Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 150554

Area Forecast Discussion
1154 PM CST Sun Dec 14 2014


Issued a Dense Fog Advisory earlier this evening across northern
portions of the forecast area where fairly widespread 1/4SM mile
visibilities had developed. The advisory currently runs until 8 AM
Monday, but if recent trends persist, it may be able to be
canceled early.

A vertically stacked low pressure system is centered over the
central Plains this evening, with a band of showers on its eastern
periphery. These showers will eventually arrive in the forecast
area by late tonight or during the day Monday, but they will not
be breaking speed record getting here given the vertically stacked
nature of the system. In any event, the pressure gradient will
gradually increase with the approach of the system, resulting in
increased wind speeds. These increased winds, along with a
slightly drier airmass that precedes the rain area, may help to
diminish the fog across the area as the night progresses. This
gradual diminishing should also preclude the need for an expanded
Dense Fog Advisory, and may allow the early cancellation of the
current one.

Aside from the previous need to add the Fog Advisory, going
forecast is in pretty good shape. Plan to make a few tweaks for
the latest trends, but no significant changes are needed at this


.SHORT TERM...(Tonight)

20z/2pm visible satellite imagery continues to show low clouds
blanketing much of central Illinois, with the exception being the
far southwest KILX CWA around Winchester.  Large area of clearing
that developed over central/eastern Missouri earlier in the day has
worked its way northeastward and now extends along/southwest of a
Winchester to Mount Vernon line.  Clearing has behaved much like the
HRRR has predicted, generally reaching the far SW CWA then coming to
a halt.  Will continue to carefully monitor satellite trends to see
if clearing can make it any further northeast, but with the sun
going down soon, this does not seem likely.  As a result, will
feature mostly clear skies across the far SW into the early evening
followed by a return to overcast conditions across the board.
Visibilities will be a big concern tonight, as current obs show some
sites already dropping into the 1-2 mile range.  HRRR has
consistently shown vsbys dropping below 1 mile by 00z, with patchy
dense fog possible after dark.  Confidence is not great enough for
another Dense Fog Advisory at this time, but the potential certainly
exists as low-level moisture continues to increase and surface
dewpoints approach the 50 degree mark.  Due to the clouds/fog,
overnight low temperatures will be several degrees above numeric
guidance in the middle to upper 40s.  Vigorous upper-level low noted
on latest water vapor imagery over the Texas panhandle will begin to
lift northeastward tonight, with model consensus spreading showers
into the SW CWA after 3am, then to the I-55 corridor by 6am. Further
northeast, dry conditions will prevail across the E/NE CWA until
after dawn Monday morning.


.LONG TERM...(Monday through Sunday)

Models in good agreement with the upper low pushing northeast into
Iowa by Monday afternoon with strong 850 mb theta-e advection/convergence
pointed right into west central Illinois in the 09z-12z time frame.
Highest POPs will be over west central Illinois during that time period
with a progressive shift east with the categorical POPs as the morning
wears on. May have some thunder in the precip band associated with
the warm conveyor belt in the morning, but mid level lapse rates really
don`t ramp up until the afternoon hours as the closed system at 500
mb approaches. Then it appears the better threat for thunder will be
over west central Illinois tomorrow afternoon which will be closest
to the closed 500 mb low. High resolution reflectivity simulations
suggest a band of showers sweeping across the area in the morning,
with a break late morning into the early afternoon hours, before
more showers push across the area as the upper low and deeper forcing
approaches. Will keep likely POPs going across the northern third
of the forecast area tomorrow evening, close to the track of the upper
wave, with POPs starting to decrease late Monday night into Tuesday
from southwest to northeast. The system should continue to push away
from our area on Tuesday with lingering POPs only in the far north
and east during the morning with colder air filtering into our area
as the surface low pushes into lower Michigan by late Tuesday afternoon.

The upper wave will merge with a northern stream shortwave with the
system slowly pulling off to our east thru the end of the week. High
pressure at the surface will shift slowly southeast across the northern
Plains while another piece of energy translates east across the
southern Plains late Wednesday into Thursday with its precipitation
shield possibly affecting southern Illinois during the day Thursday
into Thursday night. Still seeing varying solutions with the initial
wave coming out of the southwest and into the lower Mississippi
Valley late Wed thru late Thursday with the higher POPs continuing
over our far southern counties. Having a hard time seeing precip shift
too far north into a more confluent flow over the southern Great Lakes
with surface high pressure holding firm over the upper Midwest.

A second and stronger wave is then forecast to eject east and then
northeast across Texas on Friday with a large area of rain to its north.
The latest ECMWF, GEM global and UK models were more suppressed with
the low level baroclinic zone/frontal boundary, and as a result, are
more suppressed with the track of the surface low than the current
operational GFS. However, the high resolution GFS (not yet ready for
prime time model) suggests a more suppressed track as well with the
system late in the week. So will go with the consensus for the late
week system and keep any likely or higher POPs to our south late
Friday thru Saturday. The threat for precip will shift well off to
our east on Sunday as the storm system skirts around our area and
then heads up the East Coast later Saturday into Sunday while high
pressure settles into our area for Sunday.

After the unseasonably mild temperatures on Monday, colder temperatures
will filter back into the region starting on Tuesday and remain with
us through the rest of the period. We should see temperatures gradually
return closer to seasonal levels for the middle of December Tuesday
night and hold over the region thru the period.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)

Still looks like IFR or lower conditions will prevail across the
central Illinois terminals through the 06Z TAF valid time. While
visibilities have trended up a bit at most local TAF sites (except
KPIA & KBMI), CIGS are still quite low. Southeast winds have been
picking up into the 10-15 kt range ahead of an approaching storm
system, and this has been an aid to the higher visibilities.
Showers will break out during the morning hours Monday, and they
should be on and off for the rest of the forecast. Winds will pick
up further Monday morning and become gusty at KSPI, KDEC, KCMI due
to their location further south of the track of the surface low
associated with the storm system. Should begin to see winds shift
more south or southwest Monday evening as the surface low begins
to pass to the north of the area. This passage should also taper
off the threat of showers. While a thunderstorm can`t be ruled out
with this system, especially Monday afternoon, expected coverage
is too low to include in terminals at this time.


DENSE FOG ADVISORY until 8 AM CST Monday FOR ILZ027>031-036>038.



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