Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 202000
AFDILX

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

.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 18Z TAFS through 18Z Sunday afternoon)
Issued at 1222 PM CDT Sat May 20 2017

Starting to see some lifting of ceilings as warm front pushes
northward across central Illinois, with widespread VFR conditions
near and south of KDEC-KCMI. While IFR conditions continue to the
north, improvement will occur through early afternoon, and all
sites are expected to be VFR by 21Z. Next round of convection
expected to start firing around 21Z. Coverage should be
scattered, and will only go with VCTS at most sites, although
there is more confidence of showers/storms at KCMI. A cold front
will sweep across the area this evening and generally end the
thunder threat by about 06Z. While some clearing will take place
overnight into Sunday morning, the wraparound circulation around
this storm system will likely result in MVFR ceilings dropping
southward to KPIA and maybe KBMI by mid to late morning.


&&

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

&&

$$

SHORT TERM...Geelhart
LONG TERM...Geelhart
AVIATION...Geelhart



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