Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231220

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
620 AM CST Thu Feb 23 2017

326 AM CST

Through tonight...

A backdoor cold front will slide down the lake and move through the
forecast area this morning. Guidance seems rather consistent with it
arriving at 9-10AM, but sometimes these types of fronts are quicker
than forecast. The main impact from the front will be much cooler
temperatures. Highs along the lake will be in the low 40s, and could
be even lower. Locations along the lake will likely see their highs
very early this morning with temperatures falling into upper 30s by
the afternoon.  Locations away from the lake will warm until the
front moves through. Highs away from the lake will vary from the
upper 40s near RFD to the low to mid 60s south of I-80, and then
temps will drop in the afternoon.

Precip ahead of the next low forms over the region this evening and
overnight. The main question is how far north will the warm front
get tonight. Guidance seems rather consistent with the front
reaching I-80 before stalling Friday morning. This is further south
than models were predicting 24-hours ago.  Models feature enough
elevated CAPE to warrant a chance of embedded, non-severe
thunderstorms throughout the forecast area tonight.

The benefit of that warm front is temps are expected to rise



326 AM CST

Friday through Wednesday...

The extended will be a tale of two seasons. A true spring finale,
severe storms and all, occurs Friday then we return to snow and
below normal temps Saturday.

Friday bottom line up front: Severe storms with damaging winds and
maybe small hail are possible along and east of a McHenry, IL to
Gibson City, IL line Friday afternoon. Confidence is high in
thunderstorms occurring but medium in the storms being severe.

Friday Details: The surface low passes over northern IL Friday
afternoon providing ample shear for severe storms. The main question
regarding severe weather is how much CAPE will we have to work with?
 It looks like Friday afternoon may be pretty cloudy limiting
destabilization and CAPE.  Even the typically overdone NAM only has
CAPE values around 500 J/kg, with higher values over north-central
IN and points eastward.  The second question is where will the line
of storms form initially and how quickly will grow upscale?  60-80
kt of shear would certainly support structured storms, but CAPE may
limit growth.  Thinking the initial line will form roughly along a
McHenry, IL to Gibson City, IL line in the early afternoon and then
intensify as it shifts east.  SPC has this region under a slight
risk with an enhanced risk over north-central IN. Given the CAPE
questions, this seems reasonable.  Main threat from these storms
would likely be damaging winds. The storms may be able to mix down
strong winds aloft, and soundings feature inverted V`s at the
surface. CAPE is narrow, but with ample shear, can`t rule out small

Not overly concerned about flooding due to the recent dry spell and
thawing ground.  Could see some rivers rise, but widespread flooding
is not expected sine the storms should be moving at a good clip.

Also lowered high temps slightly Friday due to cloud cover.

The line of storms grows upscale as it moves in to north-central IN,
and the low`s cold front sweeps through.  Temps will fall below
freezing Friday night and 30-35 kt gusts are possible Fri night
through Saturday.  Models continue to feature wrap around precip
that will likely fall as light snow late Friday night into Saturday.
Given the recent warm spell and rain, only expecting snow to stick
to grassy surfaces. Snow totals through Saturday morning will be up
to an inch at RFD and less than inch for locations north of I-80.

High pressure spreads over the Gulf coast Saturday night, and the
bulk of cold upper level air associated with the upper level trough
move over the region. Lows Saturday night will be in the low 20s.

I have much more uncertainty regarding the next low Sunday night
through early next week. The ECMWF has a few periods of showers due
to upper level shortwave troughs/vort streamers followed by a strong
low that passes over Wisconsin Tuesday night. The GFS, on the other
hand, has a much stronger upper level vort streamer and surface low
that passes south of the region and across the Ohio Valley Monday.

Due to the wide range of solutions, didn`t make many changes to the
temps and precip chances SuperBlend put in.  Have multiple periods
of rain or snow through mid next week. While I have very low
confidence in the exact details, the general story should be periods
of precip with above normal temperatures.



For the 12Z TAFs...

The main forecast concerns through the period include:
1.) The exact timing of northeasterly wind shift this morning.
2.) Potential for low MVFR CIGs this morning, followed by LIFR
conditions tonight into Friday morning.
3.) Showers this evening and overnight, along with a threat for
some thunderstorms later this evening and overnight.

A cold front is currently shifting over the area early this
morning. While this will initially result in a period of north-
northwesterly winds for the next couple hours, expect winds to
quickly shift out of the northeast by late morning and increase
this afternoon. Some lower CIGs in the 1,000-1,500 FT range are
also likely to drop southward over the terminals in the wake of
this front, by or shortly after 14z. However, IFR to LIFR CIGS
are likely over and near Lake Michigan, and some of these CIGS may
impact KGYY later this morning as the winds turn onshore.

The main story this afternoon through Friday will be the impacts
of a significant storm system expected to shift east-
northeastward from the Plains across northern Illinois by late
Friday afternoon. This storm system will lift a warm front
northward over northern Illinois by early Friday morning. To the
north of this front, expect east-northeasterly winds to remain
breezy, with gusts exceeding 20 KT by this evening. Showers will
also be likely over the area very late this afternoon through
tonight as deeper moisture begins to surge northward over the
area. The threat for thunderstorms will also increase over the
area later this evening and overnight. At the present, I have
added a prob30 for several hours tonight for this possibility
until a better timing can be pinned down.

CIGs and VISBYs are also likely to become LIFR overnight and
continue Friday morning as the surface low and warm frontal
boundary becomes oriented right over the area. VLIFR conditions
are also possible. These very low conditions are also likely to
continue into at least Friday morning and could linger into the
afternoon if the warm frontal boundary never shifts north of the



227 AM CST

The main forecast concerns continue to focus on the significant
storm system expected to impact the region Friday into Saturday.

An area of low pressure currently shifting over northern Lake
Michigan, will drive a cold front southward down Lake Michigan
this morning. This will set the stage for increasing northerly
winds, likely up around 25 KT during the afternoon. These
northerly winds will allow waves to build above 4 feet over
southern lake Michigan this afternoon, so a small craft advisory
will be issued for the near shore waters this afternoon into

The main storm system expected to impact the lakes region, will be
shifting out over the central Plains later today, then begin to
shift east-northeastward towards the lake late tonight into
Friday. As this occurs, winds will increase more and shift
northeasterly in response to strong pressure falls occurring over
northern Illinois and eastern Iowa. As a result, it appears
likely that northeasterly gales of 35 to 40 kt will develop over
the northern 2/3rds of the lake later tonight, and persist on
Friday over the northern half of the lake. The strongest winds
will likely be on the western half of the lake. For this reason,
we have issued a gale warning for all but the southern 4 open
water zones of Lake Michigan for late tonight through much of
Friday. Note that this warning does not include the Illinois or
Indiana near shore waters, as the stronger winds should be north
of these areas.

Expect the surface low to shift over the lake on Friday
afternoon/evening. As this occurs, a period of lighter winds may
setup over portions of the lake. However, as the low continues to
the northeast, expect a period of strong west-northwesterly winds
to setup over the lake late Friday night and on Saturday. These
west-northwest winds could result in the need for another gale
warning for the lake during this period.

Gale force winds should ease over the lake Saturday night as a
surface ridge of high pressure shifts eastward from the Plains to
the Lower Mississippi Valley by Sunday morning. This high is then
expected to continue towards the southeastern Atlantic coast by
Sunday evening. Ultimately, this pressure feature will allow the
winds over the lake to back southwesterly over the lake by Sunday
afternoon. While wind speeds will likely ease early Sunday
following the northwesterly gale event, southwest winds will be on
an upward trend Sunday afternoon as a fast moving clipper system
tracks eastward over the Upper Midwest. Therefore, a period of ~30
KT southwesterly winds are possible late Sunday into Sunday

While forecast uncertainty increases in the extended period, it
does appear that another storm system could take aim on the Great
Lakes region next week, either on Tuesday or Wednesday. As a
result, this could set up another period of strong winds over the



LM...Small Craft Advisory...LMZ740...3 PM Thursday TO 3 PM Friday.

     Small Craft Advisory...LMZ741-LMZ742-LMZ743-LMZ744...3 PM
     Thursday TO 9 AM Friday.




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