Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260536

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1236 AM CDT Wed Jul 26 2017

308 PM CDT

Through Wednesday...

Most of the forecast concerns will be after the short-term period
by Wednesday evening, with the main focus on very warm and humid
conditions on Wednesday. In addition, can`t rule out a few
thunderstorms making it into portions of northern IL (mainly far
NW and north central IL portions of the CWA during the late
afternoon). A persistent cluster of showers and thunderstorms to
our northwest finally is dissipating, with only high cloud cover
spreading over the area.

The rest of the night will be quiet with increasing high clouds
from convection to our north and over the Plains. Overnight and
early morning temperatures will be milder than today`s lows and
mainly in the 60s. A warm front will lift north on Wednesday,
bringing a return to uncomfortable humidity levels, with dew
points rising to the upper 60s to lower-mid 70s. There should be
plenty of convective mid and high cloud debris overspreading the
area, which will limit temperatures somewhat. Progged 850 mb/925
mb temperatures would typically support around 90, but with the
cloud cover limitations, expecting mid to upper 80s (peak heat
index values in the mid to high 90s over IL corn belt areas).

Forecast soundings indicate fairly stout capping over the local
area into the afternoon, which should effectively prevent
thunderstorms until the mid to late afternoon. With the arrival of
main short-wave from the west toward evening, have indicated
slight to low chance category PoPs for the potential of lead
convection spreading into portions of northern IL as mentioned
above. Will need to watch for a slightly quicker onset of higher
coverage showers and thunderstorms just prior to 00z. The Long
Term discussion below contains full details on the risk for heavy
rain/flooding and strong to severe storms Wednesday evening and



311 PM CDT

Wednesday night through Tuesday...

Main forecast concerns/challenges are at the start of the period
with chances for thunderstorms, and potential for strong to severe
storms with heavy rainfall Wednesday evening through the
overnight hours Wednesday night.

Details of the forecast to fall out with later updates, but with
the general idea still on track with this morning`s guidance.
Period of interest during this time frame is Wednesday evening,
when guidance fairly consistent in ushering energetic shortwave
and associated surface low through the Midwest and Great Lakes
region. Still come uncertainty with exactly how the mid and lower
levels of the atmosphere will respond that include strength and
placement of surface low, as well as the strength of the LLJ. This
will play a part to the extent/magnitude of the strong/severe
storms and heavy rainfall. Nonetheless, instability axis shifts
back overhead during this time, along with PWAT axis of around 2
inches. All of these ingredients coming together will support a
continued chance of thunderstorms for much of the CWA Wednesday

Will likely see showers and thunderstorms developing across the
central CONUS early in the day Wednesday, and then continuing to
move east into the afternoon. Some diminishing may occur early on,
but as this precip/forcing encounters increasing instability,
will likely see expansion and strengthening of these thunderstorms
by mid to late afternoon just west of the CWA. Some slight
variability with timing, but most guidance in fair agreement with
these thunderstorms not reaching the western CWA (West of I-39)
until after 00z. Given the time any storm would reach the CWA,
wonder how widespread the severe weather threat will be and also
window that this will occur Wednesday evening. At this time, the
building instability will support the highest threat of severe
weather (damaging wind and hail) across the west and southwest
portions of the CWA. This would be along/west of I-39 and south of
I-80. However, don`t want to exclude any remaining areas in
northeast IL and northwest IN at this time, as there is an
isolated severe threat in these locations with possible additional
thunderstorm development to the north/northwest along the
approaching frontal boundary. Also, strengthening LLJ could offset
any diminishing trend with instability and support an additional
severe threat into the overnight hours area wide, with
thunderstorm coverage also possibly increasing. At this time, it
appears setup will favor continued thunderstorm development across
much of the CWA into early Thursday morning, though a favored
location would be south of I-80

Heavy rainfall and possible flooding are concerns Wednesday
evening through early Thursday morning. Still too early to
finalize an exact location for highest rainfall amounts and
although south of I-80 appears to be favored location, its still
possible for higher amounts to the north. With the high PWATS,
high rainfall rates are a concern area wide. Have continued to
highlight these threats with an ESF, but as was noted with the
previous forecast, could see the need for a flash flood watch for
a portion of the area Wednesday night.



For the 06Z TAFs...

The concerns with the aviation forecast today are:

- Chance for MVFR ceilings this morning after sunrise

- Southeast winds becoming west of south near or shortly after
  sunrise, then becoming southeast again this evening

- Showers and some storms in the area tonight

Southeast winds early this morning will shift southwest as they
feel the influence of an approaching low pressure. This low will
continue areas of showers and scattered storms eastward across
Iowa and southeast across Wisconsin today. A likely re-
intensification of convection will occur late this afternoon in
the IA/MO/western IL area, and these widespread storms should
remain south of the TAF sites tonight. However, enough moisture
and focus from this weather system will be present for likely
showers and some storms in the area. Confidence in evening rain
coverage is low, and in timing is medium. Behind the low pressure,
winds may be variable in direction and should be light as well at
daybreak Thursday before becoming northeast and increasing in



311 PM CDT

Despite high pressure just to the east of the lake this
afternoon, seeing surface gradient and winds beginning to increase
across the lake. This is occurring as surface low and trough
track through the upper Midwest and southern Canada. Expect 15 to
25 kt over the northern two thirds of the lake this afternoon and
evening, with winds continuing to increase over the northern half
through the night. As gradient further tightens, winds to 30 kt
will are likely over the north half later tonight. These stronger
winds will persist Wednesday morning but will see a diminishing
trend through midday and the afternoon as the surface trough moves
overhead. Expect these lighter winds to persist for a time
Wednesday night but as the surface trough/low depart to the
southeast Thursday, will see increasing northerly winds across the
entire lake. Winds to 30 kt return Thursday afternoon and
evening. Northerly winds will then likely continue into the start
of the weekend, with speeds periodically bouncing from 15 to 25
kt, to 30 kt during this time.






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