Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 181749
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
1149 AM CST Thu Dec 18 2014

.UPDATE...
ISSUED AT 1037 AM CST THU DEC 18 2014

16z/10am water vapor imagery shows a weakening short-wave over
northern Missouri. This feature has been responsible for an area of
light snow across mainly west-central Illinois this morning. Latest
radar loops continue to show the snow spreading northeastward:
however, surface obs indicate it has only reached a Macomb...to
Lincoln...to Decatur line. All models suggest the snow will
quickly taper off to flurries by early afternoon as the upper wave
weakens further and the precip struggles to develop/spread into
the dry airmass across north-central and eastern Illinois. Have
already made updates to the PoPs, mainly to sharpen the NE/SW
gradient across the area. Am maintaining dry conditions
along/north of I-74, but going with categorical PoPs just S/SW of
there. So far reported amounts have been quite light, with an
observer in Taylorville measuring 1/2 inch. Will continue to
forecast less than 1 inch across the W/SW CWA. Cold and dry
conditions will return by mid to late afternoon.

&&

.SHORT TERM...(Today)
ISSUED AT 331 AM CST THU DEC 18 2014

An elongated high pressure ridge remains over IL this morning while
a shortwave is approaching from the west, moving into MO at this
time. The shortwave will weaken considerably today as it moves into
the ridge over Illinois. The result will be a good chance of
precipitation west of Springfield and I-55, but lower chances to the
northeast of Springfield. Only a slight chance as far northeast as
Galesburg to Decatur to Lawrenceville. Precipitation will be in the
form of snow given the depth of moisture extending well above the
-10 Celsius level, although some drizzle could develop during
periods where showers are less active. Snowfall amounts should be
quite light with this weakening system, reaching only up to around
one half inch near Jacksonville. Temperatures should only reach to
around the freezing mark today throughout central IL, with light and
variable to light northwest winds.

&&

.LONG TERM...(Tonight through Wednesday)
ISSUED AT 331 AM CST THU DEC 18 2014

Shortwave currently swinging through northern Baja California on
track to reach southern Texas by Friday afternoon, where the
associated surface storm system will be developing. The models have
been continuing their trend of pushing this system further south of
our area, with only the GFS producing some flurries as far north as
I-70. Have thus removed the remaining snow mention over the
southeast CWA Friday night and Saturday morning.

Primary focus is thus with the active pattern setting up ahead of
the Christmas travel rush next week. A broad upper trough will be
digging across the Plains Sunday and Monday. Strong jet stream of
120-160 knots will be coming down from the Pacific Northwest toward
the southern Plains early next week, helping to develop a closed low
over the northern Plains and a strong surface cyclone under it. Bulk
of the precipitation into Tuesday morning looks to be all rain, with
some snow mixed in at times in the far north. Complicated part comes
behind the cold front passage on Tuesday. Given the strength of the
system, the surface and upper low will be slow to move out,
lingering over northern Wisconsin or the Upper Peninsula through
most of Christmas Eve. The core of the heavier precipitation will be
east of our area by evening, but precipitation will wrap around the
low through this period. The GFS is faster in bringing in the colder
air, with 850 mb temperatures down to -10C by Wednesday morning,
while the ECMWF is a couple degrees warmer. Rain will transition to
snow most areas Tuesday night, although a mix may persist south of
I-70 late into the night. Have kept snow in most areas on Wednesday,
although a rain/snow mix again possible across the south, although
obviously some adjustments may later be needed with the system that
far out.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
ISSUED AT 1149 AM CST THU DEC 18 2014

The widespread light snow from earlier this morning has now
diminished to scattered snow-showers, primarily along and south of
a KSPI...to KDEC...to KCMI line. Will carry MVFR ceilings and -SN
at those 3 terminals through 20z accordingly. Further north, VFR
conditions will prevail at both KPIA and KBMI through the day.
Once the light snow completely ends, clouds will persist across
the entire area through Friday morning. NAM RH profiles show IFR
ceilings remaining in place across southern Iowa into Missouri
through the period, while MVFR/VFR ceilings prevail further east
across central Illinois. Winds will remain light/variable beneath
the surface ridge axis through 18z Fri.

&&

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

$$

UPDATE...Barnes
SHORT TERM...Onton
LONG TERM...Geelhart
AVIATION...Barnes





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