Area Forecast Discussion
Issued by NWS Central Illinois

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038
FXUS63 KILX 252335
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
635 PM CDT Tue Jul 25 2017

.SHORT TERM...(This evening through Wednesday)
ISSUED AT 300 PM CDT Tue Jul 25 2017

Generally quiet weather is expected across central and southeast
Illinois through the period as high pressure drifts off the east
and a cold front approaches from the northwest. However, the
southerly low-level flow will push very warm and humid air back
into the area on Wednesday. Surface dew points should climb back
into the 70s areawide, with afternoon high temperatures near 90.
This will produce afternoon Heat Index values near 100 degrees.

There are varied model solutions in how how quickly and far ahead
convection will develop ahead of the cold front. However, the
current consensus is that most areas should stay dry during the
day.

.LONG TERM...(Wednesday night through Tuesday)
ISSUED AT 300 PM CDT Tue Jul 25 2017

Showers and thunderstorms will spread across the area Wednesday
night into Thursday as the cold front moves through the area from
northwest to southeast. While model agreement in the cold front &
driving upper level wave timing/track, and high precipitable
water values (>2") are good, agreement in the placement of the
potential heavy rainfall leaves much to be desired. The best model
consensus still favors a heavy rainfall axis along the I-74
corridor, although the axis is now more spread out and generally
less than 2". Given how moist the airmass is, would be surprised
if local rainfall totals didn`t exceed 2" (possibly by a
significant amount). However, there is very little skill is saying
where this will occur at this time. Considering how dry the local
area has been lately, most of the area has Flash Flood Guidance
for 1 hour in excess of 2". So, expect most areas should be able
to take the rain that falls outside of normally problematic low-
lying and/or urbanized areas. Severe storms are also possible with
this heavy rain event given the moderate instability (CAPEs
2500-3500 j/kg) and moderate to strong bulk shear (30-50 kts).
Damaging wind gusts should be the primary threat given deep warm
layer and weak lapse rates.

The rainfall threat ends by Thursday afternoon/evening as the
front departs the area. Cooler (slightly below normal) and less
humid air returns to the area for the remainder of the forecast
period with persistent ridging over the western U.S. and
persistent troffing in the east. A couple weak disturbances in the
northwest upper-level flow may bring a low chance of
precipitation to the area around Friday, and again around Monday
night, but nothing of significance is expected at this time.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 635 PM CDT Tue Jul 25 2017

VFR conditions expected the next 24 hours as a high pressure ridge
aloft crosses over the area and a cold front approaches from the
NW. Nevertheless, there is some potential for thunderstorms
developing across the area during the afternoon Wednesday,
although too small a probability for mention in TAFs at this time.
Shortly after the TAF forecast period, thunderstorms will likely
begin to move into the area from the NW with the approaching cold
front. Winds SE 4-8 kts overnight, becoming S 6-10 kts by around
18Z.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

SHORT TERM...Bak
LONG TERM...Bak
AVIATION...37



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