Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260233

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

933 PM CDT

For Evening Update:

Minor changes to going forecast mainly to bump min temps a bit
with readings slow to fall with modest but persistent south flow
and cloud cover. Also tweaked pops a bit, focused mainly on
western IL after midnight.

Elongated trough of low pressure was analyzed this evening from
West Texas to the Mid-Mississippi Valley. Low level south winds
in advance of this feature were tapping Gulf moisture along the
Texas and Louisiana coasts, and this moisture was beginning to
stream northward into the Midwest. A few high-based sprinkles have
already developed in an isolated fashion across parts of the
region this evening, though mosaic radar presentation is likely
over-representing the lower level returns and little precip is
reaching the ground. Warm/moist ascent will persist overnight
however, in association with a series of mid-level disturbances
ejecting from the base of an upper trough noted across the Rockies
and Plains. Moisture transport and advection of higher thete-E
air mass is progged to increase overnight with development of a
30-35 kt low level jet, beneath an elevated mixed layer (EML)
based around 700 mb as evident in 00Z DVN sounding. This increase
in low level moisture, and modest mid-level cooling associated
with a short wave feature currently lifting northeast across
eastern KS/OK, should tend to support elevated convective
development into parts of far western/northwest IL after midnight
tonight as depicted by several convective-allowing models. Capping
inversion looks to hold a bit stronger farther to the east per
forecast soundings though, and recent trends in CAM guidance is to
weaken/dissipate activity as it tries to spread east late
tonight. Guidance is also in good agreement however, in spreading
convection northeast across the area after sunrise Wednesday, with
model simulated reflectivity suggesting a remnant MCV (emanated
from current thunderstorms over southeastern KS) may lift across
the area during the morning. Otherwise, guidance appears to focus
on the very late afternoon and more so the early-mid evening hours
of Wednesday for the greatest potential for widespread
thunderstorms across the area. Deep south-southwesterly flow
develops across the region, resulting in sufficient speed shear
for strong to severe organized convection later in the day.

In the near term, have bumped min temps overnight based on
observed trends this evening, with temps slow to fall off under
cloud cover and with persistent south winds. Will likely trim pops
a bit in the east through Sunrise too based on recent high-res CAM
guidance as discussed above.



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 00Z TAFs...

While the weather across the area will remain quiet for much of
tonight, an active weather pattern is likely to result in
inclement weather over the terminals on Wednesday and Wednesday
night. The primary concerns during this period will be the
potential for two periods of showers and thunderstorms, with
moderate to heavy rain possible Wednesday evening, along with the
threat for low CIGs by late Wednesday.

Thunderstorms are expected to continue to develop well to our
southwest over western Missouri tonight as a storm system begins
to impact the Midwest. This activity is likely to shift
east-northeastward overnight, and the remnants of these storms may
shift over northern Illinois and northwestern Indiana early
Wednesday morning (~13 to 16Z timeframe). Confidence is not the
highest that this will result in thunder at the terminals at this
time. For this reason, I have only mentioned a PROB30 for TSRA
early Wednesday morning.

After this period of morning activity, it appears there will be a
break in the activity until sometime late Wednesday afternoon into
the evening, when our next shot of showers and thunderstorms looks
to enter the picture. During this period, another potent weather
disturbance is forecast to shift north-northeastward across
Illinois, and this looks to set the stage for a good moderate to
heavy rain event for portions of the region. There will also be
good chances for thunderstorms, especially over eastern Illinois
and into Indiana. However, farther west towards RFD, the threat
for thunder looks lower late Wednesday into the evening, and we
have therefore left the mention out for this period.

Otherwise, expect gusty south-southwesterly winds during the day,
then shifting westerly Wednesday evening a surface boundary
gradually shifts over the area. CIGS will also likely become MVFR
or IFR Wednesday evening.



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...nearshore waters
     until 11 PM Wednesday.




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