Area Forecast Discussion
Issued by NWS Central Illinois

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930
FXUS63 KILX 030526
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1126 PM CST Fri Dec 2 2016

.UPDATE...
Issued at 831 PM CST Fri Dec 2 2016

Persistent cloud cover continues to affect central and southeast
IL to near the southern forecast area boundary and looks like it
will have little change in coverage overnight. This will also keep
temperatures somewhat mild overnight with lows generally near the
freezing mark or possible to around 30 toward the less cloudy
areas on the southern periphery from Springfield to Lawrenceville
southward. Small tweaks to short term temperature and cloud cover
trends tonight but otherwise not much in the way of updates
needed.

&&

.SHORT TERM...(This evening through Saturday)
ISSUED AT 256 PM CST Fri Dec 2 2016

Southern extent of the persistent stratocumulus deck continues to
hug the southern edges of the forecast area this afternoon, although
some thin spots have developed as far north as Havana. This edge has
started to look more diurnal in nature, and skies from about
Jacksonville-Lawrenceville will be partly cloudy for a time early
this evening, before the clouds start to ooze back southward and
cirrus streams in from Missouri. While the lower deck should finally
retreat on Saturday morning, the high clouds will quickly replace
it, so mostly cloudy skies will continue to be the net result.

Went a few degrees above MOS guidance for tonight underneath the
cloud cover, with lows mainly in the lower 30s. Highs tomorrow
should not be too far off of what we have seen today, upper 30s-
lower 40s.

.LONG TERM...(Saturday night through Friday)
ISSUED AT 256 PM CST Fri Dec 2 2016

An active weather pattern is on tap for the extended forecast, with
3 waves of precipitation possible. There are indications in the 12z
models for diminished precip amounts in our forecast area for the
second and third systems, mainly due to a southward shift in the
forecast tracks of the best forcing for precipitation. Continued
trends for much colder conditions starting the middle of next week.

Clouds will generally dominate the skies from Saturday through next
Friday, with brief periods of sunshine possible Monday morning and
then next Friday.

Saturday night, a fast moving 500mb trough and inverted surface
trough will progress toward Illinois. Clouds will increase during
the evening, with precipitation developing after midnight. Low temps
will bottom out in the low to mid 30s, which will be conducive for
snow to start earlier on in the event. Snow is primarily expected
north of a line from Rushville to Effingham, with a rain/snow mix
south of there. Based on the amount of dry air below the cloud
bases, it will take a while for precip to reach the ground.
Therefore, we limited likely PoPs to just our far western areas from
Rushville to Galesburg late Sat night, with chance PoPs as far
east at Champaign to Lawrenceville. Sunday morning, precip chances
ramp up to Likely and Categorical /60-90%/ for areas north of
I-70. Snow accumulation early Sunday could reach up to an inch,
and possibly 2" near Galesburg, before warmer air arrives from the
south and changes the snow to rain. Precipitation will be ending
Sunday afternoon from west to east, with colder air returning and
snow possibly mixing in again at times before precip ends. Any
snow accumulations that occur from this system will be highest on
grassy surfaces. The warmer roads and ground conditions will
reduce the time any snow remains on the ground, steadily
diminishing snow amounts. In areas where snow changes to rain,
snow amounts will diminish as well.

Sunday night and Monday will feature the passage of a surface high
pressure center directly over Illinois. Some clearing of the skies
Monday morning should help high temps climb into the mid 40s in most
areas, with some upper 40s possible south of I-70. Clouds will begin
to fill in again as low pressure approaches from the south. A
surface low is projected to move from Louisiana to the northeast,
progressing just south of the Ohio River. That path is a
southward shift from previous model runs, which would limit the
amount of precip that occurs in central Illinois. Areas northwest
of I-55 would see the biggest reduction in precip, with areas
southeast of I-55 still expected to see some rain Monday night
into Tuesday morning. If the southward trends conditions, even our
southeastern counties may not see much in the way of rain. Have
trended downward with PoPs, but the blended initialization still
contained an area of Likely PoPs southeast of Mattoon late Monday
night and Tuesday morning. Thermal profiles support mainly rain
for this event, with low chances of snow north of I-72. Little
snow accum is expected in central Illinois, due to the southward
shift of the system.

A break in the precip is projected for Tuesday night and Wed
morning, under weak high pressure. However, the third system will
take aim on Illinois Wed afternoon and Wed night. There are large
model differences to the surface pressure pattern with that system.
The ECMWF develops a more prominent surface wave in Arkansas Wed
evening and lifts it quickly to the northeast south of the Ohio
River Thursday morning. The Canadian brings a 500mb shortwave across
Illinois with little surface reflection, generating a period of
precipitation Wed aftn and evening. The 12z GFS is much weaker with
any upper level and surface features, and now keeps our area dry
through this event. We trended lower with PoPs, but the consensus
kept chance PoPs in a large portion of central Illinois. Thermal
profiles support snow in a majority of our forecast area, with rain
mixing in Wed aftn and early evening as far north as I-72. Snow
amounts are in question, due to uncertainties in available
moisture, but accums could reach up to an inch or possibly 2 based
on the ECMWF, especially on grassy areas.

Much colder air will arrive in the wake of the mid week system,
causing high temps to be limited to the 20s on Thursday, with lows
dipping into the 12 to 17F range Thursday night. Despite
increasing sunshine on Friday, highs will remain only in the mid
to upper 20s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1125 PM CST Fri Dec 2 2016

Ceilings will continue to hover near MVFR/VFR thresholds through
much of the night with the southern edge of the cloud shield near
to just SW of KSPI. Based on latest trends in ceiling
height...conditions should remain predominantly VFR although some
localized MVFR possible. The low cloud shield should pull
northeast with the departing low Saturday morning. Winds N up to
8 kts overnight becoming light and variable by Saturday morning as
high pressure moves over the area.

&&

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

&&

$$

UPDATE...37
SHORT TERM...Geelhart
LONG TERM...Shimon
AVIATION...37



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