Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231018

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
518 AM CDT Sat Mar 23 2019

304 AM CDT

Through Sunday...

This early spring day will start off on the chilly side as high
pressure lingers overhead, making for optimal radiational cooling
now that our wind field has eased. It is a cold 22 in Waukegan as
of 255 AM. Not seeing much fog out there, but given some outlying
areas with low dewpoint depressions and little to no wind,
suspect we will have some patchy fog, especially in river

Meanwhile, fairly amplified flow is in place aloft with a closed
across the Colorado Rockies, a ridge across the plains into
central Canad, then downstream trough across the northeastern
United States. The upper ridge across the plains will spread
aloft shoving the surface high to the eastern Great Lakes. This
will open door to more southerly low level flow and warming up to
near normal temperatures today. The ridge should hold off
significant cloud cover. The south/southwest flow should
generally limit the inland progress of an afternoon lake breeze,
so the lake cooling will be much less extensive. The Saturday
afternoon lake breeze will some inland progress across northern
Cook County and Lake County (IL), but will likely struggle in
northwest Indiana with opposing synoptic flow. Anticipate highs in
the low to mid 50s away from the lake with 40s near the lake.

Our next precipitation maker will arrive later Saturday night into
Sunday as the low across Colorado will approach. Initially the
leading ridge and downstream flow across eastern Canada will slow
and weaken the low. We get a decent shot of warm/moist advection
ahead of the low late tonight into Sunday, combined with PWATs
approaching 0.75" should lead to a period of rain during this
period. QPF numbers are not too high given the limited period of
organized forcing, given that the forcing weakens a bit during
the day following this initial surge. It appears there is enough
continued warm advection induced lift ahead of low, renewed height
falls, and a southward moving cold front for continued damp
conditions at least at times on Sunday. Thunder chances appear
greater across central IL where they are still quite low.



330 AM CDT

Sunday night through Friday...

Initial concern is for a short window of potential wintry precip
for parts of the area Sunday evening into the overnight. As the
weak surface low passes to our south, cold front will move across
the area by early evening, flipping winds to northeast. Strong
high pressure spreading toward the region from central Canada will
tighten the northeast pressure gradient between it and the low to
the south, resulting in quickly strengthening northeast winds and
a much colder and drier air mass rushing in.

The big question is exactly how fast temperatures drop and how
much forcing is left for precip when temperatures dip to near or
below freezing. 00z NAM was by far the most aggressive on both
counts, yielding a period of freezing rain into parts of northern
IL due to warm layer aloft slower to cool than surface temps
cooling to or below 32F. With other guidance slower to cool
surface temps and less aggressive with precip hanging back,
played it more conservative in the grids and just carried slight
freezing rain chances toward and after midnight. With some minor
elevated instability in central IL to west central IN during early
evening prior to cold advection aloft commencing, maintained
slight chance for thunder there, though confidence is low in this.
Northeast winds will gust to 30 mph inland and potentially as
high as 35-40 mph lakeside on Sunday night, and will need to watch
for minor lakeshore flooding potential given high lake levels.

Monday will clear out with the surface high moving toward the
area, though northeast winds will be slow to ease. It will be
unseasonably chilly, especially near the lake in the mid to upper
30s, with lower 40s inland and perhaps mid 40s in spots well
inland. The western ridging and eastern troughing responsible for
the fairly chilly conditions of late will finally break down early
in the work week, supporting a rare stretch of above normal temps
mid to late week. This will be accompanied by rain/shower
potential from Wednesday night onward as multiple short-waves
emanate from trough in west that will then eject and bring a
return to cooler conditions next weekend. There`s plenty of
uncertainty in exactly how the pattern will evolve in the
Thursday-Friday stretch with large variance in operational models
and ensembles. Rain chances currently appear to be highest next



For the 12Z TAFs...

Concern today will be if the lake breeze will make it to ORD/MDW,
then on timing our next batch of rain late tonight and Sunday.

High will shift to the eastern Great Lakes today. Therefore light
winds and mostly clear skies will begin this operational period.
Winds will likely veer a bit west of south today, and then drift
closer to south through the day. We still feel the lake breeze
will just impact GYY and the lakeshore areas, however this will
bear watching as any minor change in the strength of the low level
winds could result in a flip over to an east wind direction at
ORD and MDW. MDW (and maybe ORD) also stand the chance to shift to
more of a southeast wind late this afternoon/early evening, but
speeds should remain on the lighter side.

Precipitation in the form of rain will hold off until overnight.
Expect that it will continue into Sunday morning, with some breaks
possible at times Sunday. But expect low VFR or even MVFR cigs
once the rain begins.




Hoisted a Gale Watch for the entire Illinois and Indiana nearshore
for Sunday night into early Monday. A strong cold front passage
with unseasonably strong high pressure spreading toward the
region will result in northeast winds rapidly increasing Sunday
evening. Much colder air moving in will also aid in creating more
unstable conditions and deeper mixing with time. Guidance is not
in lock step agreement on this potential gale event, with the NAM
model more supportive than the remaining guidance, so confidence
is only medium. However, opted to issue the watch for northeast
gale to 35 kt potential in collaboration with WFO Milwaukee. If
the gales occur, they should ease by sunrise Monday, followed by
conditions hazardous for small craft that should subside sometime
on Monday night.



LM...Gale Watch...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...9 PM
     Sunday to 7 AM Monday.




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