Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 191740

Area Forecast Discussion
National Weather Service Lincoln IL
1240 PM CDT Wed Jul 19 2017

Issued at 1051 AM CDT Wed Jul 19 2017

Isolated convection associated with a decaying MCV is currently
exiting the Wabash River Valley into western Indiana. Once this
activity comes to an end, partly to mostly sunny skies and dry
conditions are expected over the next couple of hours before
isolated thunderstorms re-develop by mid to late afternoon. HRRR
has been consistently focusing isolated cells along/north of I-74
and east of I-55. Have carried slight chance PoPs across these
areas accordingly. The main weather story for today will be the
increasing heat and humidity. Based on the 12z KILX upper air
sounding, mixing should take place up to about 850mb today, which
will yield afternoon high temperatures in the lower 90s. Have
maintained lower 90s across much of the CWA, but lowered them into
the upper 80s near the Indiana border where clouds and precip have
lingered throughout the morning. The hot temperatures combined
with dewpoints in the lower to middle 70s will create maximum
afternoon heat index values ranging from the upper 90s near the
Indiana around 105 west of I-55.


.SHORT TERM...(Today and tonight)
ISSUED AT 324 AM CDT Wed Jul 19 2017

Even in the short term, the main story is the heat over the
Midwest for the next few days. Aloft, the deep warm air over the
SW shifts into the Midwest as the upper ridge starts to dominate
the Mid/Lower Mississippi River Valley. Midlevels warm a couple
more degrees temps at the surface will move into the
low to mid 90s. Remnants from last nights convection, however,
will stall the diurnal trend this morning with cloud cover and a
few showers with some isolated thunder, mainly east of Interstate
55. Plenty of moisture in the low levels will keep the dwpts in
the 70s and push the heat indices into the low 100s. The Heat
Warning/Advisories in the area start today and are expected to
last through the end of the work week. Trending the isolated pops
east and south through the morning...and expecting a quick
recovery of the atmosphere...cannot rule out some more quick
convective showers in the east and southeast on the edge of the
older activity. Winds will be light and somewhat variable again
today...but trending more southwesterly as a whole.

Tonight, the activity shifts to northern IL again as another wave
moves into the upper Midwest and spawns more showers and
thunderstorms. Given the models poor performance in handling last
nights convection and/or overestimating the inhibition offered by
the ridge...have brought slight pops down into ILX forecast
area... particularly after 06z...which seems to be in line with
SPC Day One, with the Marginal Risk category dominating the
northern half of the state.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 324 AM CDT Wed Jul 19 2017

Summary of the rest of the week... hot and humid...with stormy
chances mostly confined to the north...mostly. The waves will
produce nearby convection, leaving much of Central IL subject to
more mesoscale boundary impact from any stronger storms that may
develop...through the end of the week. Best chances to see precip
will remain north of Interstate 74. Guidance is coming
together...resulting in a solution far closer to this weeks ECMWF
solutions with temps on Thu-Fri soaring into the mid to upper 90s.
The low level moisture is not moving in a relatively static
airmass. As a result, heat indices from 100F-110F dominate the
week. Overnight lows will generally not drop below the lower 70s.
Only minor adjustments to the temps through the end of the week
with the blend at this point. Heat headlines in good shape and
changes are not needed.

Central IL remains on the edge of the rising heights over the
southern half of the country...and the slightly cooler air to the
north as a series of waves move through the zonal flow to the
north. The developing boundary is expected to settle into the
region over the weekend, developing a more widespread chance for
showers and thunderstorms...and somewhat cooler max temps. Models,
however, are struggling with the timing of the boundary`s southern
progression...and Saturday looks to remain in the heat threat with
any delay. Heat abates for the first part of next week behind the
boundary...should the boundary actually remain on the move through
the end of the weekend.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1240 PM CDT Wed Jul 19 2017

VFR conditions will prevail through the entire 18z TAF period.
HRRR continues to suggest isolated storms will develop near the
I-57 corridor this afternoon...near a differential heating
boundary that has developed as a result of morning clouds/precip
near the Indiana border. Have therefore carried VCTS at both KDEC
and KCMI through the afternoon. Any diurnal clouds/convection that
develop will quickly dissipate by sunset, followed by mostly clear
skies overnight. A large complex of thunderstorms will develop
across the Dakotas into Minnesota this evening, then track E/SE
across Wisconsin overnight. Most models keep this convection well
to the north of central Illinois, so will only add a deck of high
clouds to the I-74 terminals as the system skirts by to the north
Thursday morning.


Excessive Heat Warning until 8 PM CDT Saturday for ILZ027>029-

Heat Advisory until 8 PM CDT Saturday for ILZ030-031-038-043>046-



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