Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241806

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
106 PM CDT Fri Mar 24 2017

1015 AM CDT

Have bumped up forecast highs some for today, with mid 70s for
almost the entire CWA. Still think the daylight hours today will
be dry, but watching the frontal location to our north closely, as
it will lead to an abrupt temperature drop in northeast Illinois
when it heads south at some point this afternoon into early

A large part of the CWA has been under little to thin cirrus and
upstream conditions would lend that to continue through early-mid
afternoon, though there are signs of mid-cloud development on
recent GOES-16 imagery in northwest/far north central Illinois.
Within the overall upper diffluence, it`s tough to peg where mid
and high clouds may blossom, and that will impact overall
temperatures. Far northern locations right now look like they
could hang a few degrees below elsewhere given the clouds.

South-southwest surface winds have been gusting to 20 to 30 mph
across the area this morning, a sign that mixing and advection are
ensuing well. The 925mb temperatures at DVN and ILX were both 18C,
about at record levels for late March. While that will cool a
little this afternoon, it`s a sign of the potential aloft. Given
the temperature climb this morning and RAP/HRRR trends, have felt
comfortable inching temperatures up a couple/few degrees. Given
forecast mixing profiles, it would seem like locations east of
I-55 may have potential for gusts to 40 mph.

The stationary front to the north is draped across southern
Wisconsin, and lake enhanced southward to just north of the state
line. Pressure tendencies are fairly neutral on both sides of the
front, and with the surface low over the OK/TX panhandles
expected to not move north of east this afternoon, and only
slowly move at that, would expect the lake enhancement of the
front to increase. This should drop southward into northeast
Illinois, and right now lean toward this being a hair earlier than
initially forecast. With this being near peak heating in Lake
County, Illinois, the magnitude of temperature drop could be 20
degrees in 10-15 minutes. Confidence is pretty low on when
precisely this will reach the Chicago lakefront, but presently
have that forecast somewhere between 6-8 pm.



313 AM CDT

Through tonight...

Warm and windy conditions will be in place today with showers and
thunderstorms overspreading the area this evening and tonight.
Early this morning, a 992mb low is analyzed over southeastern
Colorado while a sharp negatively tilted trough is moving across
the Southern Rockies. The surface low will dip across the
panhandles today then shift east across Oklahoma this afternoon. A
tight pressure gradient will set up over northern Illinois and
northwest Indiana in between the approaching low and high pressure
off the east coast. NAM/GFS forecast soundings indicate deep
mixing developing this afternoon with mean gusts of 20-25kt
through much of the afternoon. Meanwhile, to our north, another
low moving east across Hudson Bay will push a cold front south
across Lake Michigan today with the front expected to settle over
northern Illinois by mid to late this evening. Winds should begin
to diminish some mid to late this afternoon from the north and
northwest as the frontal trough nears the area with temperatures
cooling off across far north and northeastern Illinois as winds
turn northeast behind the front.

Broad southerly flow aloft ahead of the Plains low will transport
1.1-1.2 inch PWats up the Mississippi Valley into the local area
by late this evening. Despite poor mid level lapse rates resulting
in weak instability over the area, should be able to overcome
this given the strong kinematic environment and moisture transport
allowing for thunderstorms to develop. Scattered showers and
thunderstorms could develop along the southward moving cold front
late this afternoon into the evening, but expect more widespread
convection late this evening and overnight in advance of the upper
low as it pushes east across the Central Plains. In addition to
the aforementioned broad 50-60kt southerly flow resulting in
moisture rich air over the region, upper level diffluence and
strong mid level height falls will overspread the region providing
broad ascent and resulting in scattered to numerous showers and
thunderstorms, albeit disorganized given the lack of any well
defined focus.

Temperatures today will be unseasonably warm across the area.
Mixing down temps from the GFS and NAM indicate highs into the
low to mid 70s, which is in line with the MAV which has been
trending slightly warmer as well. Any breaks in the cloud cover
could result in temperatures going even warmer. Temperatures are
expected to fall behind the cold front from the north late this
afternoon and evening which will eventually set up a sharp
temperature gradient over the area which will be in place into



313 AM CDT

Saturday through Friday...

Northern Illinois and northwest Indiana have a wet weekend ahead
with active weather expected to continue through the upcoming
week. A closed 500mb low is progged to lift from southern MO
Saturday morning to northern Illinois midday Sunday. Showers and a
few thunderstorms will be possible at times throughout the
weekend as the low slowly makes its way across the region. Not
really any strong signals at this point on timing any peaks or
lulls in the precipitation, but would anticipate some waves in
intensity as shortwaves pinwheel around the low. The 500mb low
eventually opens up over the area on Sunday with the trough axis
passing overhead Sunday afternoon which should allow for
precipitation to wane from the west Sunday afternoon and evening.
At the surface, expect a stationary front to persist over far
northern Illinois on Saturday which will result in a temperature
gradient over the CWA with upper 50s into the 60s south of the
boundary while 40s and low 50s are in place to the north. The
front should lift back to our north by Sunday with upper 50s to
low 60s area-wide.

Another low pressure system is expected to track to our south
Monday into Tuesday followed by yet another low late in the
week. With storm tracks to our south next week and high pressure
building from the Canadian Prairies early next week to Quebec late
in the week, northerly flow will dominate the local area through
the week which will keep temperatures cooler, albeit still near
or slightly above seasonal norms, except near the lake where
onshore flow will knock temps down even more. Monday afternoon and
evening appears to be the best timeframe for showers during the
first low passage with chances ramping up again Thursday and
Friday with the second low.



105 PM CDT

A record high of 80 (thus far) has been reached at Chicago O`Hare
early this afternoon. The record high had been 79 set in 1910 and
1939. Rockford`s record high of 80 in 1939 is also in jeopardy
before the clouds move in by 2-3 pm.



For the 18Z TAFs...

Concerns with the airport forecasts are:

- Gusty south-southwest winds today turning north-northeast early
this evening.

- Low clouds and MVFR, possibly IFR visibility likely to move in
within a few hours after the wind shift this evening.

- Scattered showers this evening becoming more prevalent
overnight, with some thunder in the region.

- LIFR conditions, possibly to 200 ft, on Saturday morning and
probably into the afternoon, along with areas of showers and

Strong southwest winds today have already gusted sporadically to
over 30 kt at several area airports and such gusts are possible
prior to 22Z. These winds have held the cool front to the north
of the local area thus far, but the front should inch southward
later this afternoon into early this evening. Expecting that it
passes ORD sometime early this evening, but confidence is low on
precisely the hour when the wind shift to northeast occurs. It
could be as early as 23Z, and as late as 0230Z.

Cooler air will immediately move in behind the front but moisture
ahead of the boundary is not plentiful. Because of that, believe
that low cloud development, at least to the broken/overcast
level, could lag a few hours and have continued that approach. Do
think IFR is likely once clouds do develop later this evening,
with LIFR possible out the gate. LIFR cloud bases are expected
with high confidence Saturday morning, though just how low is
somewhat uncertain. As for visibility, confidence is lower in
this, as it will likely be dictated by rain shower intensity.

Showers are expected to expand over north central Illinois this
early to mid evening, and then into northeast Illinois late this
evening into overnight. Thunder is certainly possible but
confidence was not enough to include in the TAFs at this time.
Saturday should see arcs of showers rotating across the area,
with light rain and drizzle in-between, so that makes it
challenging for specifics on visibility.



313 AM CDT

Southerly winds are in place across much of the lake this
morning, but will begin to see this change over the next several
ours as a front is expected to move south down the lake. With the
passage of the front this morning, expect winds over the north
half to turn to the north but don`t anticipate an initial increase
in speeds. Do expect an increase though by early afternoon, but
will continue to monitor this situation as it is quite possible
for this increase to occur earlier. For the south half and
nearshore waters, southerly winds are expected and will likely be
on the higher side. Still anticipate winds hazardous to small
craft to occur today, with the Small Craft Advisory in place still
valid. Uncertainty does get higher later today into this evening,
owing to timing and placement of the front. Expect the front to
quickly push down the lake with northerly winds in place for much
of the lake including the Illinois nearshore, while the Indiana
side possibly stays more southerly. With the front possibly
stalling out near the southern end of the lake early Saturday
morning, winds may be highly variable in this location. However,
do think the Illinois nearshore could get into some stronger
northerly winds and then higher waves. So have extended the Small
Craft Advisory over the IL nearshore through tonight, with
stronger winds and higher waves likely persisting. Will need to
monitor this situation as well as these hazardous conditions could
last longer into Saturday morning, as well as the IN nearshore
possibly observing these hazardous conditions.



LM...Small Craft Advisory...IN nearshore waters until 7 PM Friday.

     Small Craft Advisory...IL nearshore waters until 4 AM Saturday.




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