Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 111112
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
512 AM CST Sun Dec 11 2016

.SHORT TERM...(Today and tonight)
ISSUED AT 217 AM CST Sun Dec 11 2016

Challenging forecast this morning with respect to thermal profile
today and resultant precipitation type forecast.

In a lull at present across most of forecast area as area of
frontogenesis pushes northeast of area ending pcpn temporarily.
Weak shortwave over Rockies will push into the plains this morning
with resultant surface low developing along the central high plains
and then tracking into the Mid-Mississippi valley today. Track of
sfc system will have a significant impact on precip type forecast
today.

Strong warm advection already occuring across area ahead of system.
However, dew points are still well below freezing even in areas
where the surface temperature has risen to freezing or above. As
precipitation moves back into the area later this morning under
strong isentropic lift, temperatures will likely fall back below
freezing as column saturates and temp approach wet bulb temperature.
As warm advection continues, precipitation should transition from a
mix to mostly rain from I-74 southward by early this afternoon.
Changeover is expected to stall over north central Illinois as the
weak shortwave moves by and the associated cold front moves across
the area. The cold air behind the front may change the precipitation
briefly back to snow before ending this evening in areas that saw
rain this afternoon.

Models differ on how far north the changeover will get with NAM
suggesting that Peoria and Bloomington may be the northern extent
with the HRRR bringing rain all the way north of the forecast area
by mid-afternoon. Previous package general split the difference
between the warmer and colder profiles and will continue that
approach until the trend becomes more evident.

In the area where the transition does not completely changeover,
snow accumulation will be capable of reaching several inches and
given the added potential for freezing rain/sleet, the location of
the current advisory in Stark and Marshall Counties still looks
reasonable. We will have to watch closely areas to the south where a
brief period of freezing precipitation may be possible and may
warrent an advisory for icy roads. Any QPF of freezing rain will
likely be quite light, but it doesn`t take much to produce a thin
layer of ice and impact driving conditions. Given the uncertainty of
the evolution today will hold off on extending the advisory at this
time and monitor closely.

.LONG TERM...(Monday through Saturday)
ISSUED AT 217 AM CST Sun Dec 11 2016

Main focus in this part of the forecast is with the next shots of
Arctic air, as well as precipitation trends toward the latter part
of the week.

Expansive area of Arctic air covers most of Canada early this
morning (850 mb temperatures generally below -20C), and lobes of
this air will be shoved southward this week as a strong 500 mb low
drops south out of the high Canadian latitudes. Greatest thrust of
this bitter cold for us will be swinging through later Wednesday
with 850 mb temps down into the -18C range, although a deeper
thermal trough will be digging southward across the Plains next
weekend. Fairly large temperature spread expected over the CWA on
Tuesday, ranging from the teens over the northern CWA to the mid 30s
south of I-70, but teens/20s will dominate everywhere Wednesday-
Thursday.

Latest models continue to weaken the fast-moving wave mid week, with
only the Canadian model still showing some semblance of the
precipitation streak. Have lowered PoP`s down to 20% Wednesday
morning, but would not be surprised to see them disappear in the
next package. Thus, the major focus will be with the late week
system.

Latest ECMWF/GFS dig a trough along the West Coast Thursday night
and early Friday, which will shift our flow out of the southwest
U.S. for a time. While both models spread precipitation over the
area on Friday, the low track Friday night and Saturday has shifted
more northwest, which would draw warmer air northward and result in
precip type issues. Not going to get cute that far out and will
largely go with snow for now, with a rain/snow mix south of I-70.
Speed differences in this system are resulting in a wide temperature
range as well (ECMWF for Springfield has a high in the upper 20s on
Saturday, while the GFS is close to 50), which is another reason for
remaining more on the conservative side.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 502 AM CST Sun Dec 11 2016

Precipitation expected to develop by mid-morning across central
Illinois terminals. Although temps are above freezing currently at
KSPI and KDEC, dew point depression remain 10-15F in those areas
and when precipitation begins, temperatures will likely fall below
freezing. Expect precip to start off as snow at most locations
this morning and then transition to rain from south to north as
warm advection on south winds continue. Once precip stops, IFR
Cigs are likely. Winds should keep low-level moisture in the form
of stratus rather than fog.

Probably will be a brief period of freezing precip or sleet
during the transition and will try to cover that with tempo
groups.

Winds will shift around to northwest overnight with gradual
clearing toward the end of the TAF terminal period.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight CST tonight for ILZ028-
030.

&&

$$

SHORT TERM...Barker
LONG TERM...Geelhart
AVIATION...Barker


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