Area Forecast Discussion
Issued by NWS Chicago, IL

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275
FXUS63 KLOT 241749
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1149 AM CST Fri Feb 24 2017

.UPDATE...
915 AM CST

The main batch of showers and storms has exited well east of the
area. We still have shortwave energy rotating northeastward this
morning, but the better upper support and deeper moisture is
generally shifting southeastward where the colder cloud tops
exist. We could still see some occasional spurts in shower
activity with decent lapse rates aloft and a strengthening mid
level speed max, but with the shallower moisture expect there to
be period of lighter showers or drizzle north of the warm front.
This front should remain south of the Chicago metro area and
really south of the tip of Lake Michigan.

The slight risk area has shifted farther east than in previous
days, and thus most of the area will not see much more activity
today. There is a narrow window of higher instability air co-
located with the better shear and steep lapse rates ahead of the
advancing cold front late this morning/early afternoon where
storms could pick up a bit, but by mid to late afternoon and on
the threat is east. The Marginal risk/slight risk areas capture
these areas pretty well. Gusty winds remain a concern south of the
front, with hail again possible north given the steep lapse rates.

KMD

&&

.SHORT TERM...
345 AM CST

Through Saturday...

Main forecast concerns are with showers and thunderstorms early
this morning with additional chances for stronger development and
hail, additional showers and thunderstorms this afternoon into
early evening some of which could be strong to severe, and then
potential for snow mainly across northern Illinois tonight into
Saturday morning.

Periodic shower and thunderstorm development has occurred for
much of the night, and continues early this morning. This
development is largely due to a steady stream shortwave energy
lifting out ahead of the upper level low over the central Plains,
and with strong isentropic in place. With stronger energy lifting
through the area and with isentropic ascent further increasing,
had continued to see isolated stronger thunderstorm development
early this morning. Any thunderstorm development at this time
remains elevated, as the surface low and warm front are situated
to the southwest and south of the current development. However,
MUCAPE axis continues to shift overhead, while steeper mid level
lapse rates advect across the region. This will continue to
support additional isolated stronger thunderstorms early this
morning, mainly across northern Illinois through the 12-13z time
frame. Am monitoring a stronger wave lifting through west central
Illinois, and have timed to reach the CWA in the 10-12 time frame.
Short term guidance has a decent handle on the current precip
development, which closely matches my time of arrival into the
CWA. With the arrival of this stronger forcing and thunderstorms,
do think the potential for stronger storms and hail will be
possible once again. Timing looks to be in the 10-11z time frame
for areas along the I-39 corridor, and then 11-12Z time frame for
remaining areas in northeast Illinois. Current track would keep
most of these storms in Illinois, however, the tail end could clip
Lake IN and Porter counties.

The departure of this next area of showers and thunderstorms
after 12z should bring about some dry conditions across the CWA
through the remainder of the morning. However, isentropic ascent
does appear that it could continued to be focused over northern
Illinois, along and north of I-88. So do lower pops for much of
the CWA this morning, but do keep some higher pops confined in
this location for additional development. Surface warm front
lifting north across the region will complicate temp forecast for
today, as well as the potential for additional fog some of which
could be dense this morning. Have continued previous trends with
this front with the idea that it will likely get hung up around
the I80 corridor. However, given that it has made significant
progress here this morning with locations across the southern CWA
in the mid/upper 50s, did bring the gradient and higher temps
further to the north today. Dense fog does not appear to be too
prevalent across the area at this time. However, would think that
as the low continues to the northeast, dense fog would become more
probable north of the front in northern Illinois this morning.

Focus then turns for the potential for additional thunderstorms
this afternoon, some of which still have the potential to be
strong to severe. Although the greater threat for more widespread
severe weather does appear to stay to the east of the CWA, can`t
rule out some isolated strong/severe storms given that the eastern
CWA will be right on the fringe of storm initiation. The previous
evening`s upstream soundings depicting anticipated cap to be in
place over the region later today. This will aid to suppress
development, but some eroding of this cap with some warming and
increasing ascent is still appearing possible. Short term guidance
indicating that this looks to erode in the 19-20z time frame with
focus for thunderstorm development staying in the warm sector,
along and east of areas from Pontiac IL to Valparaiso IN. As the
cold front quickly pushes through, it appears that these areas
would have small window of 2-3 hours of observing this development
before this shifts further to the east. Instability axis will be
shifting to the east during this time, but still anticipate
sufficient instability to be in place. This along with very strong
mid and upper level wind field would support organized
development. Despite this small window, still have some concerns
for this stronger development given the strong wind field and even
situated boundaries. Can`t rule out initial discrete development,
but would anticipate more of a broken line to develop. Main
hazard would be high winds, but can`t rule out an isolated
tornado. Once again, convective window quickly ends early this
evening, likely around the 23-00z time frame.

Cold air still anticipated to wrap around this system, and spill
southeast across the region later this even. Column will quickly
cool this evening, and then continue this trend tonight with
frozen precip type becoming more likely. Although anticipated
pattern would suggest that the bulk any precip on the backside of
this system would stay just to the north, guidance continues to
indicate that at least northern Illinois will get into some of
this precip. Given that this this strong upper level center will
push overhead tonight, additional precip chances are warranted
tonight into early Saturday morning. Column will support snow
later tonight, however, the transition period may offer a brief
window of a wintry mix. Cooling surface temps and lacking deeper
moisture could support this wintry mix, before all snow is then
anticipated. Have maintained similar snow amounts, keeping them
minor with the highest around one inch closer to the IL/WI border.
Areas elsewhere, should stay under an inch. Snow will exit to the
east Saturday morning, but highs in the 20s and low 30s with
gusty northwest winds will make for a rather chilly day.

Rodriguez

&&

.LONG TERM...
347 AM CST

Saturday Night through Thursday...

The beginning of the period will be dry as surface ridge axis
builds overhead. With a colder airmass in place and weakening
surface winds, anticipate night time lows in the low 20s Saturday
night. This high quickly shifts to the east Sunday with some
moderating of the air mass possible ahead of an approaching wave.
Guidance varying to the extent of any precip Sunday, but did keep
some precip mention with at least some weaker ascent moving
overhead. Precip chances quickly lower Sunday night into Monday,
but pattern remains active with another developing low expected to
move across the region Monday night into Tuesday. Warming ahead of
this system will likely support liquid precip, but with some
variability in place, and maintained a mention of snow during this
time.

Rodriguez

&&

.AVIATION...
For the 18Z TAFs...

Trends in LIFR cigs and IFR vsbys are the primary aviation
forecast concern in the near term, eventually shifting focus to a
period of light snow late tonight into early Saturday morning with
west-northwest winds becoming gusty 25-30 kts.

Surface low pressure was analyzed just south of the terminals at
midday, along a sharp frontal zone which runs from north of KPNT
to just south of KGYY. North of the front/low track, very moist
low level air mass was trapped beneath the strong frontal
inversion, with cool northeast surface winds and patchy drizzle
helping to maintain LIFR cigs of 300-400 ft and vsbys around 2SM
across the Chicago terminals. Surface low is expected to track
just south of KGYY over the next few hours, with low level winds
backing to north-northwest by late afternoon. Not much in the way
of drying is expected, preventing much in the way of improvement
in the near term. Guidance shows a little variation, perhaps 100-200
ft in cig height over the next few hours, though eventually should
see some slow improvement as winds shift more west-northwest by
early this evening when improvement to IFR cig and MVFR vis is
expected.

The surface low pulls away from the area this evening, while the
upper trough axis approaches and eventually passes across the area
late tonight into early Saturday morning. Deformation precipitation
currently from western Iowa into northwest Wisconsin will move
east across the area during the late evening/midnight time frame,
and is expected to produce a period of light snow through 7-8 am.
Precipitation will likely begin as a mix of rain/snow, before
transitioning to all snow after an hour or two. IFR vsby expected
with some minor accums mainly on grass areas. Snow should taper
off to flurries and end during the mid-late morning. Winds will be
the main concern beyond Saturday morning, with 290-300 direction
and sustained speeds 15-20 kts gusting 25-30 kts.

Ratzer

&&

.MARINE...
220 AM CST

An active end of February will continue over the lake. A strong
low that will move across northern Illinois this morning will then
track over the far southern part of the lake this afternoon.
Northeast winds to the north of the low will remain stout, with
gales likely today, especially over the northern half of the lake
where some 40 kt are possible. There will be a period when wind
speeds drop slightly tonight, but then will begin to increase late
as the cold air wraps in behind the low. There is a fair amount
of uncertainty on whether this will bring another period of
widespread gales, though at least occasional during Saturday
morning has high confidence. Have at this time issued a Gale
Watch.

As for the Illinois and Indiana nearshore, specifics on direction
today get a little uncertain given the low passage over the area.
This is especially true at and south of Chicago along the
northwest Indiana shore. If the warm front reaches offshore of
Indiana, south wind gusts are likely to be close to Small Craft
criteria late this morning and afternoon. Otherwise, the northwest
winds late tonight into Saturday will require another Small Craft
Advisory.

Beyond, a weaker low will pass over Lake Superior on Sunday still
causing a decent pressure gradient over the lake. Wind gusts look
to approach gales during the day Sunday over the open water.
Another northeastward moving low is expected sometime midweek that
could present another gale threat.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...NEARSHORE WINTHROP TO WILMETTE HARBORS UNTIL 3 PM Friday.

     Dense Fog Advisory...LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779 UNTIL 3 PM Friday.

&&

$$

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