Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 211449

Area Forecast Discussion
National Weather Service Lincoln IL
949 AM CDT WED SEP 21 2016

Issued at 936 AM CDT Wed Sep 21 2016

Convective issues causing a challenging forecast for today.
Outflow boundary, from the earlier line of storms which passed
through Chicago, is currently approaching Galesburg, Peoria and
Bloomington from the northeast. This caused the line to weaken
rather quickly as it began to outrun the line. However, convection
from Wisconsin has now settled into far northern Illinois in an
west-east configuration in the border counties. HRRR has been
struggling to handle the storms over the last several hours, and
has this secondary line weakening as well before reaching us.
Forecast was updated earlier to add some 20-30% PoP`s across the
far northern CWA for a few hours this morning, which appears
sufficient for now. Will monitor the secondary line for a couple
hours to see if any additional PoP`s need to be added for midday
or afternoon.

Temperatures will also be a challenge as well, between the
outflows and the cirrus shield from the decaying MCS to our north.
Already up to 80 degrees across the far southwest CWA, which is
in line with the previous forecast. Have made some adjustments to
the hourly trends across the north to reflect the outflows and
increased cirrus, but the remainder of the forecast area still
expected to get to near 90.


.SHORT TERM...(Today and tonight)
ISSUED AT 325 AM CDT Wed Sep 21 2016

The recent stretch of unseasonably warm weather will persist across
central and southeast Illinois today (and several more days
actually). These warm conditions will be supported by persistent
southerly low-level flow and the continued building locally of a
flat upper-level ridge. Based on yesterday`s high temperatures and
minimal airmass change, boosted today`s highs into the 90s areawide.
The airmass has become more humid over the past 24 hours and may
not heat quite as quickly today, suggesting going even warmer for
highs today would not be prudent.

Aside from the heat, only other concern of note this period is the
threat for showers/storms across the north. A storm complex is
currently chugging across the upper Midwest, and it has shown some
tendency to turn toward the right into the richer
moisture/instability. Some guidance suggests outflow from this
complex may fire off a few storms across our far northern counties
later today or this evening. At this point, think it will be a close
call, but favor keeping forecast dry with any activity staying just
north of the area. Convective trends will need to be monitored
closely today in any event.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 325 AM CDT Wed Sep 21 2016

Forecast remains unseasonably warm and dry for the rest of the week
in Central Illinois.  Temps in the mid to upper 80s through the
weekend.  With the persistent delaying of onset of precip with the
next system...think the guidance is actually going a little low on
the max temps this weekend and think the forecast will trend to near
90 in some locations for Fri/Sat.

Main issue with the forecast remains the trof that moves into the
west coast on late Thu/Fri.  The models have been modulating back
and forth btwn phasing the wave into the northerly synoptic stream
and producing a more cut off low over the SW, delaying the onset of
precip to the Midwest.  Latest GFS and ECMWF runs are more of a
hybrid, progressing a weakly phased wave across the Plains, and
almost stalling in the mid Miss River Valley for late in the
weekend/beginning of next week.  As a result, the pops start up Sun
night and continue through at least Tuesday at this point.  With the
models struggling with the pattern shift overall, pops are confined
to 20-30 percent for now.  After the system arrives Monday...temps
cool off considerably with the trof.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 635 AM CDT Wed Sep 21 2016

Generally quiet/VFR aviation weather conditions expected across
the central Illinois terminals through the 12Z TAF valid time. The
only potential exception would be if some convection manages to
fire across parts of the area along an approaching outflow
boundary from northern Illinois. This would mainly be a threat at
KPIA, KBMI, or KCMI, but feel chances of TSRA are too low to
mention at this time.


.ILX Watches/Warnings/Advisories...


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