Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Lincoln Illinois
1111 PM CST Tue Dec 17 2013
issued 815 PM CST Tue Dec 17 2013
Surface high pressure over eastern Kansas and western Missouri
early this evening will track to our south tonight. A very dry
air mass was advecting southeast into central Illinois this
evening with mid to upper teens common over the northern half of
the forecast area. The dry air combined with just enough wind
overnight should keep any significant fog problems at Bay. Last
few runs of the hrrr model have backed off with the low vsbys in
parts of our area but with the ridge axis expected to be right
over our area by 12z, not that confident to remove the current
patchy fog wording in the grids late tonight.
As the surface high shifts off to our southeast tomorrow, we can
expect an increasing southerly wind across the entire area which
should help push temperatures into the upper 30s north to the
middle 40s southwest. Have made some minor adjustments to the
evening dew points and temperatures, as well as backing off the
fog wording until the 4am to 8am time frame. Updated zones should
be out soon.
issued 1110 PM CST Tue Dec 17 2013
Except for the potential for a brief period of fog in the 10z-14z
time frame...VFR conditions are expected to prevail. Another issue
to address will be the threat for low level wind shear tomorrow
night as the surface winds decouple while winds around 1500 feet
remain out of the southwest at 40-50 kts. Surface winds tonight
will continue from a westerly direction but continue to slowly
decrease as the night wears on with the surface ridge axis passing
thru central Illinois around dawn. Then, winds will back into a southwest
to south direction Wednesday morning and increase to between 12 and
17 kts by early afternoon, with a few gusts up to 22 kts thru the
mid afternoon hours before diminishing to around 12 kts after
dark. With surface temperatures expected above freezing in all areas
tomorrow, there should be quite a bit of melting of the snow, which
in turn may bring about an increasing chance for the development
of fog and low clouds late Wednesday night. However, it appears
southerly winds will hold up enough to keep it out of the forecast
thru 06z Thu.
issued 300 PM CST Tue Dec 17 2013
The short term will be highlighted by a warming trend through mid
week. Fog may become an issue as the snow melts over the next few
days. A cold front is still slated for Friday morning, with a
major winter storm taking aim on the area for the weekend. Precip
type issues remain the biggest challenge with that storm. The 12z
guidance has generally trended warmer on Sat and Sat night, which
means rain longer and farther north, with a band of freezing rain
in our northern area. Snow chances may hold off until very late
Sat night and possibly until Sunday as the colder air finally
flows into central Illinois. Snow amounts on the northwest side of this system
could be very high, with the latest trends putting that band just
to our northwest. Any shift in the placement of the stationary
front and track of the low up along the front will be critical to
precip type distribution and amounts of snow and ice.
Short term...tonight through Friday night.
The quick clipper snows from today should be out of our southeast
counties by evening. We will not mention any snow in the worded
zones as a result.
Fog potential tonight does not appear as high as previous model
runs, although the models have not been handling the ground level
moisture very well at all. We kept a mention of patchy fog in our
grids for tonight, but do not expect any widespread dense fog
despite very light winds and clear skies.
Wednesday will see high pressure sliding east of Illinois and southerly
winds increasing. Warm advection flows will intensify in the
afternoon, helping to push temperatures into the 40s in many
areas. Snow cover will reduce the effective heating of the sun,
with energy going to melting the snow. That may keep highs in the
upper 30s over areas with deeper snow. The additional ground level
moisture should help fog develop Wed night, despite some steadier
south winds overnight.
Fog should linger Thursday morning as mid and upper level moisture
increases ahead of the approaching cold front. Rain showers should
hold off until afternoon when isentropic lift intensifies due to
45kts of flow up the 295k surface. Rain should generally remain
north of Lincoln in proximity to the developing mid-level warm
front and nose of the Theta-E ridge.
There remains some concern as to the extent of freezing rain later
Thursday night into Friday morning as surface temps northwest of
Peoria dip below freezing behind the cold front. We kept a mention
of freezing rain in that area, with a thin layer of icing possible.
Precipitation chances should become confined mainly to the
southeast half of our area Friday afternoon and Friday night as
the cold front and shortwave shift east. Once again, any rain
falling into the night could transition to freezing rain along
and north of I-72.
The cold front is projected to stall out just to the southeast of
Illinois, in prep for a return push northward when the weekend
Long term...Saturday through Tuesday.
A surge of warm air and moisture in the mid levels will strengthen
on Saturday, pushing the front back north as a warm front. Rain
will re-develop or advance northward during the day, with freezing
rain possible in the morning north of I-72 before surface temps
climb above freezing. All or our forecast area except maybe around
Galesburg should climb above freezing during the afternoon,
keeping all precipitation as rain. The timing and speed of the low
riding up the front will dictate how soon that colder air will be
drawn into the northern edges of the storm and change the rain to
freezing rain and eventually snow. Saturday night could see an
extended period of freezing rain in a band from SW to NE across
our counties, including areas from Rushville/Jacksonville/
Springfield north to Canton/Peoria and east to Bloomington and
Champaign. Ice amounts could reach over a quarter inch in some
areas, depending on how soon the cold air arrives at low levels.
Snow is expected to develop on the northwest fringes of our area as soon
as Saturday evening, but more likely after midnight into Sunday. A
couple of inches of snow could accumulate on Sunday in areas west
of I-57, with lesser amounts toward I-57 and east. A band of
deformation snows could increase those amounts in bands west of
There remains a great deal of uncertainty with the weekend storm
system, but the trends have been for a warmer solution in our
counties, even from the previously colder European model (ecmwf).
Dry and colder conditions will follow the storm, and should set up
better travel conditions for Christmas eve day through Christmas,
and possibly longer.