Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 010452

Area Forecast Discussion
1152 PM CDT Sun Aug 31 2014


Aside from a few spotty showers in east central Illinois, quiet
conditions prevail across the forecast area this evening, and
should for most if not all of the night. The main thing that may
disrupt the quiet weather before morning, and it looks rather
impressive at the moment, is the storm complex approaching from
the Plains & upper Midwest. This complex is being driven by a
seasonably strong upper level jet/wave, and 45-50 kt low level
jet. Expect this complex, although it should be on the downswing
in intensity, to begin to impact areas west of the Illinois River
after 4 AM.

Going forecast has overnight scenario well covered. Only minor
tweaks have been needed to the hourly trends.


.SHORT TERM...(This evening through Monday)

Vigorous short-wave evident on 18z/1pm water vapor imagery over
Colorado will be the primary weather-maker across central Illinois
over the next 24 hours.  As this feature pushes eastward, showers
and thunderstorms will become more numerous across western
Iowa/Nebraska this afternoon.  A complex of strong to severe
thunderstorms will eventually get organized, then begin pushing
eastward across Iowa overnight.  Most model guidance keeps this
convection west of Illinois until around 12z.  Have therefore
gone with a mostly dry forecast for tonight, with chance PoPs
arriving across the Illinois River Valley well after midnight.

Overnight thunderstorm complex will likely spill into west-central
Illinois Monday morning as it undergoes diurnal weakening.  While
gusty winds and brief heavy downpours may be possible with the
storms, the severe weather threat will remain low during the morning
hours.  Have gone with likely PoPs along/west of I-55 through
midday, with just chance PoPs further east and south.  Main question
on Monday will be when/where thunderstorms will re-develop during
the afternoon.  Colorado short-wave and accompanying cold front will
approach from the west late in the day, while outflow boundary from
morning convection will be around as well.  With plenty of
boundaries to develop on and strong upper support arriving, think
showers and thunderstorms will become likely across the northern
two-thirds of the CWA during the mid to late afternoon hours.
Forecast soundings indicate the atmosphere will have enough time to
destabilize after the morning clouds/showers, with CAPE values
reaching the 2000 to 3000J/kg range.  More impressive is the
projected 0-6 km bulk shear values, which climb into the 30 to 50kt
range.  If airmass can destabilize as expected, strong to severe
thunderstorms will be a good bet Monday afternoon/evening.  Any
storms that develop will be capable of producing large hail and
damaging wind gusts.  Have added severe/heavy rain mention to the
forecast accordingly.


.LONG TERM...(Monday night through Sunday)

A line of strong to severe storms is expected to in progress at the
start of Monday night, as a cold front pushes southeast across
Illinois. Forecast soundings indicate around 2000 J/kg MLCAPE and
0-6km shear of 40kt will continue into the evening. Those params
coupled with abundant moisture in the 1.7" to 2" PWATs will be
enough to fuel some rotating storms with hail and damaging winds.
Flash flooding will be a concern as well, especially as Monday
night progresses. The 300mb jet is expected to increase over the
western Great Lakes, putting central IL under the right entrance
divergent region. The line of storms is expected to become
oriented parallel to the steering flow later at night, allowing
for more training echoes, back-building, and very high rainfall
amounts. The higher threat for flooding will be mainly southeast
of I-72 Monday night, and heavy rain was added to the forecast
during that time frame.

Storm chances will continue into Tuesday for our southeast counties
due to the slower progress of the front, along with heavy rain
threat. Zonal flow will bring a few shortwave toward Illinois on
Wednesday, but any storms associated with them appear to lose
support as they depart farther from the thermal and theta-e ridges
in the Plains. We kept chance PoPs west of Springfield for Wed, with
slights east to I-57. Variable coverages of slight chance PoPs were
left in the extended through Thursday due to periodic waves of
energy that could trigger isolated showers/storms.

Storm chances increase in the north Thursday night as the next cold
front and upper trough approach IL. Widespread chance PoPs were left
in the forecast for Friday into Saturday as the cold front slowly
progresses southeast across the area.

Humidity will remain high from Tuesday through Friday, as afternoon
dewpoints climb from the upper 60s Tuesday to the low to mid 70s on
Thursday and Friday. High temps will be in the low to mid 80s on
Tue-Wed, but will warm into the upper 80s and low 90s Thur and
Friday ahead of the cold front Friday night. Much cooler temps and
dewpoints are forecast for Saturday and Sunday next weekend, with
highs on Sunday in the 70s and lows in the mid to upper 50s.


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

Convective complex making steady progress toward central Illinois
from the west, although it has been weakening with time as
expected. Cigs will develop/thicken/lower across the terminals
overnight as the complex draws closer/pushes in, but predominantly
VFR conditions are likely aside from any heavier precipitation.
Still some uncertainty when/if the storm complex will dry up
completely, so have only included a TEMPO at KPIA overnight.
Shower/storm redevelopment is likely across central Illinois by
Monday afternoon, focusing along an outflow boundary from
tonight`s storms and/or an approaching cold front. This
precipitation is apt to be more widespread, so have included
predominant showers with VCTS beginning in the afternoon and
continuing into the evening hours Monday. At this point, only have
VSBYs falling to MVFR levels in the steadier rainfall, but the
CIGs certainly may as well.




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