Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 271939

Area Forecast Discussion
239 PM CDT Sat Sep 27 2014

.SHORT TERM...(Tonight)

A weak 580 dm 500 mb low spinning over nw IA will stay over western
IA through Sunday morning, and then starting drifting se into MO and
sw IL by Sunday evening. Isolated convection associated with this
feature will stay west and south of central IL tonight & Sunday.
Few cumulus clouds this afternoon especially over western IL will
dissipate at sunset, with patches of mid/high clouds drifting
northward across central IL into Sunday. A 1026 mb surface high
pressure ridge over the Great Lakes region will stay near IL through
Monday, and continue fair weather with light ENE winds into Sunday.
Dewpoints currently averaging in the mid 50s, and lows overnight will
be close to these readings with lower 50s by Danville and Paris.
Patchy shallow ground fog will develop near river valleys again
during overnight (IL and Wabash rivers) and dissipate by 830 am


.LONG TERM...(Sunday through Saturday)

Models in good agreement in evolving longwave ridge over Central
North America into a ridge west/trough east pattern by the end of
the forecast period. Although there are still some detail
differences, particularly with respect to timing, models are in
better agreement in the extended period than yesterday.

Cut-off upper low over middle Missouri Valley embedded under the
ridge will gradually become absorbed back into the northern stream
before moving across the Midwest as a weak open wave Sunday Night
and Monday.  Moisture should be quite limited and lift is weak
suggesting that an increase in cloudiness will likely be the major
impact with any precip staying to our northeast.  Weak cold advection behind
the system may bring slightly cooler temperatures to the area on
Tuesday, but with 850 temps remaining generally in the 11-15C range,
the mercury should remain above seasonal normals.

Energy now over Sierra Nevada will slowly eat away at ridge through
midweek and as surface ridge retracts into the northeast, Gulf of
Mexico should open enabling moisture flow to increase prior to the
incoming wave.  Latest model suites are in better agreement today
with timing of precip onset with operational models keeping forecast
area dry until late Wednesday Night.  Frontal passage looks to be
Thursday Night with showers lingering behind the front until the
passage of the main trough axis Friday.  This seems reasonable given
the cool 500 mb temps in the trough with 12z GFS indicating as low
as -25C over the area at 00z Friday evening.

Latest model runs suggest moderate instability will be in place just
ahead of the boundary Thursday evening and given the significant
shear expected with GFS forecasted 40kt+ 850 mb winds at 00z
Thursday Evening may have some stronger storms. Later forecasts will
need to monitor this potential closely.

Strong cold advection behind the front is also now more consistent
between the operational models with drops of 10C over a 24 hour
period Thursday Night and Friday. MOS Guidance from 12z GFS and 00z
ECMWF are beginning to show this drop below normal for the end of
next week and will be reflected in forecasted temps.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)

VFR conditions expected to continue across the central IL airports
through Sunday. The exception will be possible patchy shallow
ground fog producing possible MVFR vsbys from 09-13Z especially
along IL river at PIA with a bit higher dewpoints in the mid to
upper 50s with SPI dewpoint currently at 62F. Few to scattered
cumulus clouds 5-6k ft this afternoon mainly at western TAF sites
of PIA and SPI will dissipate by sunset and redevelop by midday
Sunday. Otherwise patches of mid/high clouds to drift northward
across central IL the rest of the weekend. A 580 dm 500 mb low
over western IA to keep its isolated convection nw of IL as it
meanders over western IA through Sunday morning then begins to
drift se thereafter. Large 1027 mb surface high pressure to remain
over the Great Lakes this weekend, keeping light east winds close
to 5 kts during daytime hours and light to calm at night.




LONG TERM...Barker
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