Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 240159
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
859 PM CDT TUE AUG 23 2016

.UPDATE...
Issued at 845 PM CDT Tue Aug 23 2016

Low level moisture continues to increase across the region this
evening with surface dew points already into the low 70s over
extreme west central Illinois, while mid 60 degree dew points
were common over far eastern Illinois. 00z ILX sounding showing
quite a bit of difference in the moisture profile from what we
saw yesterday at this time with precipitable water values already
up to 1.70 inches with the SPC mesoanalysis depicting 2+" PWATs
stretching from southern IL west-northwest into parts of northern
Missouri. The better 850 theta-e advection/moisture transport is
still west of our area but is expected to shift over Illinois
late tonight/Wed. morning as the low level jet begins to veer
more into a southwest to west direction.

Both the RAP and HRRR have backed off considerably with respect
to timing of a complex of storms that was forecast to be in our
area starting around midnight. Now, the hi-res models have pushed
the start times back to between 08z-11z across most of the
forecast area. Starting to see the 925-850 mb winds starting to
increase from a southerly direction out in central Mo with the
better speed convergence noted over southern Iowa into northeast
Mo where convection is expected to increase later this evening.
With the better instability and shear located to our west, it
appears our main threat with any of the stronger storms will be
with torrential rainfall. Have pushed the timing of the showers
and storms back a few hours overnight, otherwise the rest of the
forecast is in good shape. Should have the update out soon.

&&

.SHORT TERM...(This evening through Wednesday)
ISSUED AT 300 PM CDT Tue Aug 23 2016

Developing storm system moving into the region from the west this
evening.  Relatively quiet this afternoon and early evening, with
increasing clouds ahead of the rain and potential thunderstorms
later tonight after midnight.  Towards morning, the models are
having trouble working out a consistent solution.  Plenty of
instability available in the lower levels in a warm and muggy
airmass, but how much severe weather will manifest will be
contingent on a diurnal component out far ahead of the synoptic
boundary.  So, tomorrow is currently in a marginal risk, with much
of the risk dependent on any clearing/holes in the cloud cover after
the first round of showers. NAM at least has some breaks in the
activity, but not enough cloud clearing.  If the clouds break,
daytime heating could result in some quick development of activity
again through the day.  Either way, PW values in the 1.5-2.0" range
will allow for some heavy rain threat under any of the more prolific
cells.

.LONG TERM...(Wednesday night through Tuesday)
ISSUED AT 300 PM CDT Tue Aug 23 2016

Wed evening looks to start out with an ongoing or developing storm
complex across central Illinois, due to the close proximity of an
approaching cold front from the W-NW. Areas north of I-72 will have
the better chances of storms, due to proximity of the 500mb
shortwave NW of our forecast area. Have trended with the highest
rain chances (80%) in our NW tier of counties, with likely 60-70%
chances down to the I-72 corridor. Severe potential will be marginal
Wed evening, but brief strong downbursts could occur due to precip
loading and subsequent storm tower collapse. PWAT values will remain
around 2" Wed eve, which would support locally heavy rainfall and
flooding of low lying or poor drainage areas.

The next wave of rain/storms beyond that should be tied to the
passage of the cold front on Thursday. The frontal progression will
be slow, due to the front becoming parallel with the upper flow.
However, there is agreement that the front will eventually move
south of our counties by Thursday evening. Due to variability in
timing and coverage of any particular waves of rain, we kept PoPs in
the chance category on Thursday, with diminishing chances NW of the
IL river Thursday afternoon.

Some cooler air is expected to arrive for Friday and Friday night,
under high pressure centered to the north of Illinois. The dry
period may be short-lived, as low pressure in the plains approaches
on Saturday. Warm frontal precip could advance as far east as I-57
Saturday afternoon. The warm front is forecast to stall out north of
Illinois, putting our area in a warm and humid summer-time air mass
for several days. Afternoon and evening instability params show
storms could develop each day from Sunday through Tuesday. Varying
intensity of the storms would be tied to any shortwaves that pass
during the more unstable periods over those days. PoPs were kept in
the 30-40% chance range for now, due to uncertainties.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 600 PM CDT Tue Aug 23 2016

Conditions will continue to slowly deteriorate overnight as low
level moisture increases from the south helping to fuel showers
and scattered thunderstorms across the forecast area, especially
after 05z. Already have seen a band of MVFR cigs track north into
our TAF sites roughly along and west of a TAZ to PIA line with
the last few runs of the HRRR model suggesting DEC and BMI may see
at least some tempo MVFR cigs at times early this evening. As the
complex of showers and storms increases in coverage later tonight
we expect cigs at all sites to eventually drop to MVFR and
possibly some tempo IFR cigs in the heavier precipitation aftr
08z. MVFR cigs should gradually improve to VFR after 16z as the
area of showers and storms shifts off to our east and north.
Surface winds will be out of a southerly direction tonight at
around 10 kts, and south to southwest at 10 to 15 kts late Wed
morning through the afternoon hours.


&&

.ILX Watches/Warnings/Advisories...
NONE.
&&

$$

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



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