Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 250818

Area Forecast Discussion
318 AM CDT Mon Aug 25 2014

.SHORT TERM...(Today and tonight)

Very hot and humid conditions will continue today under the dome of
high pressure. No storms are expected across central and SE Illinois
due to the synoptic subsidence. Highs will climb a couple degrees
above Sunday, with heat index readings in the 105 to 112 range
across the board. All heat headlines look on track and no
adjustments will be needed with this update.

A cold front to the NW of IL will slowly approach over the next 24
hours. A 500mb shortwave is projected to move northeast along the
front tonight as the front reaches near the Illinois river by
12z/7am Tuesday. Storms will initially develop in NE Iowa and NW
Illinois this evening and advance with the front toward our
counties. We kept slight chance PoPs for the evening for areas NW of
Peoria. Then after midnight we went with chance PoPs for the same
area, and slight chance as far southeast as Rushville to El Paso. No
severe weather is expected, but some locally heavy rainfall could
develop in the very moist air mass.


.LONG TERM...(Tuesday through Sunday)

Placement of the frontal boundary will be a challenge through the
period. The upper ridge will become flattened a bit by mid week as a
shortwave tracks across the upper Great Lakes region. An upper low
currently evident on water vapor imagery over southeast Oregon will
slowly track eastward through the week, likely not reaching our area
until the weekend. Evening model suite in general agreement with
pushing the frontal boundary southward to around I-72 by early
Wednesday, as surface high pressure builds southeast into the Great
Lakes. However, they start to diverge a bit after that as heights
begin to rise again ahead of the Oregon wave.

Main convective bands through midweek will track across Iowa and
northern Illinois, but rain chances will spread southward Tuesday
afternoon and evening. Latest ECMWF and GEM models have rain chances
as far south as I-70 by afternoon, but the NAM/GFS hold the cap that
far south in place until evening. The cloud cover with these storms
has some potential for limiting the high temperatures on Tuesday,
especially over the northern CWA, with heat index values a bit lower
as well. High dew points have been the trend the last couple days,
and should see that continuing to bring heat index values into the
103-107 range. No changes anticipated to the heat headlines at this
time. The cloud cover should be more extensive mid to late week as
highs drop back into the 80s, so heat index values should remain
below 100 for the most part.

Longer range models trying to show a trend toward a brief drier
period Thursday night and Friday morning ahead of the wave moving
into the central U.S.  The latest ECMWF would suggest the first half
of the weekend could end up dry as well, while the GFS drags the
front through early Saturday as it is picked up by a shortwave
moving through the Great Lakes. The upper features of the two models
are continuing to show more agreement through Friday at least, but
confidence wanes again during the holiday weekend.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)

The main forecast concern continues to be coverage of MVFR or
lower vsbys overnight in fog. Already seeing some patchy fog
east of I55 late this evening with CMI at 4SM with lower vsbys
well south and east of the TAF sites. Surface obs continue to
indicate winds above 5 kts at most locations along and west of
I55 which should keep any widespread low vsbys at a minimum.
Short Range Ensemble forecasts have indicated areas east of
I-55 have the higher probabilities for vsbys less than 3 miles
from 08z-12z so will continue to keep the lowest vsbys out east,
with higher range MVFR vsbys to the west.

Any fog should quickly lift by 14z with VFR conditions expected
for the remainder of the forecast period. We will have to watch an
area of showers and storms that will push across Iowa into Minnesota
and southwest Wisconsin which may turn more east and then southeast
into parts of northern IL during the morning hours of Monday. At
this time, it appears the better threat for convection Monday will
remain to our north as upper level ridging holds across the region.
Southeast winds of 3 to 7 kts will continue tonight and then shift
more into a south to southwest direction on Monday with speeds of
10 kts or less.


EXCESSIVE HEAT WARNING until 7 PM CDT Tuesday FOR ILZ042-047>054-

HEAT ADVISORY until 7 PM CDT Tuesday FOR ILZ036-040-041.

HEAT ADVISORY from Noon today to 7 PM CDT Tuesday FOR ILZ027>031-



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