Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 252049

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
349 PM CDT Tue Apr 25 2017

208 PM CDT

Through Wednesday...

For the remainder of the afternoon and overnight, conditions will be
relatively quiet.  Main forecast concern will be timing and
intensity of tsra expected to move into the region tomorrow.

With low pressure tracking north along the Atlantic coast, a ridge
of high pressure will remain parked from New England through the ern
Gulf Coast.  With deepening low pressure over the central plains and
a trough extending north into the upper Mississippi Valley, low level
flow will be off of the Gulf of Mexico and deep layer moisture will
be on the increase with sfc dewpoints climbing into the lower 60s,
temperatures in the mid to upper 70s and pwats approaching 1.5
inches. With the increasing moisture and instability as well as a
wind profile with a 50 kt low level jet extending into nrn IL ahead
of the approaching cold front, severe thunderstorms will be a
possibility and SPC has extended the Slight Risk for severe
thunderstorms through central IL into the far southern portions of
the Chicago Metro area with a Marginal Risk over the remainder of
the CWA, including the Rockford area and northwestern Indiana.  At
this point, the main severe threat should be for large hail and
strong straight line winds. Through the day, isolated severe storms
could evolve upscale into more linear bow echoes and strong winds
becoming the main severe threat.  However, the progression of the
pattern is looking to become less progressive through the day
tomorrow and heavy rainfall and flooding may become an increasing
concern, especially into the evening hours.


329 PM CDT

Wednesday night through Tuesday...

Main forecast concerns/issues are with a continued severe and
heavy rainfall threat Wednesday evening, as well as returning
chances for showers and thunderstorms over the weekend which could
provide periods of stronger development as well as periods of
heavy rainfall.

As was mentioned in the short term AFD, the severe threat may
transition over to heavy rainfall and possible flooding threats in
the evening. The severe threat will likely continue for a time in
the evening though, especially across the eastern CWA, far
northeast Illinois and northwest Indiana. This is as a front
pushes through the remaining CWA, but with a weakening trend of
the overall instability while this occurs. Once again, showers
likely to continue for much of the night. Strong ascent owing to
negatively tilted trough overhead, lingering instability, as well
as high PWAT axis still in place will support this setup. Have
increased pops during the Wednesday night time frame, along with
QPF. Given the convective nature of the anticipated precip, likely
did not increase enough. Most locations will likely get into this
continued precip axis, however, it does appear that northeast
Illinois and northwest Indiana will be a continued favored
location. System will finally kick out of the region Thursday with
any lingering showers in the morning exiting to the east.

After somewhat of a lull period, active weather returns on Friday
with showers returning in the afternoon and evening. At this
time, not overly concerned for hazardous weather, either stronger
storms or additional heavy rainfall during this period. However,
this begins to change on Saturday as attention turns towards
expected large system to slowly move through the central CONUS
through the weekend. Increasing moisture and instability will
assist in returning thunderstorm chances across the area Saturday
and Saturday night, and then likely continuing Sunday and quite
possibly Sunday night. Details with exact solution/evolution still
to come, but still monitoring for possible periods of stronger
thunderstorms and heavy rainfall that could produce flooding



For the 18Z TAFs...

Forecast concern initially will focus on winds for the remainder
of the afternoon and through the night and then shift to pcpn/tsra
chances for tomorrow.

With a ridge of high pressure extending from New England through
the central Gulf coast and a trough of low pressure extending from
the central plains to the upper Mississippi valley, winds across
the region have been generally southerly through much of the day.
As the low pressure deepens over the plains, wind direction will
gradually back to sely this afternoon and overnight tonight. With
filtered sunshine, modest mixing will allow for ocnl gusts to arnd
20-25 kt through sunset. The pressure gradient should strengthen
overnight, keeping winds in the 10 to 15 kt range overnight and
then becoming gusty again tomorrow, to arnd 25 kt.

Warm/moist advection will increase through the morning hours and
isentropic lift in the warm sector ahead of an approaching cold
front will bring increasing chances for pcpn/tsra. The window of
opportunity for the greatest amount of ts coverage and intensity
will be through the afternoon and evening hours though there will
be a chance for at least some sct shra/tsra by late morning.
Progression of the frontal system is expected to slow durg the
afternoon and into the evening, so locally heavy rainfall may
contribute to lower visibility than currently indicated in the
TAFs. Also, while widespread winds should be in the 15g25kt range,
localized winds invof tsra will likely be much stronger and of
variable direction. Ocnl mvfr cigs are likely invof tsra, but
should otherwise gradually lower to lower end vfr as moisture
streams into the region.


340 PM CDT

A trough of low pressure is situated just to the north and
northwest of the lake, while low pressure is over the central
Plains. As this low slowly lifts northeast toward the lake
tonight, expect gradient and winds to increase over the entire
lake tonight and Wednesday. These increasing winds will likely be
around 30 KT over the open waters, with hazardous conditions for
small craft developing tonight and continuing into Wednesday
evening. Winds will diminish throughout the night Wednesday night
as this low then moves over the lake and then to the northeast.
Winds are not expected to be overly strong on the backside of this
system, but it is quite possible for them to be stronger than
currently forecast.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...11 PM Tuesday to 11 PM Wednesday.




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