpn should change over to snow at the
Chicago area terminals shortly after midnight. This is expected to
produce a period of light snow through tomorrow morning. Snow
should taper off to flurries and end by mid-late morning. Winds
will be the main concern through the day tomorrow, with 290-300
direction and sustained speeds 15-20 kts gusting to 25-30 kts.

&&

.MARINE...
143 AM CST

The pattern remains active over the Great Lakes this weekend.
Departing low pressure from the central lakes this morning will
continue cold advection and at least occasional northwest gales
are likely across the north and central, while more marginal
south. Weak ridging tonight will ease the winds only temporarily,
as a quick moving low over the northern lakes on Sunday will
result in stout southwest winds. The air mass looks inherently
cool enough going into Sunday to prevent an inversion and likely
support southwest gales during the day Sunday, especially central
and south. Some near gale force gusts are possible in the Illinois
and Indiana nearshore on Sunday.

The next chance for higher wind speeds comes Wednesday behind a
northeastward moving low pressure which passes during or near
Tuesday night/early Wednesday. Still a decent amount of spread in
our computer model guidance on this system, but this could
support some near gales, as well as more of a due north wind
direction.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Gale Warning...LMZ777-LMZ779 UNTIL 3 PM Saturday.

     Small Craft Advisory...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 9 PM
     Saturday.

     Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 3 AM Sunday.

&&

$$

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