909 
FXUS63 KILX 130900
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
300 AM CST Sat Dec 13 2014

.SHORT TERM...(Today)
ISSUED AT 300 AM CST SAT DEC 13 2014

High pressure centered just southeast of Illinois with a 500 mb 
ridge axis drifting slowly toward the Midwest will maintain a strong 
inversion based at 900 mb today. This inversion will trap moisture 
near the surface over central Illinois, while warm moist advection 
at low levels begins to push additional moisture into the region. 
The result will be continued low cloudiness and several degrees of 
warming compared with Friday. By evening, there could even be some 
areas of drizzle as weak low level isentropic lift takes place. 

Although there have been some breaks in the cloudiness overnight, 
these are likely to continue to erode as has been the trend on 
satellite imagery the past few hours. Widespread thin fog has been 
observed overnight, but this should erode by late morning with 
daytime heating. Winds will continue to be light southwesterly.


&&

.LONG TERM...(Tonight through Friday)
ISSUED AT 300 AM CST SAT DEC 13 2014

Main concern for the remainder of the weekend is with the plague of 
clouds that refuses to go away. With upper ridge only drifting 
through the Mississippi Valley, model forecast soundings are in no 
hurry to get rid of the low level inversion. Some weak lift will be 
approaching from the west late Saturday night, and with the shallow 
moist column (generally 3000 feet or less), this should be enough to 
wring out some mist/drizzle and keep the haze or light fog in place. 
Have focused the better chances of this in the grids for late 
Saturday night and early Sunday. The extent of clouds puts a bit of 
a question on high temperatures Sunday, but it still appears that 
50s should be widespread.

A piece of energy from the deep trough currently over California and 
Nevada will intensify into a closed upper low, as it moves through 
the 4 corners region tonight. It will then begin to slide northeast 
before reaching Illinois Monday night. Evening model suite in 
reasonable agreement with bringing the surface low into eastern Iowa 
by Monday evening. Have made some adjustments to the timing of the 
associated rain shield, increasing PoP's late Sunday night to around 
60% in the extreme southwest CWA near Jacksonville and Rushville, 
then continued 60-70% across the board throughout Monday. Steadiest 
rain will linger north of I-74 into the evening, then diminish from 
southwest to northeast overnight as the low pulls into the western 
Great Lakes. 

Model agreement begins to break down as the week progresses. At 
issue is a northern stream trough/low that will be dropping out of 
the Canadian prairie provinces on Monday, reaching North Dakota on 
Tuesday. The ECMWF and GEM models shunt this eastward into the Great 
Lakes, while the GFS gets it caught in a bit of a Rex block 
configuration before retrograding it back northwest later in the 
week. This causes some complication with a broad trough in the 
southern stream, which will be emerging onto the southern Plains. 
The GFS is more progressive and organizes a surface cyclone which 
would be spreading precipitation into our area as early as Wednesday 
night. The latest ECMWF catches the state in a narrow upper ridge 
into early Friday, and shunts the entire storm system to our south 
late week as a stronger piece of energy dives southeast through the 
central Plains. The model solutions this far out have been less than 
stable, so am not inclined to go with more than slight chance PoP's.


&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
ISSUED AT 1141 PM CST FRI DEC 12 2014

Problematic aviation forecast for at least the next several hours
across the central Illinois terminals. A large hole in the
expansive low cloud deck across the Midwest continues to spread
east toward the terminals. This hole had previously been expected
to fill in, and the bulk of the forecast guidance suggest this
should still occur. However, confidence in this occurring is
diminishing so have included a TEMPO group to scatter out the
clouds for a time overnight. While the clear area upstream has not
yet seen thick fog problems, good radiational cooling conditions
could allow thicker fog to develop during the clear period. Tried
to cover this potential in the TEMPO group as well. The clouds
should eventually return/fill in area-wide and linger through the
remainder of the 06Z TAF valid time. Light south to southwest
winds will prevail through the period.


&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Onton
LONG TERM...Geelhart
AVIATION...Bak