Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLOT 180902

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
302 AM CST Thu Jan 18 2018

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.



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

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

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.


For the 06Z TAFs...

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

Gusty southwest winds have redeveloped over the area this evening.
Expect these winds to continue to be gusty at times into Thursday
before abating by early Thursday evening. Wind speeds of 45 to 50
kt will continue around 2,000 ft AGL, so we will continue the
mention of LLWS in the tafs through early to mid morning Thursday.

Otherwise, expect VFR conditions through the period.



300 AM CST

A broad ridge will remain in place over the Deep South through the
remainder of the week and into the weekend while a series of lows
move east across Canada. Expect a west to southwest gradient to
prevail over Lake Michigan through at least Saturday. Low pressure
will move from Alberta this evening to near James Bay Friday
evening. Winds will pick back up to gale force across the Lake
Friday afternoon through Friday night, and should ease back below
gale force during the predawn hours Saturday. Will hoist a gale
watch this morning for the open waters to cover this time frame.

Another low will take shape over the central Great Plains over the
weekend then lift over or near Lake Michigan on Monday. Models
continue to have modest differences with timing, strength, and
the exact track of the low so the details are still a bit unclear.
In general though, winds are expected to turn south to southeast
on Sunday ahead of the low. A more northerly low track will also
favor high dew point air lifting into the region resulting in
stable conditions over the lake and likely fog. Gales or near
gales are possible Sunday and Sunday night. As the low shifts east
of the lake on Monday, gales then appear likely and could
potentially be strong as cold air spreads back over the region
behind the deepening low.



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

     Gale Watch...LMZ777-LMZ779...noon Friday to 3 AM Saturday.




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.