Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 182012

Area Forecast Discussion
312 PM CDT Mon Aug 18 2014

.SHORT TERM...(This evening through Tuesday)

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


.LONG TERM...(Tuesday night through Monday)

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 18Z TAFS through 18Z Tuesday afternoon)

Lowest ceilings/visibilities have lifted from the earlier fog, but
rapid diurnal development resulting in MVFR ceilings at midday,
This will continue for a couple more hours before ceilings lift
above 3000 feet.

Main concern later in the period is with convective potential
overnight with a storm system moving through the upper Mississippi
Valley. Most of the 12Z models form a line of convection across
Iowa this evening as an upper wave drops southeast, but weaken the
line as it crosses the Mississippi River. Have included a PROB30
group at KPIA and KBMI to address potential MVFR visibilities with
the storms moving through northern Illinois, but currently feel
potential further south is still too uncertain to go more than the
existing VCTS.




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