Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 020824

Area Forecast Discussion
324 AM CDT Tue Sep 2 2014

.SHORT TERM...(Today and tonight)

18z/1pm surface analysis shows outflow boundary from late night
convection across Iowa extending from near Kankakee to just west of
Taylorville.  Airmass behind the boundary has stabilized greatly,
with the special 18z KILX upper air sounding showing less than
200J/kg of CAPE and a strong cap at around 850mb.  Meanwhile ahead
of the boundary, LAPS soundings indicate CAPES of 2000-2500J/kg and
little to no capping.  Will be watching the E/SE CWA for potential
development over the next couple of hours, but window of opportunity
will be small and any convection that fires will quickly push into

Next concern will be the heavy rain threat tonight.  As has been
advertised by the last several runs, latest models continue to show
showers/thunderstorms becoming widespread along the trailing boundary
tonight as 40-45kt LLJ develops from the Southern Plains
northeastward into the Ohio River Valley.  Still some questions as
to exactly where the axis of heaviest rain will set up, with the NAM
appearing to be a bit too far north.  GFS/HRRR seem to have a better
handle on the rain given current scenario.  Think scattered
showers/storms will develop across the southern half of the CWA
early this evening, then as LLJ jet energy increases, areal coverage
will expand toward midnight.  With storms becoming parallel to upper
flow and likely training over the same locations, am concerned about
flash flood potential across the E/SE overnight.  Precipitable water
values ramp up to over 2 inches, while upper jet strengthens over
the Great Lakes providing enhanced synoptic lift.  Based on GFS/HRRR
QPF fields, have decided to issue a Flash Flood Watch for all
locations along/south of a Paris to Shelbyville line.  Rainfall
rates may exceed 2 inches per hour at times, leading to a flash
flooding risk.  Have gone with categorical PoPs across the Watch area,
tapering down to just slight chance across the far north around
Galesburg. Have lingered PoPs across the E/SE into Tuesday morning:
however, all model guidance suggests frontal boundary will push
south of the area and bring an end to the rain chances by midday.


.LONG TERM...(Wednesday through Monday)

The frontal boundary is expected to return north as a warm front in
Missouri and western IL during Wednesday. The 00Z model guidance has
trended a bit quicker with a short wave moving out of MO into IL Wed
afternoon and into IN Wed evening. Best chances of convection still
apears to be over sw IL into MO where better 850 mb low level jet
will be and more moisture transport. Confined pops to areas from
I-74 sw on Wed. Warmer and more humid air starting to return back to
central/se IL with highs in the mid to upper 80s and dewpoints
elevating into the upper 60s and lower 70s Wed afternoon and
highest in sw counties.

Have 20-30% chance of showers and thunderstorms over northern
counties by IL river valley Wed night mainly after midnight as the
instability axis, thermal ridge, and nose of the 850mb LLJ advance
toward northwest IL. Thu still appears to be the warmest day this
week with highs in the upper 80s and lower 90s and dewpoints in the
low to mid 70s with breezy sw winds and well entrenched in the warm
sector. Peak afternoon heat indices may reach into the upper 90s to
around 100F on Thu. Most areas should be dry Thu with just isolated
convection near the Wabash river Thu afternoon.

Low pressure deepening ne from the northern plains to north of the
Great Lakes will drag a cold front into the IL river valley early
Friday morning and into southern Illinois by Friday evening. Have
20-30% chance of showers and thunderstorms after midnight Thu night
over the IL river valley. Then a good chance of showers and
thunderstorms spreading se across central/se IL during the day
Friday and continue Friday night. Still a very warm and humid day
Friday with highs in the 80s and dewpoints in the upper 60s to mid
70s, with southeast IL near 90F southeast of I-70. Lingered chances
of showers and thunderstorms Saturday south of I-72, with
diminishing chances in southeast IL during Saturday afternoon as
frontal boundary pushes se into TN river valley.

A significant airmass change will follow that frontal passage, as a
1024 mb Canadian high pressure builds southeast into the western
Great Lakes. Less humid air along with cooler highs Sunday and
Monday in the 70s, with lows Saturday and Sunday night in the 50s.
This will be the coolest air since mid August to visit central IL,
the last time we had below normal temperatures.


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

A band of showers and thunderstorms will continue to brush past
KSPI, KDEC, and KCMI for the next several hours, but KPIA and KBMI
should be largely dry. Conditions will likely degrade to MVFR as the
rains continue and/or as fog develops in the case of KPIA/KBMI.
Quieter weather and VFR conditions will prevail for most of the
daytime hours Tuesday and into the evening as drier air filters
into the area behind a cold front.


FLASH FLOOD WATCH until 7 AM CDT this morning FOR ILZ056-057-



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