Area Forecast Discussion
Issued by NWS Central Illinois
FXUS63 KILX 171751
Area Forecast Discussion
National Weather Service Lincoln IL
1151 AM CST Tue Dec 17 2013
ISSUED 906 AM CST Tue Dec 17 2013
Short-wave trough currently evident on water vapor imagery over
northern Iowa is triggering light snow/flurries across much of
eastern Iowa into west-central Illinois. While both the 12z KILX
and KDVN upper air soundings show quite a bit of dry air between
950 and 700mb, a few upstream reports are indicating light snow.
Based on current obs and the latest HRRR, have added scattered
flurries to the forecast across the Illinois River Valley this
morning and through much of the afternoon further southeast across
the remainder of the area. Once this wave passes, skies will
gradually clear from northwest to southeast by mid to late
afternoon. Zone update to better reflect sky trends and to add
flurries has already been issued.
ISSUED 1151 AM CST Tue Dec 17 2013
Band of snow-showers with visbys down to 1SM or less is currently
moving across central Illinois. 1744z radar indicates that the
snow-showers are mainly along and east of I-55, with areas to the
northwest now dry. Based on radar timing tools, will carry -SN
through 19z at KBMI, KSPI, and KDEC, and through 20z at KCMI. Once
the snow ends, VFR ceilings will persist for a couple hours before
clearing arrives from the northwest. Clouds have already scattered
at KGBG and will clear at the TAF sites between 19 and 21z. After
that, mostly clear skies will prevail tonight into Wednesday.
Winds will generally be from the west today/tonight, but will
become southerly at 10 to 12kt by Wednesday morning.
ISSUED 333 AM CST Tue Dec 17 2013
Weak high pressure over much of the center of the country and
edging into the Midwest in the wake of an exiting clipper system.
West/northwesterly flow aloft slowly modifying to more
southwesterly through the 7 day forecast. Brief showers coming to
an end this morning over SW ILX. First problem with the forecast
is the potential for fog tonight as the winds come around to more
swrly and push warmer and higher RH air in over the snow pack. WAA
being countered by the snow on the ground and not handled well in
the models. GFS and ECMWF getting far more continuity with each
run with respect to the initial front, but the timing of the main
storm system for the weekend as energy ejects out of the SW along
the boundary is nowhere near consistent. Thermal profiles btwn the
GFS and the ECMWF still making precip type an issue. Though first
glance at the models point to a major snow event, too much warm
air moving into the midlevels pushing the threat to more of a
potential freezing rain event. Not too many changes to the
forecast from the 12z run.
SHORT TERM...Today and Tomorrow...
Cold and cloudy conditions continue as another clipper pulls away from
the region. High pressure building into the Midwest, and winds
slowly becoming more southwesterly this evening. Temperatures
modifying, and concern for fog in the overnight as any melting is
only going to enhance RH in the boundary layer. Lack of clouds
tonight and certainly a brief pd of light winds will only add to
the threat. Same story for tomorrow as the WAA fights with
existing snow field to bring max temps up a couple degrees.
Overnight, the fog threat continues. This may be a recurring theme
through the end of the week if the snow field remains.
LONG TERM...Thursday through Monday...
Low confidence continues with the temperatures through the end of
the week. Will depend greatly on the persistence of the snow. After
the main trof pulls to the east, more southwesterly flow sets
in as a deep trof develops over the swrn CONUS. Deep upper low
now remaining open and more diffuse in the GFS and ECMWF as the models
continue to converge. SWrly flow putting ILX in a transition zone
temperature wise with the initial passage of the front on Thurs
night/Friday. Front moving into and stalling out just to the south
over the Ohio River Valley. Location of the boundary as a series
of upper waves eject out and along said front will make a major
difference on the forecast. Northerly extent of the precip with
respect to the front is the main issue for the forecast as well as
how deep into the cold air the precip reaches. Latest ECMWF still
maintaining ice as the cold air takes a little longer to move in
and midlevels remain warm. GFS still looks like mostly snow after
a transitional mix. Precip type is still ambiguous, timing is all
over the place btwn the two models into the extended. In addition
to all the mentioned problems, growing concern for the projected rapid
development of the surface system with an open wave aloft. The
confidence in the model performance is lacking considering the
dynamics look a bit off.