Area Forecast Discussion
Issued by NWS Central Illinois

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804
FXUS63 KILX 170530
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1230 AM CDT Thu Aug 17 2017

.UPDATE...
Issued at 946 PM CDT Wed Aug 16 2017

Updated forecast, primarily for the latest shower/storm trends. A
fairly unorganized area of showers/storms has spread/is spreading
across mainly the southern half of the forecast area this evening.
This convection was well ahead of the approaching cold front,
which currently extends from low pressure over southern Minnesota
into the central Plains. A line of storms exists closer to the
frontal boundary, and this line is likely to impact at least a
portion of the area into the overnight hours. However, this line
is not handled terribly well by current guidance, and confidence
in its evolution is not great. A general weakening trend seems
most likely since diurnal heating is waning, and the storms will
be moving into a more stable environment in general.

&&

.SHORT TERM...(This evening through Thursday)
ISSUED AT 230 PM CDT Wed Aug 16 2017

Moisture channel imagery depicts the upper wave rotating to the
northeast into central NE this afternoon. Diffluence ahead of
wave seen firing convection over MO in the frontal convergence.
Good deep moisture convergence along front over MO with MUCAPE
ahead of the MO line around 3000. Less instability over IL today
due to cloud cover and lower moisture values than to the west,
with less shear also noted than to the west associated in area of
front.

Short range models have been very inconsistent today in the
convective development trends and so has been difficult to come up
with timing on any significant development or any rain free
periods. Therefore expect the trend to continue into the evening
of scattered showers and storms developing over parts of the
warm sector over eastern MO and central IL, with no well defined
significant complex. Models do finally agree on convective line or
complex moving into IL during the late evening hours, but timing
differences are still seen. Have tried to use blend of HRRR, NAM
3km and ARW to get a forecast of pops and for TAFs in the timing
on the main pcpn to move through late this evening and overnight.

.LONG TERM...(Thursday night through Wednesday)
ISSUED AT 230 PM CDT Wed Aug 16 2017

By Thu night, upper low closed off in Great lakes region and
frontal zone has moved through. This will push pcpn east of area
and some drier air into area. Weak short wave shows up in all
operational models, sliding down back side of upper trof and
bringing quick chance of more pcpn into central IL Fri night.

Then next short wave rotates down into area Mon night into Tue
night, bringing another chance of pcpn with it. This system drags
another front into the area and so a better chance of rain will be
the case with this system. Temperatures through the period will
remain generally in the normal for mid August.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1230 AM CDT Thu Aug 17 2017

Area of showers and isolated thunderstorms is advancing into
western parts of central Illinois, and should soon be at KSPI,
reaching the other TAF sites over the next couple hours. Bulk of
the rain appears to be in the period through about 10-11Z, and
isolated MVFR ceilings are expected to become more widespread in
the wake of the rain. A cold front will begin moving into central
Illinois late morning, resulting in improving ceilings from west
to east, as well as a period of gusty southwest winds in the
afternoon. These winds should subside toward 22-23Z.

&&

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

$$

UPDATE...Bak
SHORT TERM...Goetsch
LONG TERM...Goetsch
AVIATION...Geelhart



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