Area Forecast Discussion
Issued by NWS Central Illinois

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KILX 241801

Area Forecast Discussion
National Weather Service Lincoln IL
101 PM CDT SUN JUL 24 2016

Issued at 1045 AM CDT Sun Jul 24 2016

Updated the forecast to remove the patchy fog wording in southeast
IL this morning. Also nudged dewpoints up a tad today as dewpoints
currently 75-81F, similar to this past Thu levels. 10 am temps
were 84-87F while heat indices were 93-104F. Highs still expected
to be 90-95F this afternoon combined with high dewpoints will give
peak heat indices of 102-112F similar to Thu levels. Updated NPW
product to continue the excessive heat warning across CWA through
7 pm today as heat indices do not slip below 100F until between
730-8 pm. Most high resolution models keep CWA pretty dry through
7 pm today though a few members bring some qpf north of I-72 late
this afternoon, as cold front over central parts of IA/WI tracks
southeast into NW IL. SPC continues marginal risk of severe storms
as far southeast as I-70 for mainly tonight period and primarily
for strong gusty winds.


.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

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 18Z TAFS through 18Z Monday afternoon)
Issued at 1259 PM CDT Sun Jul 24 2016

A cool front over central WI and southeast IA into nw MO will
track southeast across central IL late this evening and early
overnight. A band of showers and thunderstorm is expected to
develop ahead of this front late this afternoon and evening over
central IL, reaching PIA and BMI between 23-24Z and to I-72
between 00Z-01Z. Have VCTS for a few hours early this evening due
to uncertainty of coverage, though latest HRRR model showing
better cover over BMI and CMI around 00Z/7 pm. Carried VCSH late
this evening and early overnight until frontal passage to the
south of I-72 by 08-09Z. MVFR conditions are briefly possible with
thunderstorms. Fog formation later tonight/early Monday morning
looks mainly south of I-70 but did carry vsbys 5-6 miles from
09Z-14Z. Breezier SSW winds 10-13 kts and gusts of 16-20 kts this
afternoon to diminish under 10 kts after sunset. Winds to switch
NW after frontal passage and be northerly 5-10 kts on Monday


.ILX Watches/Warnings/Advisories...
Excessive Heat Warning until 7 PM CDT this evening for



AVIATION...07 is the U.S. government's official web portal to all federal, state and local government web resources and services.