Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 130306

Area Forecast Discussion
National Weather Service Lincoln IL
906 PM CST Tue Dec 12 2017

Issued at 905 PM CST Tue Dec 12 2017

A warm advection band of precipitation will pass by mainly to the
northeast of our forecast area. We will see clouds increase along
and north of I-74, with even some radar returns showing up in that
cloud layer. However, we do not expect precipitation to reach the
ground in our counties with the overnight WAA precip band ahead
of the surface low pressure system. The lighter winds of this
evening will give way to an increasing pressure gradient after
midnight. South-southwest winds will increase before sunrise,
helping to produce rising hourly temps after midnight. Therefore,
we are still on track for low temps to occur this evening.

Precip chances will increase for our northern counties as the
clipper system reaches near Chicago tomorrow afternoon. The
temperature profiles support primarily rain, but with FROPA mainly
between 17z and 21z, we could see some light snow mix in for areas
north of I-74 as the afternoon progresses. No accumulation of snow
is expected, but very windy conditions could make any snowfall
look worse than it is.

Strong winds tomorrow will approach wind advisory levels of
sustained 30 mph or gusts over 45 mph. It appears we should remain
below those numbers, but it will be windy, none-the-less.

Updates this evening were mainly to sky and hourly temps, as the
remainder of the forecast database looks on track. Updated
forecast info will be available shortly.


.SHORT TERM...(This evening through Wednesday)
ISSUED AT 258 PM CST Tue Dec 12 2017

A surface high pressure ridge and associated light winds will cross
over central IL this evening. After this, the clipper system
currently over Alberta heading toward northeastern IL will approach,
increasing southerly winds for early in the morning. Temperatures
will drop into the low 20s early in the night, steadying through
early morning with the increase in southerly winds. By noon
Wednesday, the cold front associated with this fast moving system
should already be approaching the Illinois River, with temperatures
peaking for the day early just before frontal passage, and winds
shifting to NW behind the front. Expect highs ranging from around 40
on I-74 to mid to upper 40s from Jacksonville to Lawrenceville.
Boundary layer momentum looks to be quite strong, with 925 mb winds
up to 35 kts. Have therefore raised wind speeds for the Wednesday
forecast, with sustained wind speeds reaching 20-25 mph and some
gusts reaching 40 mph. Also included a chance of precipitation
mainly along and north of I-74, and a slight chance extending
southward to Havana, Lincoln, and Paris. Precipitation should be a
mix of rain and some snow from I-74 northward. Amounts of
precipitation should be only a few hundredths or less, with no
expected snow accumulation.

.LONG TERM...(Wednesday night through Tuesday)
ISSUED AT 258 PM CST Tue Dec 12 2017

The next system looks on track for Thursday night into Friday
morning, although models show little indication of precipitation
this far south. Highs expected in the low to mid 30s Thursday and
Friday, and lows in the low to mid 20s Thursday morning through
Saturday morning.

Starting Saturday, a warmup looks on track as the longwave ridge
over the west moves eastward and milder pacific air advects into the
central and southeast U.S. ahead of a trough moving digging into the
mountain west. Highs well into the 40s can be expected over the
weekend into early next week as a result. Uncertainty develops in
how the trough evolves into next week, however fairly good agreement
that a disturbance embedded in southwesterly flow will bring some
precipitation to areas south of I-70 Sunday or Monday.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 548 PM CST Tue Dec 12 2017

A brief break from gusty winds is expected this evening. However,
another low approaching from the NW will bring increasing S-SW
surface winds and winds aloft by late tonight into early Wednesday
morning. As a result of strong SW winds aloft, have included low
level wind shear from 10-15Z at all TAF sites due to 45 kt flow
at 1500 ft AGL. CMI and DEC will be the latest to see LLWS, but a
few hours from 12z-15z or so could see those conditions as well.
As a cold front approaches and then passes tomorrow, winds will be
shifting from S-SW winds to start the day, then become westerly in
the afternoon while continuing at 15G30kt. Winds will eventually
become NW by late in the TAF period, once FROPA occurs.

Cloud heights look to approach MVFR by mid to late afternoon
tomorrow, with the TAF sites along the I-74 corridor having the
better chances of dropping down into MVFR levels. No precip of
significance is expected, although a few sprinkles could develop
with the frontal passage.




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