Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 252110

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
Issued by National Weather Service Lincoln IL
310 PM CST Tue Feb 25 2020

300 PM CST

Tonight through Wednesday night...

Guidance has shown another lowering of the overall snowfall totals
for this winter event. The placement of higher snowfall totals
remains in our eastern CWA, primarily in NW Indiana. That prompted
a replacement of the Winter Storm Watch for our Indiana counties
with a Winter Weather Advisory for 3-5" of snow. The Winter
Weather Advisory counties in Illinois are now expected to see
slighly less snowfall than previous forecasts, with the northern
tier from La Salle to Cook possibly remaining in the 1-2" range.
However, based on impacts from the combination of strong north winds
and snowfall, we kept the original advisory counties as is. Our
southern IL counties from Livingston to Iroquois still have
potential to climb into the 2-4" range. The 5" amount potential
should be primarily in the Indiana side of our CWA, based on the
intensification of a few snow bands Wednesday morning as
differential PVA from the upper trough enhance precip production
in that area. The gusty winds could create some drifting problems
for west to east roads tonight and Wednesday.

After the upper trough axis passes Illinois, subsident isentropic
motions and dry air intrusion should help to shut down snowfall
from west to east. Dry conditions should become established by
Wednesday evening. Surface winds will shift to the northwest and
decrease overnight.


Lakeshore flooding...

As winds and waves continue to increase tonight, and especially
Wednesday, lakeshore flooding is expected along the entire
Illinois and Indiana Lake Michigan shoreline. The worst conditions
are expected from daybreak to mid-afternoon Wednesday when wind
gusts (from the north northeast) will exceed 40 mph and waves 8
feet. Experimental model guidance also suggests a "storm surge" of
1-1.5 feet is possible Wednesday which will exasperate impacts
from the waves. The main limiting factor with this storm is that
the strongest winds and waves will not be directed at any portion
of the lakeshore for a prolonged period of time, as the direction
from which the winds (and hence waves) will be coming will
transition from northeast to north as Wednesday progresses. As
such, this storm will not be as bad as the storm earlier this
year. Even so, significant impacts are expected along the
lakeshore and a Lakeshore Flood Warning will remain in effect.
Hazardous conditions along the lakeshore will abate Wednesday
night into Thursday as winds become offshore.



300 PM CST

Thursday through Tuesday...

Northwest flow in the upper levels will help reinforce below
normal temperatures in the wake of the winter storm system, with
highs remaining in the 20s Thursday and Friday. Despite a typical
flow of shortwaves across the area during winter-time northwest
flow, we are continuing with a dry forecasts for the end of the
work week.

However, the GFS and Canadian are hinting at a shortwave pushing
across NE Illinois/NW Indiana on Saturday, triggering a few snow
showers. The ECMWF dissipates the wave before it reaches Illinois,
and the resultant model blend has kept the CWA dry even on
Saturday, for now.

Upper level ridging and rising heights will trigger a warming
trend starting on Saturday. More noticable warming will develop on
Sunday as highs return to the 40s.

Precipitation chances return to the forecast on Monday, as an
upper trough develops in the Rockies and southwest flow ahead of
the trough taps into a moisture source along with much warmer
air. The precipitation may start out with a few flakes Monday
morning, but quickly transition to all rain, as highs climb into
the upper 40s. Rain chances become likely Monday night and
Tuesday, as a surface low advances across Illinois. Despite the
clouds and precip, highs on Tuesday will still reach the lower
50s across a majority of the CWA.



For the 18Z TAFs...

Initially the brunt of light snow with vsbys 2-3 miles has been
mainly affecting GYY at midday, while staying se of the Chicago
metro airports (MDW and ORD) aside from scattered flurries. MVFR
ceilings have returned at midday except still VFR at MDW but
should become MVFR in next hour or so. Snow with MVFR
visibilities will spread northeast into Chicago metro late this
afternoon starting between 21-22Z and could be mixed with light
rain for a few hours with surface temps near 40F. As mixed pcpn
changes to all snow around 00Z, IFR vsbys of 1-2 miles and
ceilings expected to develop. RFD will be on nw fringe of this
storm system and mainly MVFR conditions there with light snow or
flurries tonight from mid evening on. Snow to diminish to flurries
during mid/late Wed morning lasting longest at GYY into early Wed
afternoon. Brisk NE winds 14-20 kts with gusts of 23-30 kts this
afternoon to veer more NNE tonight and northerly Wed morning. IFR
ceilings should lift to MVFR as snow diminishing Wed morning from
the sw and VFR vsbys return. Maintained MVFR ceilings of 1.5-2.5k
ft at ORD and MDW Wed afternoon.




230 PM...

Low pressure will linger to the east of Illinois tonight,
deepening on Wednesday as an upper level trough approaches
Illinois. High pressure over southern Canada will build into the
northern Plains tonight and Wednesday and the gradient will
tighten between these two systems. Northeast gales to 35kts are
already occurring, so we went with a Gale Warning starting already
this afternoon. There may be a brief decrease in winds tonight
but as the low deepens Wednesday...winds will turn more northerly
with additional gales expected. Winds will slowly turn northwest
by Wednesday evening with gales diminishing by mid/late Wednesday
evening. Northwest winds may remain gusty to 25kt through Friday



IL...Lakeshore Flood Warning...ILZ006-ILZ014 until 3 PM Wednesday.

     Winter Weather Advisory...ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039...6 PM Tuesday to 6 PM

IN...Lakeshore Flood Warning...INZ001-INZ002...6 AM Wednesday to 9 PM

     Winter Weather Advisory...INZ001-INZ002-INZ010-INZ011-INZ019...6
     PM Tuesday to 6 PM Wednesday.

LM...Gale Warning...nearshore waters until 9
     PM Wednesday.



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