Area Forecast Discussion
Issued by NWS Central Illinois

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904
FXUS63 KILX 240825
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
325 AM CDT SUN JUL 24 2016

.SHORT TERM...(Today and tonight)
ISSUED AT 325 AM CDT Sun Jul 24 2016

Abnormally hot conditions will continue for one more day, before a
cold front pushes through IL later this afternoon and tonight. There
is enough evidence that heat indices will reach advisory criteria
(105F) even across our northern counties, so we decided to add the
northern tier of counties back in the excessive heat warning (due to
cumulative effect of 4 days of heat advisory levels). High temps
should top out in the lower 90s, and dewpoints should reach the mid
to upper 70s, producing peak heat index readings of 102 to 107F. The
heat warnings will end at 7 pm for all of our counties.

Despite minor timing differences for frontal progression in the
short term models, the overall consensus has the cold front reaching
near the IL river around 00z/7pm this evening. The RAP and HRRR
models seem to be acknowledging the capping inversion across central
IL, because they keep storms at bay ahead of the front for much of
the afternoon. The forcing along the front should be enough to
trigger a line of convection that may contain strong winds and hail.
We remain in the SPC Day 1 Marginal Risk for severe storms north of
I-70. The uncertainty in breaking the cap sufficiently is one
factor that has kept us below the Slight Risk to this point.

In light of the latest model trends, have kept the forecast dry
until this afternoon. We started with slight chances NW of the IL
river at 20z/3pm, then ramped up to high chance PoPs by 00z/7pm,
while advancing the storm chances to the southeast as the front
progresses toward SE IL. The most active period for storms in our
area appears to be this evening, as the low level jet intensifies
ahead of the front just after sunset. The main storm axis may be
along I-72 at that point, then progress into SE Illinois after
midnight.

The airmass change behind the front should be delayed enough to keep
low temps in the low to mid 70s, with a few upper 60s possible
farther NW toward Galesburg.

&&

.LONG TERM...(Monday through Saturday)
ISSUED AT 325 AM CDT Sun Jul 24 2016

The slowing boundary moving through the Midwest Sun/Mon will linger
the precip through Monday, slowly retreating to along and south of
the I-70 corridor by the end of the day. High pressure building just
to the west will keep a slight northerly component to the winds.
Temperatures retreat into the 80s for the first part of the work
week, and reduced RH will keep heat indices in the 90s.  However,
the 850mb temps only varying a few degrees throughout the week. Once
the ridge passes Wed night, more southerly direction to the winds
sets up and temps may start to edge up again.  Concern with the
extended guidance at this point is that it is not handled well, and
the perpetual convective feedback at the end of the week is keeping
the guidance artificially low.  That being said, a warm and muggy
summertime airmass will keep plenty of instability in place for
mesoscale influences, and pops returning to the forecast in an
increasingly northwesterly flow over the building western ridge.
Latter half of the week may see rounds of thunderstorms as
shortwaves top the ridge yet again. Whether or not the convection
materializes enough to warrant the lowered guidance in max temps
remains a question.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1120 PM CDT Sat Jul 23 2016

We may see some patchy MVFR vsbys in fog/hz in the 08z-12z time
frame, but mainly VFR conditions are expected this forecast
period. Convection to our north looks to remain just north of the
BMI and CMI TAF sites, but will hold on to a VCTS mention in those
locations in case the storms track a bit further southeast than
currently projected. Any fog/low vsbys should burn off quickly in
the morning, after 13z, followed by some cumulus developing by
late morning. A cold front is expected to approach the area from
the northwest tomorrow night and that appears to be the best
chance for seeing some scattered TSRA but this far out and with
coverage still very much in question, will only go with a VCTS
late tomorrow afternoon into the evening hours. Light and variable
winds tonight will pick up a bit from a southerly direction on
Sunday with speeds in the 8 to 13 kt range.


&&

.ILX Watches/Warnings/Advisories...
Excessive Heat Warning until 7 PM CDT this evening for
ILZ040>057-061>063-066>068-071>073.

Excessive Heat Warning from 11 AM this morning to 7 PM CDT this
evening for ILZ027>031-036>038.

&&

$$

SHORT TERM...Shimon
LONG TERM...HJS
AVIATION...Smith



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