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Area Forecast Discussion
1128 AM CST Sun Jan 25 2015


Surface low located just northwest of St. Louis late this morning.
Widespread precipitation continues over our northern CWA in the
deformation zone, with dual-pol radar data from showing the
rain/snow line roughly along a Canton to Lexington line, although
some mixed precipitation is showing up in the latest observation
from Champaign. Freezing level will continue to lower as the low
tracks toward Carbondale by midday, bringing the rain/snow line
closer to Springfield by early afternoon, where temperatures are
still currently in the lower 40s. 

Will need to watch snow rates across the north closely. Already
have had 1 inch reported just northeast of Galesburg where light
to moderate snow continues. NOAA Satellite Analysis Branch shows
this area on the periphery of an area of enhanced snow production
over the next few hours, although drier air is being advected in
from the northeast where dew points in north central Illinois are
down to the low-mid 20s. 

Have sent some updated zones/grids to update the precipitation and
transition trends, and to tweak the hourly temperatures especially
across the northern CWA.


.SHORT TERM...(Today)

3am surface analysis has low pressure centered over southern Iowa 
with the latest 3 hourly surface pressure falls suggesting a 
continued southeast movement to the low this morning. Not a lot
of cold air associated with this particular storm system as surface
temperatures across Iowa, southern Minnesota and southeast South
Dakota were still above freezing after the low shifted away from
there area. 

By 12z, the low is forecast by most models to be just west-southwest 
of Quincy. Temperatures ahead of the low continue to slowly rise 
early this morning with readings ranging from the mid 30s over far 
eastern Illinois to the middle 40s over southwest Illinois. With the 
low expected to push to near Lawrenceville by late this afternoon, 
we expect the warmest temperatures across the north this morning 
with steady or slowly falling temperatures this afternoon as winds 
turn into the northeast and increase to between 15 and 25 mph with 
gusts up to 30 mph at times. 

The high resolution short term forecast soundings continue to show 
temperatures at the surface too warm to support much in the way of 
snow until later this morning across the north, and not until later 
this afternoon over central through east central Illinois. Further 
south, any changeover looks to hold off until the low shifts to
the south of the area this evening. Am a bit concerned with our 
northern counties...roughly along and north of I-74...later this
morning and especially this afternoon when the low shifts further
to our southwest, surface winds up to 900 mb switch into the northeast
with soundings showing 30-40 kts around 900 mb. That may be enough
to bring surface temps down sufficiently for a brief period of time
for a period of moderate snowfall before soundings indicate we lose
the ice crystals in the upper layers of the clouds. However, except
for the NAM, which once again appears to have initialized too cold
in the lower levels of the atmosphere, all other forecast soundings
indicate surface temperatures should stay above freezing during the
daylight hours. Highest totals, in the 1 to 2 inch range will be 
along and north of the I-74 corridor with lighter accumulations
to the south.


.LONG TERM...(Tonight through Saturday)

Surface low pressure between Evansville and Louisville KY at sunset
and then into eastern KY by midnight tonight. Have increase chances 
of mixed precipitation to likely over southeast IL during first half 
of evening, as light rain changes to light snow before diminishing 
during late evening. Snowfall amounts in southeast IL should be less 
than a half inch. Gusty NNE winds linger during the evening before 
diminishing overnight as low pressure pulls away from IL with weak 
ridging into IL by dawn Monday. Lows tonight in the upper teens to 
near 20F north and mid 20s south of I-70 in southeast IL. 

Another clipper low pressure system will dive quickly southeast into 
the upper MS river valley by sunset Monday, though this time best 
chances of light snow to be NE of central IL Monday afternoon and 
Monday night. Carried slight chances of light snow and flurries over 
northern and NE counties Monday afternoon into Monday night. Highs 
Monday range from lower 30s from Bloomington and Champaign NE to 
near 40F from Jacksonville SW. 

1025 mb surface high pressure drifts over IL Tue night and into the 
eastern Ohio river valley Wed while upper level ridge noses into IL. 
This to bring quiet weather with temps moderating again with highs 
in the mid 30s to lower 40s Tue and 40s Wed and near 50F from 
Jacksonville SW. 

Low pressure off the Pacific northwest moves east into IL and Ohio 
river valley Wed night and Thu returning a chance of mainly light 
rain, though could be a mix of light snow northern counties before 
ending. Highs Thu around 40F northern areas and mid 40s in southeast 
IL. Cooler highs in the mid to upper 30s Friday which is close to 
normal for late January. Extended models dig an upper level trof 
over the Great Lakes and eastern states next weekend bringing 
chances of light snow showers especially NE areas. Temps also cool 
some more Sat night and Sunday.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)

Several challenges with this TAF set. Widespread precipitation
north of a KIJX-KDNV line has been changing from rain to snow.
Radar data indicates the transition line is just south of KBMI to
about 30SW KPIA, although a bit of mixed precipitation has also
been reported near KCMI. Still seeing temperatures in the upper
30s to lower 40s from KSPI-KCMI so the transition is still a
couple hours away. High-resolution model data suggests some
erosion in the precip shield as the afternoon goes on, as a
surface low travels through the southern tip of the state, so most
of the TAF sites should see the precip end by 00Z. Strong
northeast winds kicking in on the north flank of the low
circulation, so winds 25-30 knots will be spreading over the TAF
sites over the next couple hours and persist much of the evening.
IFR/LIFR conditions will be occurring as the snow moves through.
Some improvement expected this evening, but have largely kept MVFR
ceilings at all TAF sites through the night, as the wind flow does
not favor rapid clearing. Did scatter out KPIA for a time after
06Z, as suggested by a narrow clear slot on the RAP model. Winds
to turn more southwesterly Monday morning ahead of a storm system
which will pass to our north.





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