Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 172328
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
528 PM CST Wed Jan 17 2018

.SHORT TERM...
256 PM CST

Through Thursday...

The main concern is the potential for patchy blowing and drifting
snow in open areas of northern Illinois north of I-88 and near
the lake in northwest Indiana through a portion of the overnight.
Southwest winds will peak this evening through the early
overnight, with gusts up to 25-30 mph, highest north of I-80.
With the dry, fluffy snow cover in place that hasn`t undergone
much if any melting and little consolidation, hence the concern
for patchy blowing and drifting snow. If winds are any stronger
than forecast, then blowing snow could become more of an issue,
while on the other hand, if gustiness is less frequent than
expected, blowing snow may not materialize. Despite winds
remaining steady to gusty tonight, the refrigerating effects of
the snow cover should allow for a gradual fall in temperatures,
particularly later in the overnight into early Thursday when
winds are expected to ease some. Have forecast lows in the lower
to mid teens, which with the wind will yield wind chills to start
Thursday within a few degrees of zero.

Strong warm advection will continue aloft on Thursday, forming a
stout inversion and limiting mixing to under 925 mb. With gusty
southwest surface flow over the expansive snow cover and the stout
inversion, suspect that high temperatures will again end up below
MOS guidance and on the lower end of the climatology of progged
925 mb temps. Forecast highs are from around 30 to mostly low 30s
(28-34 range). Full or nearly full sunshine and these temperatures
should start the slow melt and consolidation process of the snow
pack. Gusts topping out in the 25-30 mph range during the
afternoon will continue to make it feel more chilly than it
otherwise would be with temperatures near or around freezing, with
wind chills in the teens to lower 20s.

Castro

&&

.LONG TERM...
321 PM CST

Thursday night through Wednesday...

For the longer term forecast period, the main concern will focus on
a low pressure system dropping out of the Central Rockies to the
South High Plains Saturday night and associated impact on the local
area.

In the mean time, a generally dry pattern will continue late this
week and into the weekend as strong high pressure continues to slide
across the northern Gulf coast and off the southeast Atlantic coast
by Sunday.  As the high moves off to the east and a series of low
pressure systems track across southern Canada, a general warming
trend will set up for Friday and through the weekend as deep layer
flow trends to more southwesterly.  By Friday, temperatures should
trend to well above average as upper ridging builds across the ern
2/3 of the CONUS.

Warm, moist advection will be on the rise through the weekend,
and there is some concern that the longer term models may be
trending a bit cool given the synoptic scale pattern of persistent
warm, moist advection through the weekend. Have trended max
temperatures up from the upper 30s/lower 40s on Friday to middle
to upper 40s by Sunday. The southwestern portions of the CWA could
even see some low 50s temperatures, especially around the Greater
Pontiac Metro Area to the LaSalle/Peru area. Given the persistent
warm advective pattern, the going temperature forecast could be
trending a little low, but given some uncertainty, especially with
the impacts of increasing dense cloud cover through the weekend,
do not have enough confidence to raise temperatures any more than
already carried.

Adding to the uncertainty is that the longer range models are
having some timing issues with the associated upper low. The GFS
is trending much faster than the GEM/ECMWF, which is a bit of a
departure from earlier, more consistent model runs. In general,
prefer a slower trend of the GEM/ECMWF since the the models, in
general, are too fast to kick upper lows out of the southwest and
lift across the plains. While there are timing concerns with the
system, the general track remains consistent with the upper low
tracking either over nrn Illinois or just north of the Wisconsin
border while the associated sfc low takes a similar track as the
system becomes more vertically stacked. Given the expected track
of the system, much of the associated pcpn should be liquid, or
perhaps a rain/snow mix over the nrn portions of the CWA.

Precipitation type and amounts may have an impact on area rivers.
With the recent very cold conditions, many area rivers are either
totally ice covered or partially ice covered.  A warming trend would
melt any snow remaining on the ground.  Also, rainfall amounts will
be a concern as a frozen ground would turn much or all of the
rainfall to runoff.  The latest models and WPC guidance suggest a
swath of moderate rainfall from nwrn Illinois and across srn
Wisconsin.  Runoff of rainfall into area rivers, along with a rapid
warmup could lead to ice breakup and ice jamming on area rivers.
These conditions will need to be monitored for possible ice jam
flooding.

Following the passage of this system, conditions should trend back
to seasonal normals following the passage of an associated cold
front.  Following a few days of above normal temperatures,
conditions should quickly turn cooler again, with highs back into
the upper 20s to lower 30s by early next week.

&&

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

Overall winds and low level wind shear will be the main weather
concern at the terminals tonight into Thursday.

The gusty southwest winds have eased a bit this evening and this
may be a result of more decoupling of the winds with the cooling
surface layer over the snow pack. As a result, wind gusts may be
more occasional and given wind speeds of 45 to 50 kt are expected
around 2,000 ft AGL, we have opted to add LLWS to the tafs into
early Thursday. The surface winds are likely to become gusty
again during the morning and into the afternoon.

Otherwise, expect VFR conditions through the period.

KJB

&&

.MARINE...
326 PM CST

West-southwest gales of 35-40 kt will continue over the open
waters through late tonight, with west-southwest winds remaining
elevated at 25-30 kt through Thursday night. Freezing spray is
likely through tonight. The next period of concern for
southwesterly gales is on Friday into Friday night. Finally, a
potentially strong low pressure area will approach the lake Sunday
night into Monday and cross the lake by Tuesday. Brisk east-
southeast to south winds will occur ahead of the low pressure
area, shifting to west and northwest winds in the wake of the low.
Depending on the track and strength of the low pressure, gales
are possible both ahead of behind it.

For the nearshore waters, satellite and webcams indicate plenty
of thin ice, which will melt late this week into the weekend as
temperatures warm. Until then, the ice will limit waves despite
hazardous wind speeds for small craft tonight through Thursday
afternoon. Have some concern for a few gale force gusts this
evening.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning...LMZ777-LMZ779 until 3 AM Thursday.

     Small Craft Advisory...nearshore waters
     until 4 PM Thursday.

&&

$$

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