Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 230409
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1109 PM CDT Wed Mar 22 2017

.UPDATE...
Issued at 830 PM CDT Wed Mar 22 2017

A chilly evening across central Illinois as a large Canadian high
pressure system slowly edges away from the state. This large fair
weather system brought copious amounts of sunshine today but
rather cool temperatures to most of the area with afternoon highs
ranging from around 40 over in Danville to 51 degrees in Flora.
Clouds were on the increase across the region this evening as the
low level flow shifts into a southeasterly direction, which will
eventually bring much warmer air into the state late Thursday and
especially on Friday. With the low and mid level warm advection
pattern well underway just out to our west, we can expect the
clouds to continue to slowly increase overnight with a few showers
developing across Missouri by Thursday morning. Some of those
will edge into parts of west central Illinois during the morning,
but will encounter a very dry air mass as they track east of the
Mississippi River, but warrant at least low chance POPs during the
day Thursday.

Despite the gradual increase in high and mid level clouds,
temperatures have dropped off a bit quicker than earlier thought,
especially across the east where the clouds were not as thick.
However, the combination of the increase in cloud cover in the
east the rest of this evening along with the wind staying up just
enough overnight should keep the temperatures in check for the
rest of the night. Have made the usual tweaks to the early evening
temperatures, but other than that, no other adjustments were
needed to the evening grids that would require a ZFP update at
this time.

&&

.SHORT TERM...(This evening through Thursday)
ISSUED AT 315 PM CDT Wed Mar 22 2017

19z/2pm surface analysis shows 1039mb high centered over the Great
Lakes, with cool/dry northeasterly flow noted across central
Illinois.  As the high edges eastward, mid/high clouds currently
across Iowa/Missouri will gradually spread into the area tonight.
Skies will initially start out mostly clear during the early
evening, but will then become overcast overnight.  Low
temperatures will be coldest across the northeast KILX CWA around
Paris and Danville, where skies will remain clear longest and
readings will drop into the upper 20s.  The warmest lows in the
middle 30s will be focused across the southwest CWA where clouds
will increase soonest.

As the high moves further away from the region and boundary layer
winds veer more southerly, warm air advection will increase markedly
on Thursday.  Models continue to produce light QPF across parts of
the area: however, boundary layer airmass will initially be quite
dry, so think they may be too fast spreading precip eastward.  As
such, have kept locations east of I-55 dry through the morning
hours.  Have focused highest PoPs in the Illinois River Valley
Thursday afternoon, with dry conditions expected through the entire
day south of I-70.  High temperatures will top out in the lower to
middle 50s.

.LONG TERM...(Thursday night through Wednesday)
ISSUED AT 315 PM CDT Wed Mar 22 2017

Vigorous short-wave trough noted on latest water vapor imagery off
the coast of southern California will cross the Rockies, inducing
surface cyclogenesis over eastern Colorado Thursday night.  As has
been the case with the past several model runs, 12z Mar 22 models
continue to show the short-wave closing off at 500mb...resulting in
a slower eastward progression of the wave and its associated surface
low.  As a result, confidence is growing that any precip associated
with the slowly approaching system will hold off until Friday night.
Ahead of the wave, partial sunshine and southerly winds will help
boost high temperatures well into the 70s on Friday.  All models
suggest precip will arrive across the Illinois River Valley Friday
evening...then spread across the remainder of the area overnight.
GFS MUCAPE values remain negligible, so have opted to only mention
isolated thunder with the initial wave of precip.  As the system
comes overhead, MUCAPEs increase to 500-1000J/kg and mid-level lapse
rates steepen by Saturday afternoon, warranting an increased thunder
threat.  Will maintain high PoPs and thunder chances throughout the
day and into Saturday evening...before rain chances gradually
decrease from southwest to northeast across the area on Sunday.

After a brief lull in the rain chances Sunday night, the next
southern stream system will quickly spread showers back into the
area on Monday.  This feature has trended south with the past few
model runs, so have concentrated highest PoPs and isolated
thunder mention across the southern CWA.  After that,
considerable model spread exists for the end of the period...so
forecast confidence is low beyond Monday.  At this time, it
appears Tuesday will be a dry day before another system approaches
by the middle/end of next week.  Timing of that system is very
much in doubt, with the GFS delaying it until next Friday...while
the ECMWF/GEM are both more aggressive with its eastward progress
as early as Wednesday.  Will only carry slight chance PoPs for
Wednesday until better model agreement is achieved.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1105 PM CDT Wed Mar 22 2017

VFR conditions are expected this forecast period, although cigs
will drop to low VFR (3500-4000ft) tomorrow morning as isolated
showers track over west central IL. The threat for showers will
gradually work their way north during the afternoon and out of our
area by early evening. Coverage of any rain tomorrow looks too
isolated at this point to mention much more than VCSH. One the
showers move north of the area late tomorrow afternoon or early
evening, mainly high level cloud cover is expected across the
forecast area. LLWS potential will increase across the area after
03z as southerly winds increase to 45 kts at the 2000 foot level.

Surface winds tonight will remain out of the east at 8 to 15 kt
range with winds veering more into the southeast at 12 to 17 kts
with gusts around 20 kts at times tomorrow afternoon through
tomorrow evening.

&&

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

$$

UPDATE...Smith
SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Smith



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