Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240817

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

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 difluence 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.



For the 06Z TAFs...

Main forecast concerns are with LLWS this morning, gusty south
southwest winds today, wind shift to the northeast early this
evening, IFR conditions this evening, and rain tonight.

Relatively quiet across the terminals right now with any precip
staying to the north and northeast. This trend looks to continue
for this morning into the remainder of the day, with VFR
conditions to remain in place. Southerly winds will steadily
increase later this morning with gusts up to 30 KT likely by
midday. These stronger southerly winds will likely hold in place
through mid afternoon, but will start to see a front dropping down
the lake approach the terminals. Do think a quicker timing of this
arrival is appearing likely, and so have brought up the timing of
the wind shift to the northeast. Will need to monitor for an even
sooner time. Behind this front, ceilings will drop down to IFR and
quite possibly LIFR along with vis reduction owing to fog. Precip
chances increase tonight with thunder a possibility but with
confidence low at this time, have excluded from the TAFs.



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...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...9 AM Friday to 7 PM Friday.




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