Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 220825

Area Forecast Discussion
325 AM CDT Fri Aug 22 2014

.SHORT TERM...(Today and tonight)

07z/2am surface analysis shows a stationary front extending from
west to east across northern Illinois. A 35kt LLJ has helped trigger
a line of convection just south of that front. Those storms will
affect our northern counties early this morning, with locally heavy
rainfall rates possible. Areas near Midway airport last night picked
up 3" in 30 minutes. There most likely will be a break in the
rainfall for at least a few hours later this morning and early this
afternoon, before the next shortwave over Iowa works its way into
IL. Precipitable water values up near 2" will continue through the
day, so any convection that redevelops this afternoon into evening
will have the potential to produce very heavy rainfall. Coverage may
not be too extensive during the day, so slight chance and low chance
PoPs were used for areas north of I-72 after 7 am.

The HRRR/NAM/GFS/ECMWF all show an enhanced opportunity for storms
either later this afternoon or this evening as an MCS develops to
our west and rolls across N IL. With timing still in question, we
did not ramp up to likely PoPs in any one area even this evening and
overnight. The main theme does seem to be that the northern half of
the KILX CWA will be the main target area for storms over the next
24-36 hours.

High temps today will once again climb into the upper 80s to low
90s, with dewpoints already in the mid 70s and likely to climb into
the upper 70s in some areas. Heat index readings will likely climb
to around 105 in southern areas. We can not rule out the possible
need to expand the heat advisory northward. Precip and cloud cover
across the north should keep their heat index readings below
advisory levels in general.


.LONG TERM...(Saturday through Thursday)

Main issue through the extended remains the extreme heat for this
time of year through the weekend and into the first portion of next
week.  Models still having issues with the ultimate breaking down of
the ridge, resulting in a potential extension to the heat wave
should the trend continue. Hot daytime highs into the 90s and
dewpoints into the mid 70s resulting in heat indices over 100F for
several days on end.  Scattered chances exist as long as the
boundary remains close to the FA...and will be the only chance for
brief cool downs.  But as the high builds into the region, the
general subsidence will be hard to overcome, so the forecast for
Saturday night through Sunday night is dry for now...with some
slight chances coming back for Monday...but the models struggling
with the solutions in the mid range with the timing of a vigorous
wave to erode the ridge. GFS still far more aggressive, amplified,
and fast with the passage of the upper trof/sfc front Tues night
into Wednesday.  ECMWF starting to close the gap and has abandoned
the closed low of 24 hrs ago... but is still signif weaker,
resulting in a developing baroclinic zone at the sfc as the wave
fizzles in parallel flow and provides a focus for a longer duration
potential for precip. Model blends now through the mid/end of next
week results in more widespread chances.


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

The main forecast concerns this period include timing and coverage
of convection overnight, along with the potential for some patchy
fog across our northern TAF sites. Little overall change in thinking
from the previous discussion with some scattered showers and
thunderstorms occurring to our north and trying to make their way
southeast into central Illinois. Expect the coverage to gradually
increase over the next several hours but still not certain enough
across our northern TAF sites to include a tempo group just yet.
Will keep an eye on the radar up until issuance time and change
if need be. Further south, will continue to hold off mentioning
anything at SPI and DEC as it appears the higher probabilities
for better coverage to the rain will be to the north. Any storm
that does impact a TAF site has the capability of dropping cigs
and vsbys very briefly to MVFR or even IFR conditions before
quickly improving.

Some MVFR vsbys possible with fog as our temp-dew point spreads
were down to a few degrees, especially across the north. The
shower and thunderstorm threat looks to diminish by 15z across the
north with any fog quickly dissipating by 13z, with mainly VFR
conditions all areas thru tomorrow evening. Southerly winds of
4 to 8 kts will prevail tonight and south to southwest winds of
10 to 15 kts are expected on Friday.


HEAT ADVISORY from 1 PM this afternoon to 7 PM CDT Monday FOR



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