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 221110

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
610 AM CDT Thu Mar 22 2018

239 PM CDT

Through Thursday...

Expect mainly clear/sunny skies and dry weather this forecast
period, as high pressure builds across the region. However, could
see continued scattered clouds spilling off the lake this evening,
mainly into northwest IN. As the gradient relaxes, winds will
further diminish this afternoon and evening. Under clear skies and
light winds, expect temps tonight to fall well below normal, with
lows in the low to mid 20s. Given the setup, could see slightly
lower temps, and will monitor the potential for a few locations to
fall into the teens tonight. High clouds may try to increase on
Thursday, however, think sunny skies will prevail. Temps will
rebound well into the 40s on Thursday, with locations near the
lake observing slightly lower temps. This extent of this cooling
will be tied to lake breeze timing, that may not develop and then
push inland until later in the day.



258 PM CDT

Thursday night through Wednesday...

Main forecast concern in the extended will be the potentially
significant snowfall Friday night into Saturday.

Ridge of high pressure will begin to shift to our east Thursday
night into Friday morning while low pressure deepens over the
central plains. Southerly flow from the gulf will bring moisture
rich air north across the Mississippi River Valley with 1 inch
PWat contour spreading into portions of northern Illinois. PWats
of around 0.8 inches and mid level mixing ratios of 4.5 to just
over 5.0 g/kg will move into the local CWA by Friday evening
around the same time as deep corridor of isentropic ascent
overspreads the region resulting in quickly saturating mid levels.
Timing of low level saturation and onset of precipitation a
little more uncertain with a dry boundary layer flow out of the
east to northeast, but thinking this should occur sometime mid to
late evening for areas southwest of a Rockford to Rensselaer
line. Somewhere just north of this line, there will likely be a
sharp cutoff in snow totals.

Late Friday night into Saturday morning, forcing is expected to
peak with a number of features aligning to provide deep ascent
across the region. Synoptically, right entrance region to a strong
jet digging across the mid-Atlantic coast will move across the
western Great Lakes while the nose of a weaker jet moves into the
mid Mississippi Valley. The coupling of these two jets along with
increasing DCVA ahead of a shortwave trough poised to cross the
local area around midday Saturday will provide broad ascent across
the region.

Guidance also signaling the potential for strong mesoscale forcing
as a sloped region of strong F-gen overspreads portions of the CWA
topped by a region of negative EPV and steep mid/upper level
lapse rates which will allow for a deep vertical response to the
forcing. The combination of strong forcing and instability will
set the stage for heavy snowfall rates, which will likely be in
the form of large aggregates at times as soundings show a deep
region of saturation cutting across a broad temperature range.

Main forecast uncertainty will be pinning down the exact location
of the axis of heaviest precipitation, and, for some locations,
there will be p-type concerns in our far south. At this distance,
there is reasonable agreement between the main global models
indicating the axis of highest QPF (around 1.5 inches liquid
equivalent) falling from eastern Iowa southeast across central
Illinois, including Ogle/Lee counties southeast through
Ford/Iroquois counties. Amounts will taper to the northeast with a
likely sharp gradient down to near nothing somewhere over the
northeastern counties in the CWA, including possibly portions of
the Chicago Metro area. As is typical with events driven by strong
mesoscale features, wouldn`t be surprised to see shifts in the
precip axis 100 miles or so in either direction over the next day
or two, but in the meantime there is a good signal for some
locations seeing in excess of 6 inches of snow. SREF plumes show a
very broad range of possible values. Near Sterling, IL for
example, plumes have a range of nothing up to a foot with a mean
around 3 inches. While there is some decent clustering in the 1-4"
range, there is also a secondary cluster around 6-8" presumably
with models catching onto the mesoscale banding. This may be a
decent starting point for much of the CWA with this system
producing a broad area of 1-3 inches with a narrow corridor
perhaps 50-100 miles wide with the potential for 6+ inches. Exact
details on placement still to be worked out, but for now the
latest forecast shows a Dixon, IL to Remington, IN line as the
axis of heaviest snowfall.

Meanwhile, in our far south along a Peru, IL to Fowler, IN line,
thermal profiles are supportive of a wintry mix including freezing
rain and sleet. Will have to keep an eye out on the potential for
minor ice accumulations within this corridor.

Saturday night through Monday, ridging will re-establish over the
region providing quiet weather. Another system may impact the
region by the middle of next week with a warming trend ahead of
the system and primarily rainfall Tuesday into Wednesday.



For the 12Z TAFs...

Little change necessary to the going forecast.  Quiet weather
expected for the period with the only forecast concern being
winds, and particular, timing of a possible lake breeze passage.

As high pressure moves across the region, winds will becm lgt/vrbl
to light nwly for much of the day. Light winds and little or no
significant cloud cover will keep conditions favorable for lake
breeze formation, with the main questions remaining, will the
lake breeze push inland far enough to cause a wind shift to ely
and an increase in wind speeds to close to 10 kt. The higher
resolution guidance still indicates that the lake breeze will
push inland, with a lake breeze passage timing arnd 23z at
ORD/MDW. Confidence in these details medium. After sunset, high
pressure will still remain in place, so winds should drop off
again the lgt/vrbl with no other significant weather impacts.


410 AM CDT

High pressure extends from Hudson Bay through the western Great
lakes to the Central Gulf Coast.  The high pressure will remain
nearly stationary into the weekend while low pressure tracks
across the Great Plains.  Northerly flow will prevail ahead of
the ridge axis through Thursday and much of Friday, then winds
should begin to veer to the east later Friday into Saturday as low
pressure deepens and tracks across the Central Plains. East winds
should peak around 25 to 30 kt Saturday morning. The low will then
track across the mid Mississippi Valley Saturday and weaken.
Another low will form over the plains late in the weekend and move
across Lake Michigan around midweek.


IL...Winter Storm Watch...ILZ008-ILZ010-ILZ011-ILZ019-ILZ020-
     ILZ021...10 PM Friday to 4 PM Saturday.

     Winter Storm Watch...ILZ022-ILZ023-ILZ032-ILZ033-ILZ039...1 AM
     Saturday to 4 PM Saturday.

IN...Winter Storm Watch...INZ010-INZ011-INZ019...1 AM Saturday to 4
     PM 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.