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Area forecast discussion 
National Weather Service Lincoln Illinois
856 PM CST Thu Nov 20 2014

Update...
issued at 856 PM CST Thursday Nov 20 2014

High pressure will provide another very cold night with ideal
radiational cooling conditions. As the surface ridge axis
progresses directly over IL, winds will become light and variable
under clear skies. That will allow temps to drop into the single
digits toward Galesburg and Altona, with low to mid teens all the
way south to I-70. Some light snow cover from Paris to Effingham
will help produce slightly colder low temps in those areas.

Still seeing the potential for a few hours of freezing rain at the
onset of precipitation Friday evening. The GFS remains much warmer
than the NAM with the low level thermodynamics. Even the NAM shows
the peak warm layer temp climbing to +8c by midnight as far north
as I-74. That would diminish the potential for any continued
freezing of the rain after it reaches the ground based on expected
surface temps hovering very close to freezing at that time.

The main concern remains that any freezing rain initially would
be falling on dry and cold pavement, which could quickly produce
very slippery conditions even with just a few hundredths of
freezing rain. The warming trend from south to north the rest of
the night should help any ice begin to melt after a few hours of
icy conditions.

Overall, the current forecast grids have a good handle on the
expected trends over the next 12-24 hours, and no formal update
will be needed this evening.

&&

Short term...(tonight)
issued at 248 PM CST Thursday Nov 20 2014

Surface ridge over lower Missouri Valley will push east into
Illinois overnight. Resultant drop in wind speeds will combine with
mostly clear sky to produce near ideal radiational cooling
conditions across central and southeast Illinois. Coolest readings
should be in gbg area and in the area from Paris to Effingham where
some snow cover is still observable in visible satellite. Have
generally used mav guidance as NAM/WRF and resultant met have been
on cold side last night and today.

&&

Long term...(friday through thursday)
issued at 248 PM CST Thursday Nov 20 2014

Upper air and moisture channel imagery this morning shows main
northerly jet from Canada into northern Illinois and then curving off to
the east. Upper trof over eastern US is now moving east, as strong
closed low system seen moving coast to West Coast. That system will
be the system that will trigger precipitation (pcpn) over Midwest
into the weekend. Models similar in movement of that system, as
surface high pressure area over MO drifts off to the east.

As West Coast system digs into the southwest US, surface high moving
into central East Coast will aid in southern moisture advection from
the Gulf into Midwest N Friday. Cold air at surface will be overrun
by moisture from Arkansas and MO, increasing on Friday and reaching region
Friday evening. Warm air overriding lingering cold dome will result
in some freezing rain on Friday evening, as the warm air advects
north into area. NAM and GFS soundings, and temperature meteograms
show timing of the pcpn and The Retreat/warming of the boundary
layer will be critical in freezing rain development. NAM seems to be
too cold, but each New Run seems to warm NAM. Temperature timing in
Friday afternoon through evening and overnight was a blend of GFS
and a warming of the NAM. Freezing line is expected to retreat
northward over County Warning Area in the evening and overnight, with brief freezing
in the south sections and longer periods in the north. Pcpn amounts
however, will be light, less than tenth of an inch. However some
slick areas will be possible on secondary roads.

For Saturday into Sunday, upper system will dig into Texas and then
move north through Midwest into Great Lakes by Sunday night. This
will bring periods of rain over region through the weekend, with the
heaviest rainfall on Sunday as the low center moves through. Colder
air moves in by Tuesday will some lingering pcpn.

Models then start to diverge for Wednesday and Thursday. Coordinated
will surrounding offices and consensus to drop pcpn chances for Wed
night into Thursday night until future model runs.

&&

Aviation...(for the 00z tafs through 00z Friday evening)
issued at 552 PM CST Thursday Nov 20 2014

The surface ridge extending from southeast North Dakota across SW Illinois will
push east across Illinois over the next 24 hours. Winds tonight will
start out west-northwest at 7 to 10 kt but become light and variable by late
evening. As the ridge axis advances east of the terminal sites on
Friday, winds will increase from the southeast. Winds speeds
should climb into the 10-12kt range by 21z Friday. Mid clouds at
15k ft will advance into the region from the SW as moisture begins
to overrun the cold high pressure. Any precipitation is expected
to hold off until after the end of this taf period.



&&

Ilx watches/warnings/advisories...
none.
&&

$$

Update...shimon
short term...Barker
long term...goetsch
aviation...shimon

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