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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
321 PM CDT Mon Oct 23 2017

Short term...
212 PM CDT

Through tonight...

While the rain is ending across much of northeastern Illinois at this
time, expect more rain to develop over the area tonight into
Tuesday. This will occur as a significant upper trough digs
southeastward over the central Continental U.S. And couples with the
disturbance currently driving a weaker area of surface low
pressure over northern Indiana. Expect this surface low to
undergo explosive development tonight into Tuesday morning as it
shifts over northern portions of lower Michigan. In fact, the
central pressure of the surface low is likely to deepen over 20
mb, to near 980 mb by early Tuesday morning as the low shifts to
near The Straits of Mackinac. With moisture wrapping around the
back side of this surface low, and a closing off mid-level low,
expect periods of showers over much of the area late tonight and
continuing on Tuesday.

West-northwesterly winds are also expected to increase over the
area late tonight in response to the rapid deepening of the
surface low. These winds will remain gusty over the area on



Long term...
302 PM CDT

Tuesday through Monday...

A traditional late autumn pattern will be in place the remainder
of the week, with multiple large systems in the region providing
wind-driven temperature swings...most of them being cold surges.

The deep, almost bombing low on Tuesday referenced in the short
term discussion will move little during the day, providing a
sustained cold advection pattern with almost no temperature climb.
In fact some places could easily drop a few degrees into the low
40s during the late morning into afternoon as areas of deformation
rain rotate southeastward. Plenty of support for moderate rain
showers, with maybe the far southern and western County Warning Area remaining a
bit more scattered.

As for tuesday's northwest wind speeds, the synoptic setup with a
sub 985mb low and cold advection support at least frequent gusts
to 35+ mph. Depending on just how deep the boundary layer is
within the clouds and rain, gusts approaching 45 mph are
plausible. Sometimes guidance can underdo wind speeds with these
rapidly deepening lows. The pressure gradient tightens the most
to our north where Wind Advisory criteria (45+ mph gusts) is
presently most favored.

The low will start to pull east Tuesday evening with rain showers
inching their way eastward. Temperatures will be dropping and a
quickly decreasing freezing level, but likely there will not be
an overlap of rain and cool enough enough boundary layer
temperatures for any wet flakes to mix in Tuesday night. However,
will need to keep an eye on any back side short wave energy
Wednesday morning, as that could bring some spotty light
rain/wet flakes mix to eastern Wisconsin and far northeast
Illinois depending on how far west it were to track (presently
favored a little east of our cwa).

Temperatures on Tuesday night just by advection alone will dip
into the mid 30s, and in the west with clearing could see to
freezing or even a degree or two below. Given the winds persisting,
no frost mentioned at this juncture.

As the upper trough departs eastward, a warm advection regime
quickly spreads into the area Wednesday night into Thursday. The
GFS 850mb temperatures of 14c are quite a bit warmer/anomalous
than the ec's 9c, and have gone a middle Road toward highs with
lower to mid 60s.

Global guidance is in fairly strong agreement for four days out
in bringing the next upper low southward into the western lakes
region by later Friday, with it's surface cold advection
encompassing the area as Friday progresses. The models then spread
markedly on how quickly the system departs. Depending on this and
just how far south wraparound moisture reaches will dictate how
long rain shower chances persist and how cool Saturday is. Based
on the solutions and conceptual model, there is some possibility
of mixing in wet snow flakes, maybe more so off of Lake Michigan
into far Northwest Indiana Saturday night.



for the 18z tafs...

Periods of light rain will gradually end across the Chicago area
terminals early this afternoon. Cigs are also expected to improve
to VFR for the remainder of the afternoon and evening. However,
lower cigs and the chances for more rain come back into the
forecast tonight and for most of the day on Tuesday. Gusty west-
northwest winds are also expected over the area on Tuesday as an
area of low pressure rapidly strengthens over the upper Great
Lakes late tonight into Tuesday.

Overall, expect lower MVFR cigs (possible periods of ifr), gusty
winds, and periods of showers over the area on Tuesday.



315 PM CDT

A very active period over the lake this week, with high impact
conditions late tonight through Tuesday night.

A phasing low pressure system will significantly deepen with its
center moving north to Sault Ste Marie by late tonight. The
rapidly tightening pressure gradient, coupled with cold advection,
will support gales developing across the north tonight and then
across the entire lake on Tuesday. Storm conditions with 50 kt
gusts are expected across the north and parts of the central
Tuesday and Tuesday evening. In the very far north toward Beaver
Island and eastward -- underneath the low center -- winds will be
less. However they will sharply pick up immediately to the
west/southwest of there, so have included the entire north in a
Gale Warning. Significant waves of 12-16 ft with some higher are
expected across the eastern half of the lake.

As for the Illinois and Indiana nearshore, strong west to
northwest winds will spread over Tuesday morning. Gusts during the
late morning through early evening will at least reach occasional
gales, but may actually be more consistent. Thus have held onto
the gale watch as of 3 PM.

Winds will briefly turn southerly into Thursday ahead of the next
low. This will drop southeast over Lake Michigan on Friday, with a
cold front passing over and a wind shift to northwest. Gales look
probable at this time for a period that could last into Saturday.



Lot watches/warnings/advisories...
Lm...Gale Warning...lmz777-lmz779...9 am Tuesday to 9 am Wednesday.

Small Craft nearshore waters until 1 PM Tuesday.

Gale watch...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...1 PM
Tuesday to 4 am Wednesday.

Small Craft Advisory...lmz740-lmz741-lmz742...7 am Tuesday to 1
PM Tuesday.



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