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Area forecast discussion 
National Weather Service Lincoln Illinois
652 am CDT Fri Aug 22 2014

Short term...(today and tonight)
issued at 325 am CDT Friday Aug 22 2014

07z/2am surface analysis shows a stationary front extending from
west to east across northern Illinois. A 35kt low level jet 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
Illinois. 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 mesoscale convective system develops to
our west and rolls across north Illinois. 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 County Warning Area 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)
issued at 325 am CDT Friday Aug 22 2014

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 forecast area...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. European model (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 12z tafs through 12z Saturday morning)
issued at 652 am CDT Friday Aug 22 2014

A nocturnal low level jet has helped fuel an east-West Line of storms across
north-central Illinois. The storms with heavy rainfall will affect
mainly pia, bmi and cmi early this morning, bringing MVFR vis and
ceilings. Bmi has dropped to IFR cloud height, which could linger
through 15z before lifting to MVFR. A break in the storms is
indicated for the terminal sites for late morning into afternoon
in the hrrr and rap model output. Additional storm chances are
indicated for later this afternoon and evening for the northern
taf sites of pia, bmi and cmi. We kept precip our of Dec and spi
for now, but trends are for storms to get quite close even this
morning. Will monitor closely for changes to the dry forecast for

Winds will generally remain S-SW in the synoptic flow across our
area, south of the warm front. Local northerly winds have
developed for the northern terminals where the storms have
produced an outflow boundary moving south. The outflow could even
reach spi and Dec, but wind speeds by then should be 10kt or less.


Ilx watches/warnings/advisories...
heat advisory from 1 PM this afternoon to 7 PM CDT Monday for



Short term...shimon
long term...hjs

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