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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1128 am CDT sun Mar 26 2017

Update...
1116 am CDT

Apart from some light showers and patchy drizzle the main body of
heavier precipitation has lifted north of the area as of late
morning. Additional periods of isolated to scattered activity are
possible through the afternoon, especially into Northwest Indiana.
A stacked low pressure system gradually lifting through the area
is supporting these continued precip chances, though short term
model guidance suggests the strongest shortwave energy with this
low is directly overhead or has already moved a bit to our east.
So there remains plenty of cyclonic flow aloft to support weaker
disorganized activity through the afternoon, but focused areas of
stronger forcing aloft and moisture flux at the surface would
generally support a better chance for organized showers and
thunderstorms over eastern portions of the area.

Lenning

&&

Short term...
245 am CDT

Through tonight...

Another day of showers on tap for northern Illinois and Northwest
Indiana, with precipitation finally winding down late this
afternoon into the evening.

Slow moving upper low is crossing the Mississippi River early this
morning near far northeastern Missouri and the 500mb circulation
is expected to lift across the chicagoland area mid afternoon.
Meanwhile, at the surface, a frontal boundary is draped from pnt
to rzl early this morning and expected to lift north to near the
Illinois/WI state line this afternoon. This will allow warmer air to
overspread most of the County Warning Area with afternoon highs expected to be in
the low 60s. Southwest winds behind the front should bring the
warmer temps all the way to the lake front. NAM guidance indicates
the front may stall over the northern tier of counties which
would result in a temperatures staying cooler for areas within
several miles of the state line, but as of now the NAM is a
southerly outlier.

Isentropic ascent continues to be fairly broad across the County Warning Area
today but there is better pressure packing that lifts across the
area between 12z and 18z today which coincides with a lobe of
DCVA that lifts across the eastern half of the County Warning Area. Instability
has shifted farther east today, and is mainly over the Northwest
Indiana counties which will limit the thunder threat for Illinois.
With this in mind, have the highest pops and expected the
greatest precipitation coverage east of the I-55 corridor today
with more spotty activity expected to the west. Precipitation
chances should wind down from the west this afternoon as the 500mb
low/trough axis moves to Lake Michigan mid afternoon then to
southwestern Michigan early this evening.

Fog may be an issue throughout the day within the lighter flow of
the frontal trough with some improvement expected as southwest
flow increases from the south today. Overnight, a trailing surface
trough and eventually a col area will move over the County Warning Area which
will continue to provide light winds. Expect cloud cover to stay
locked in so we won't radiate very well, but the combination of
high dewpoints and light flow will promote fog redevelopment
overnight.

Deubelbeiss

&&

Long term...
245 am CDT

Monday through Saturday...

Upper low that provided wet conditions over the weekend is progged
to open up and shift to New England early next week with a brief
lull in precipitation first half of Monday as a transient upper
ridge builds across the Midwest. Meanwhile, water vapor imagery
this morning indicates another upper level disturbance over the
Desert Southwest which will eventually lift across the plains and
Midwest through the day Monday. A modest surface reflection will
move from Oklahoma late tonight to central/downstate Illinois
Monday afternoon with precipitation overspreading the County Warning Area
associated with a corridor of mid level warm air advection out ahead of the low.
Soundings indicate fairly shallow moisture this far north with
weak instability only clipping areas east of the I-57 corridor.
This system will quickly depart to our east Monday night with high
pressure building from the Canadian prairies across portions of
the upper Midwest Tuesday and Wednesday. Expect dry conditions
during this timeframe but north to northeast winds will suppress
temperatures near the lake front, keeping those areas in the low
to mid 40s while farther inland temperatures warm into the 50s.
Models continue to struggle handling track and timing of low
pressure late in the week. The GFS has a southerly track that
keeps some of the local area dry late in the week while the European model (ecmwf)
and Gem lift the low across portions of the Midwest which would
result in showers and possibly thunderstorms locally. While the
details are still way out of focus at this distance, the Gem and
European model (ecmwf) have been more consistent over the past few days and will
lean on those solutions with at least high chance pops late in the
week.

Deubelbeiss

&&

Aviation...
for the 12z tafs...

Main forecast concerns continue with low ceilings, reduced vis
due to rain/drizzle/fog, and wind trends.

Approaching low pressure and associated frontal boundary bisecting
northern Illinois supporting highly variable wind speed and
direction, and ceiling/vis conditions across the area early this
morning. Lowest ceilings and reduced vis is occurring near the
rfd/dpa areas this morning, with vis slightly improving further to
the east southeast. Do think these lower conditions will continue
through mid morning, but confidence does lower by late morning and
midday. Do have a slight improvement through late morning but its
possible that these lower ceilings/vis continue slightly longer
today. After any improvement today, conditions will likely come
back down once again later tonight. After continued periodic rain
or drizzle for much of today, a drying trend should occur later
this evening. However, chances for additional precip will arrive
by late Monday morning. Wind trends would indicate locations
getting into a southeast wind by mid morning and then south to
southwest by midday/early afternoon. However, these trends have
been slow, with Ord still showing a 050 direction. Based on this,
did slow the wind shifts this morning.

Rodriguez

&&

Marine...
405 am CDT

Low pressure continues to slowly lift northeast through the mid
Mississippi Valley this morning, with more of an easterly wind in
place. This low will move into northern Illinois today, and then
across the lake early this evening. This will provide highly
variable winds over much of the lake today into tonight, however,
do expect speeds to diminish today into tonight. The higher speeds
which provided conditions hazardous for small craft across the
Illinois nearshore have diminished. Higher waves are still in
place, and so have continued the Small Craft Advisory today. Will
monitor for a possible early end time though. Continue to monitor
fog over the south half of the lake, and at this time, it still
appears that the more dense fog is situated over the Western
Shores of the lake. It does look like this dense fog will continue
for much of today, and quite possibly expanding over much of the
southern half of the lake. Will monitor the possible need for
expansion of the dense fog advisory to remaining areas over the
south half of the lake.

Rodriguez

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...dense fog advisory...lmz740-lmz741-lmz742-lmz777-lmz779 until 7
PM Sunday.

&&

$$

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