Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 190452
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
1152 PM CDT Mon Aug 18 2014

.UPDATE...
ISSUED AT 900 PM CDT MON AUG 18 2014

Skies have become mostly clear across central and SE Illinois this
evening as diurnal heating ends. Although a few pockets of light
fog are possible early tonight, the next main forecast feature
will be the chance for thunderstorms overnight as an upper level
shortwave and cold front move into central IL. Have trimmed back
POPs before midnight as current radar and satellite indicate
storms are still several hours away. HRRR model simulated
reflectivity has been depicting movement of the system well in
northern IA, southern MN, southwest WI this evening and has been
used as a guide for timing through the first few hours of Tuesday.
Currently the system is associated with a slight risk (15% +) of
severe winds and hail to our northwest, but as it approaches
central IL, CAPE of 3000 J/kg should be diminishing to around 1500
or less with marginal 0-6 km bulk shear around 25-30 kts. This is
enough for only a 5% severe risk from the Illinois River
northwest. To the south, risk of severe storms is negligible, but
at least a slight chance for storms will reach as far as SE
Illinois by early morning. Have sent updates for
POP/Temp/Humidity/Wind trends this evening/early tonight.

&&

.SHORT TERM...(This evening through Tuesday)
ISSUED AT 311 PM CDT MON AUG 18 2014

Earlier stratus has given way to diurnal type clouds over central
and southeast Illinois early this afternoon, underneath a weak high
pressure axis. Leading edge of shortwave currently seen on water
vapor imagery pushing through Minnesota and northern Iowa, with a
more potent vorticity maximum moving out of North Dakota.

Convection ahead of the shortwave expected to begin over the next
couple hours over northern Iowa, where skies have been sunny and
CAPE`s are over 3000 J/kg per SPC mesoanalysis. High-resolution
models pushing the first wave of storms into Wisconsin with another
line forming in southern Minnesota/northern Iowa later this evening.
The second line is more of a concern for us as it will have a
southeast trajectory. However, most of the models show some fading
of the line with more of the energy staying to our north, and a
separate convective cluster developing over northwest Missouri
overnight. Have adjusted the PoP`s in our area to be primarily after
midnight, and largely eliminated them south of I-70.

Associated surface boundary expected to push into central Illinois
Tuesday afternoon as the upper wave arrives. Some concerns with
severe weather potential across east central and southeast Illinois,
as development takes place early afternoon and bulk shear increases
during the peak heating time. Latest Day2 SPC convective outlook
indicates a severe threat southeast of a Danville to Effingham line,
with NAM indicating a 2 pm to 7 pm time frame being the most
concern.

&&

.LONG TERM...(Tuesday night through Monday)
ISSUED AT 311 PM CDT MON AUG 18 2014

Tuesday evening will see decreasing storm chances from north to
south as a shortwave in the upper level trough over the Great Lakes
advances east of IL. We are in the day 2 slight risk in our
southeast counties, so we can`t rule out some lingering strong
storms into early evening, but the main threat should advance
southeast of IL by mid evening.

Additional storm chances re-develop across our far S-SW counties late
Tue night as a 300mb 75kt jet max in the back side of the trough
generates some lift in the left exit region. Minor rain/storm
chances will continue on Wednesday as the jet max progresses
eastward over Illinois, but only slight chances are expected.

Wednesday night, a warm front developing across our northern
counties oriented from NW to SE will become a focus for a round of
storms, with chances increasing to likely after midnight along and
north of I-74. Storms should progress northward with the warm front
on Thursday. However, storms chances are expected to linger north of
a line from Peoria to Paris Thursday and Thursday night.

Height rises and mid-level warming on Friday and Saturday should
help to keep provide a capping layer, and reduce storm chances
across central IL. The may be an isolated late afternoon or early
evening flare-up of a storm, but most areas should stay dry.
Rain-free conditions are forecast to continue into Sunday by the
ECMWF and GFS. They both show a cold front arriving later Sunday
night and wash-out as it passes across IL on Monday. The weakening
front is not expected to trigger much in the way of storms at this
point, but confidence is low on that scenario staying dry.

Heat and humidity will definitely dominate the last half of this
week, as highs climb into the upper 80s and lower 90s, with heat
index readings in the mid to upper 90s. Some relief will come on
Monday behind the cold front, with low to mid 80s for highs and
lowering dewpoints into the 60s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
ISSUED AT 1152 PM CDT MON AUG 18 2014

Patchy fog resulting in VLIFR to MVFR conditions in place from
around K1H2-KAAA-KCMI eastward, with a line of thunderstorms
approaching northern IL from the northwest. Fog expected to
dissipate as light SSW winds ahead of the line of thunderstorms
promotes mixing. This line expected to weaken as it approaches and
mainly affect TAF sites from KPIA and KBMI northward. Have
included VCTS in central IL TAFs to address potential for
thunderstorms, but likelihood of MVFR conditions looks too low for
explicit mention at this time. A break in shower/thunderstorm
activity likely in the morning, then an upper level shortwave and
daytime heating should produce a re- intensification of storms
along the cold frontal boundary lingering over central Illinois.
Have brought VCTS and ceilings near MVFR thresholds for the
remainder of the afternoon to account for this. Starting around
00Z, conditions expected to improve as frontal boundary and upper
level shortwave moves east of the area.

&&

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

$$

UPDATE...ONTON
SHORT TERM...GEELHART
LONG TERM...SHIMON
AVIATION...ONTON





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