Area Forecast Discussion
Issued by NWS Central Illinois

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247
FXUS63 KILX 210219
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
919 PM CDT Sat May 20 2017

.UPDATE...
Issued at 919 PM CDT Sat May 20 2017

Widely scattered showers and thunderstorms have developed in
advance of a cold front across the Illinois River Valley this
evening. Latest radar imagery continues to show the cells
struggling to organize as daytime instability wanes...with the
activity mainly confined to locations from Rushville southward.
HRRR has been consistently showing scattered convection spreading
E/NE across the remainder of the CWA by midnight, with showers
ending from west to east overnight.

&&

.SHORT TERM...(This evening through Sunday)
ISSUED AT 300 PM CDT Sat May 20 2017

Warm front has pushed northward across most of the forecast area,
and was just starting to come into the areas from Galesburg to
Lacon at 2 pm. Quite a bit of thinning in the clouds has taken
place south of the front, and SPC mesoanalysis shows surface
based CAPE`s have reached 1500-2500 J/kg from about Bloomington
southward, although some capping is still evident looking at
mixed-layer CAPE. Initial line of convection has formed to our
southeast and was tracking northeast into Indiana, which just some
weak showers from around St. Louis southwest.

Main issue for this part of the forecast will be with convective
redevelopment ahead of the cold front, which is currently back in
western Missouri. High-res models have been trying to focus on a
couple bands of convection, one forming near I-57 during mid-
afternoon and another one arriving early evening immediately
ahead of the front. The NAM and HRRR support CAPE`s around 1000
J/kg through sunset across western Illinois as the line arrives,
although the better shear lags behind just a bit. Still may see
a broken line of stronger storms though, with the threat waning
toward late evening. Most of the forecast area will be dry by
about 3-4 am once the front passes. While Sunday should remain
dry, the circulation around the departing low will keep a fair
amount of cloudiness across the northern CWA.

The Flash Flood Watch for Sangamon County was extended into Sunday
morning, as hydrographs downstream of Spaulding Dam continue to
show a steady rise.

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 300 PM CDT Sat May 20 2017

The large upper low will slowly lift northeast early next week,
with a secondary lobe of energy swinging into the Great Lakes
Monday night into Tuesday. Scattered showers and a few
thunderstorms will be possible with this system. The trough will
continue to deepen as it moves into Illinois early Wednesday, and
some cold-core showers will be possible mid-week as lapse rates
steepen due to 500 mb temperatures down to -20C. Have included
some isolated thunder mention in the grids Wednesday afternoon due
to the steep lapse rates as well.

The upper low will finally shift off to the east late in the week,
allowing for a narrow ridge to build eastward from the Plains.
Some discrepancies with the models late-week in terms of a
shortwave coming out of the Rockies to help disrupt the ridge,
with the ECMWF most aggressive with any associated rain activity.
Have kept rain chances in the 20% range for now.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 645 PM CDT Sat May 20 2017

Central Illinois remains largely dry early this evening: however,
latest radar imagery is beginning to show widely scattered showers
developing well in advance of an approaching cold front from near
KMQB to KPPQ. HRRR suggests these cells will become more numerous
over the next few hours as they track northeastward toward the
I-55 corridor. Will include VCTS at both KPIA and KSPI through 05z
accordingly...but will hold off on any thunder mention further
east until radar trends become better established. Any storms that
develop will diminish toward midnight as the cold front pushes
into the area. Forecast soundings continue to suggest a brief period
of MVFR ceilings immediately behind the front...mainly at the
eastern terminals. These clouds will rapidly clear out by 10z. After
that...the main aviation challenge on Saturday will be potential
MVFR ceilings spilling into KPIA/KBMI. Based on NAM forecast
soundings...have lowered ceilings to MVFR at KPIA by 14z...then
further east to KBMI by 17z. Further south...kept ceilings in the
low VFR category through the day.


&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Sunday morning for ILZ051.

&&

$$

UPDATE...Barnes
SHORT TERM...Geelhart
LONG TERM...Geelhart
AVIATION...Barnes



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