Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 260532
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1132 PM CST Sat Feb 25 2017

.UPDATE...
Issued at 830 PM CST Sat Feb 25 2017

While tonight`s forecast is a quiet one overall, the low
temperature forecast will be tricky. Winds have died off in most
areas with a ridge of high pressure building across the area, the
airmass is dry, and there is only thin high clouds across the
area. This scenario is generally good for a rapid nighttime temperature
drop. Current temperatures were only a couple degrees from
forecast lows in many areas, so have updated forecast to drop
overnight lows another couple degrees. However, the surface
ridging will quickly shift to the east overnight, with winds
trending southerly before morning. Also, an upper-level
disturbance tracking across the Plains will spread thicker clouds
into the area before morning. The southerly winds and increasing
clouds should stop the temperature fall at some point overnight.
Bottom line...overnight lows should be reached within the next few
hours, with temperatures becoming steady or rising before sunrise.
Other than the update to the overnight lows and hourly temperature
trends, only minor tweaks were needed to the going nighttime
forecast.

&&

.SHORT TERM...(This evening through Sunday)
ISSUED AT 304 PM CST Sat Feb 25 2017

High pressure will build into the region as the most recent storm
system slowly moves out of the Great Lakes.  Overnight, little more
than cirrus anticipated, and temperatures will be chilly again
tonight, dropping into the 20s.  The overnight lows are more
seasonal for the end of February, but the return of southerly flow
at the surface on Sunday will bring back a brief period of warm air
advection. Sunday, the max temps will move into the upper 40s/near
50, although the clouds will increase as the day progresses.

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 304 PM CST Sat Feb 25 2017

Our upper flow will become southwesterly once again as we head into
the new work week which will result in unseasonably mild air
streaming back north into the Midwest starting on Monday. In the
developing southwest flow aloft, a low amplitude shortwave will
shift across the southern Plains and bring some light rain chances
to southeast Illinois Sunday night. We may see a mix of rain and
snow late Sunday evening into the overnight hours, but if that
occurs, we are not looking for any appreciable snowfall on the
northern periphery of the precipitation shield.

A lead shortwave will eject quickly out of the developing longwave
trof and track towards our area late Monday night into early
Tuesday, which will bring a round of showers to central Illinois
with most areas having high chance to likely POPs with this
initial wave. A more significant precipitation event still looks
to be shaping up late Tuesday afternoon into Wednesday as a
frontal boundary edges into parts of central Illinois late Tuesday
night into Wednesday. As a stronger upper level wave ejects out
of the southwest U.S. late Tuesday several waves of low pressure
are forecast to develop along the slow moving cold front which
should be approaching our area. Showers and scattered
thunderstorms will start develop across parts of west central
Illinois later Tuesday and become more widespread Tuesday night
into Wednesday as the initial surface wave moves along the
boundary enhancing the low level convergence across central
Illinois. With most of our area in the warm sector ahead of the
system, GFS MUCAPE values increase to between 500 and 1000 J/kg in
the afternoon, especially south of I-72. Looks like the potential
for some locally heavy downpours with any thunderstorms over parts
of south central through southeast Illinois where Precipitable
Water values will be over 1.00 inch.

Medium range models are in decent agreement, although a slower trend
was noted on the 12z ECMWF and GFS, in pushing a cold front across
the forecast area early Wednesday, ahead of the significant upper
trough edging towards the Midwest. As the last of the surface
waves tracks across east central Illinois into Indiana, models
were suggesting a band of light precipitation would move across
the area Wednesday afternoon and evening with the north and
northwest edge of this rain shield possibly turning over to light
snow before ending. Still quite a bit of model uncertainty with
this feature so will just continue with a rain/snow mix or just
chance for light snow across our far northern areas Wednesday
evening.

The slow moving trof will shift east of our area on Thursday with
our flow becoming northwesterly bringing in another brief spell
of cooler weather. Embedded in the deep northwesterly flow will be
several low amplitude shortwaves, one of which will track across
the lower Great Lakes and bring low chance POPs for rain later
Thursday and possibly some rain and snow Thursday evening before
the wave quickly moves away by Friday morning. Temperatures back
to normal for the end of the work week before we see above normal
temperatures return next weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1129 PM CST Sat Feb 25 2017

VFR conditions will prevail across the central Illinois terminals
through the 06Z TAF valid time. Mid-level CIGs will be common as
an upper-level disturbance moves through the area, but heights
will be well into the VFR category. Light westerly winds will
trend southerly by daybreak, becoming gusty at times for much of
the daytime hours.

&&

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

$$

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


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