Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 160207 CCA

Area Forecast Discussion...Corrected format
907 PM CDT Mon Sep 15 2014


Showers and drizzle are departing quickly to the southeast, with
the back edge of the clouds approaching faster than previously
expected as well. The latest HRRR is indicating a band of low
clouds are expected to redevelop behind the clearing, moving into
our counties from the northeast toward Lake Michigan. Low level
wind patterns and obs from that area show some support for that
solution. Have updated the sky grids to acknowledge both the
initial clearing and subsequent clouds from the NE for a few hours
after. Any areas that clear out will have a good chance of seeing
fog develop. The light winds later tonight could set the stage for
locally dense fog, especially NW of I-55 where clearing will
develop sooner. Dewpoints will remain relatively high for enough
hours after clearing that lows will drop very close to the
dewpoint in those areas. Added fog to the weather grids
accordingly. Updated info is already available.


.SHORT TERM...(Tonight)

Storm system still impacting Central Illinois ahead of an
approaching boundary.  Some low clouds and developing fog in the
wake of the showers hint to the main issues going into the overnight
hours tonight.  Showers and thunderstorms should begin to wrap up
this evening but wet, misty, and low clouds should continue for a
little while at least.  Satellite imagery showing the low clouds
hanging around a bit after the wind shift/boundary moves through
with the NW/N winds.  Cooler tonight...but coolest in the NW where
most likely to see some clearing.  Temps may bottom out locally if
the cigs clear in the overnight.  Either way, enhanced llvl moisture
will bring with it the chance for fog development.  For now,
question is between dense fog and very low stratus.  Stratus already
developing and see no reason to break that trend, so only putting in
patchy fog mention in the overnight hours.


.LONG TERM...(Tuesday through Monday)

The medium and long range models are in generally good agreement
that a progressive upper level flow pattern will dominate through
the next seven days.

Surface high pressure from the northern Plains will work its way
toward the Midwest Tuesday, resulting in a clearing sky and a
continuation of cooler than normal temperatures. A weak shortwave
trough in the upper level northwest flow will approach the
mid-Mississippi River valley Wednesday. However, an abundance of dry
air in the lower levels should keep central and eastern IL
rain-free. Would not be surprised to see a sprinkle or two in areas
southwest of a Rushville-Springfield line. Will hold off on the
mention of that for now as the favorable moisture and lift appears
to stay along and west of the MS River.

Anticyclonic low level flow will continue to dominate through
Thursday and Friday keeping rain out of the forecast. A return,
southerly flow Friday will finally allow temperatures to climb
toward seasonal levels in the 75-80 range across most of central and
southeast IL.

Will keep the potential for showers and thunderstorms in the
forecast west of I-57 Saturday, and across all of central and
eastern IL Saturday night. The European and GFS models are actually
in quite good agreement that an upper level trough will advance from
the Rockies Friday to the Midwest later Saturday. The southerly low
level flow will advect plenty of moisture ahead of the surface cold
front, resulting in showers and thunderstorms. However, there is the
potential for heavy rain from this system, particularly according to
the GFS solution. The GFS depicts the remnants of Hurricane Odile,
currently in the southern California Baja region, ejecting toward
the central U.S. with deep tropical moisture. This would easily
result in 1-3" of rain across much of the forecast area Saturday
into early Sunday. However, the European and Canadian models are not
buying into this solution at the moment. As a result, will watch
this potential for the next several forecast runs before introducing
a mention of heavy rain.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)

Showers and/or drizzle will linger for another hour or two at the
terminal sites except PIA, which is seeing ceiling heights rise to
VFR periodically due to dry air mixing down already. Satellite and
observations are indicating MVFR will return to PIA for several
more hours before some clearing arrives. CMI is the only LIFR
site, with IFR at SPI/BMI/DEC due to low cigs and vis. Forecast
soundings and HRRR output still show progressively improving
conditions by late evening, with high pressure building behind the
departing cold front. Therefore, have kept the clearing trends in
the cloud cover, with fog formation likely in any areas that clear
out. Went LIFR or VLIFR fog FROM 11Z-13Z for the northern
terminals of PIA/BMI/CMI, and IFR at SPI/DEC. The timing may need
to be adjusted, but feeling more confident that low level moisture
will become low stratus and fog across the board.

VFR conditions should develop quickly Tuesday morning with mixing
of very dry air just above the moisture layer.

Any residual WNW winds will become N by late evening behind the
front. A few gusts to 18kt will be possible for an hour or two
past FROPA, then light winds should develop the rest of the night
and into Tuesday.




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