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Area forecast discussion 
National Weather Service Lincoln Illinois
1132 PM CDT Wed Oct 22 2014

issued at 849 PM CDT Wednesday Oct 22 2014

An upper level high pressure ridge axis will cross Illinois
tonight, becoming increasingly narrow as a fast moving shortwave
approaches from the northern plains while a deep low over the East
Coast is slow to move. Light southeasterly winds will slowly turn
southerly and increase toward morning as surface pressure
gradients increase with the approaching wave. Temperatures
primarily in the 40s already this evening will only drop into the
upper 30s to around 40 as a result of increasing wind and high
cloud cover spreading into Illinois. The approaching shortwave
will spread cloud cover and chances for light rain showers mainly
from around I-72 northward in the afternoon and evening Thursday.
Current forecast is on track with these features and no
significant updates needed this evening.


Short term...(tonight)
issued at 318 PM CDT Wednesday Oct 22 2014

High pressure ridge over the Midwest this afternoon slowly shifting
eastward with the winds slowly taking on an increasingly
south/southeasterly direction. Winds through the overnight will
continue to be light, but will help advect warmer air into the
area. However, location of the ridge axis closer to the Indiana
border with lighter winds will result in lower temps east of I-57.
Low lying areas may see some areas of frost as well in the pre dawn
hours. Not as concerned in the west where lows will only dip into
the lower 40s. Approaching system from the west slowing
considerably and precip chances not in the forecast before 12z.


Long term...(thursday through wednesday)
issued at 318 PM CDT Wednesday Oct 22 2014

The short/medium range models are in fairly good agreement with the
progressive upper level through the weekend. Differences start to
appear by early next week with the next significant precipitation
producing system.

Upper level troughing from the High Plains will approach the
Mississippi River valley Thursday, with a weakening trend the
farther east it advances into surface and upper level ridging. Based
on the 12z upper air analysis from today, there is very little deep
moisture associated with this system. Moisture advection and
isentropic lift will be quite weak, so the window of opportunity for
rain tomorrow should be pretty short. Scaled back on the rain
chances as a result, with 30-40 pops mainly along and west of the
Illinois River during the afternoon - spreading toward bmi-cmi-
Lincoln toward early evening. The clouds and spotty rain showers
will keep temperatures a bit cooler than seasonal normals Thursday.

Trimmed temperatures a bit for Friday as some low level moisture
with a light southerly flow ahead of weak shortwave trough should
produce a few more clouds as the day progresses, especially west of
the I-55 corridor. The weekend is expected to be mild with plenty of
sunshine as ridging at the surface and aloft moves toward the
Midwest. Low level warm advection Saturday will allow highs to reach
the lower 70s in most areas, although readings will be a few degrees
cooler on Sunday as the surface ridge axis moves over Illinois. A
return, southerly flow on Monday will give US unseasonably warm
readings in the 72-76 range across the forecast area.

The model differences mainly revolve around the strength and
amplitude of an upper level trough moving out of the plains Tuesday.
The GFS has more of a phased northern/southern stream system which
moves at a steady pace through the Midwest Tuesday. The European
model has a much stronger southern stream system, which develops a
surface low on the cold front, which then tracks into central Illinois by
early Wednesday which gives US heavier rainfall and precipitation
chances into Thursday morning. At this time, prefer the more
progressive/phased GFS model, and will keep the forecast dry for
next Wednesday.


Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1132 PM CDT Wednesday Oct 22 2014

Until 18z...expect VFR conditions with winds southeast 3-5 kts increasing
to S 8-10g15 kts in the morning. After 18z...scattered -shra
spreading eastward across mainly northern portions of the central
Illinois forecast area. Isold-sct MVFR ceilings possible mainly kijx-
kdnv northward as this precipitation spreads across the area, but
MVFR conditions not yet included in tafs due to timing and storm
track uncertainty. Precipitation ending kpia-kspi westward as
early as 00z Friday, ending throughout central Illinois by 06z. Low
ceilings approaching MVFR thresholds continuing.


Ilx watches/warnings/advisories...


short term...hjs
long term...Miller

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