Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180509
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1209 AM CDT Sat Mar 18 2017

.UPDATE...
806 PM CDT

For Evening update:

Some significant modifications made to hourly grids late this
afternoon and early this evening across the area, to add detail
especially to sky cover and to boost temperatures per observed
trends. Also, scaled back pops for expected sprinkles/light
showers into the overnight hours, focused mainly along the IL/WI
border region.

Early evening water vapor imagery depicts a strong mid-upper level
jet streak translating east-southeastward across the region in
association with a digging mid-level short wave. This wave is
forecast to continue to dig southeast into the Ohio Valley
overnight, amplifying an upper trough across the Great Lakes and
Midwest, east of the forecast area. At the surface, weak low
pressure was over northeast Illinois, trailing a cold front on the
leading edge of a Pacific origin air mass. Surface temperatures
had risen sharply late this afternoon near the triple point of the
cold front and warm front associated with the surface low. Winds
will shift to the west-northwest and become gusty in the 20-25 mph
range early this evening as the front moves east, with temps
gradually cooling back into the 40`s this evening and eventually
the 30`s overnight. As the mid-level vort digs southeast across
the area, clouds will spread back late this evening. Guidance
indicates some isolated/scattered showers developing in
association with this moisture return and steepening of low level
lapse rates, though forecast soundings depict this moisture as
being fairly shallow, and overall am not too impressed with
precipitation potential. Based on this, I trimmed back chance pops
from a good chunk of the cwa this evening and overnight, limiting
the mention of isolated sprinkles mainly to portions of far
northern IL especially along the IL/WI border based on plethora of
high-res model simulated reflectivity output. Primary
precipitation type is expected to be liquid, with moisture shallow
enough to not support widespread ice nuclei for any snow, even as
temps cool late tonight. Also, adjusted temps up based on
observational trends early this evening, and decreased cloud cover
where clearing had occurred behind the cold front.

Updated grids, text products available.

Ratzer

&&

.SHORT TERM...
251 PM CDT

Through Tonight...

Main forecast concerns are with potential for scattered showery
development later this evening into the overnight hours.

In the near term, low stratus and fog still situated across
northeast Illinois and northwest Indiana. However, the western
extent to this cloud shield is quickly pushing east through north
central Illinois. This is occurring as surface trough and
associated boundaries are moving through Wisconsin and Illinois.
Warm front noted across the western CWA at this time, with wind
shift but more importantly a quick rise in temps to the upper 40s
and even low 50s. Do think this clearing trend and warming trend
will continue, but the warming impact may be felt less over the
far eastern portions of the area given the clouds and time of day.
Nonetheless, still expect all locations to see a rise at least
into the 40s and possibly mid to upper 40s. After a period of at
least partly cloudy skies this afternoon, cloud cover will spill
back south over the entire area this evening with cloudy skies
then expected through the remainder of the night.

Have included scattered wording for precip development later this
evening over much of the CWA, with pattern supportive of this
showery development. With strong vort max diving south and cold
advection spreading southeast across the region, should see precip
develop. Thermal profiles supportive of all liquid and have
included this in the forecast, but do think it is still worth
monitoring for a possible brief wintry mix. Guidance varies to the
extent of the deeper moisture this evening, but with continued
CAA overnight, do think there could be a window of a rain snow, or
sleet mix. If this were to occur, don`t anticipate any impacts as
this would remain brief and isolated.

Rodriguez

&&

.LONG TERM...
332 PM CDT

Saturday through Friday...

Drier and windier conditions are expected Saturday morning.  Low
pressure continues east while high pressure builds over plains.
Northwest winds will gust to around 30 MPH in the morning but then
diminish as the high spreads east in the afternoon.  Local
climatology would suggest warmer than forecast high temps so raised
highs into the mid 40s away from the lake.  Thinning cloud cover and
light winds will allow for efficient cooling Saturday night. I don`t
think we will completely clear out so temps won`t drop too far and
kept lows in the upper 20s away from downtown.

Winds become south on the backside of the high Sunday ushering in
warmer air.  Once again raised highs and ended up with high temps in
the low 50s, except in Lake County IL where winds off of the lake
will keep temps cooler.

A large surface low reaches Hudson Bay Sunday night, and its cold
front reaches the Mississippi River late Sunday night/early Monday
morning. The GFS is still quicker with precip reaching the forecast
area Sunday evening. Kept precip chances in the slight chance range
as I do not have a ton of confidence in precip occurring before
midnight.  The ECMWF precip field features convective bullseyes, and
GFS forecast soundings feature elevated CAPE that the front should
help storms tap into.  CAPE values are low so only have a slight
chance of thunder Sunday night south of I-88.

The GFS continues to be more progressive with precip exiting the
forecast area Monday afternoon. The ECMWF holds on to precip over
the southern half of the forecast area through Monday night.
Thinking Monday afternoon could be dry, but decided to keep a chance
of precip in the forecast due to the ECMWF`s precip output.  High
temps are forecast to get into the 60s for most locations with
cooler temps along the lake.

The GFS brings in additional bands of light precip Monday night and
Tuesday morning south of I-80.  Have low confidence in precip
occurring so capped precip chances in the slight chance range.  A
secondary cold front surges down the lake Tuesday, and expecting a
dry frontal passage.  Northerly winds bring cooler air south and
high temps Wednesday will range from the upper 30s to mid 40s.  As
the high over the region Wednesday shifts east, another surface low
is progged to form over the plains and possibly impact the forecast
area Thursday and Friday.

JEE

&&

.AVIATION...
For the 06Z TAFs...

Stratus deck is likely to result in largely MVFR CIGS overnight
into Saturday morning with CIGS probably lifting to VFR with some
BINOVC developing in the afternoon. Moderate WNW winds will
continue with gustiness through Saturday morning expected to
gradually diminish in the afternoon before becoming light
northwest or light and variable Saturday night.

Izzi

&&

.MARINE...
332 PM CDT

Headlines...Issued a small craft advisory for late tonight into
Saturday morning for the IL nearshore waters and through Saturday
afternoon for the IN nearshore waters.

A surface low over southwest Ontario dives southeast over the Great
Lakes Friday night while a large high pressure ridge builds over the
plains. The tightened pressure gradient will result in northwest
winds to 30 kt, but have high confidence that winds will stay below
gale force.  Conditions hazardous to small craft are expected with
both gusty winds and high waves.

Winds diminish as the high moves over the lake Sunday morning. The
high continues east Sunday, and winds become south.  A large surface
low shifts northeast over southern Canada, and south winds increase
to 30 kt. The low`s cold front swings through Monday morning turning
winds north to northwest.

A secondary cold front surges down the lake Tuesday. Another surface
high moves over the western Great Lakes Wednesday, and a surface low
is progged to form over the plains late next week.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Gary to Michigan City IN until 10 PM Saturday.

     Small Craft Advisory...Winthrop Harbor to Gary IN until 4 PM
     Saturday.

&&

$$

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