Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 032346

646 PM CDT SAT OCT 3 2015

.SHORT TERM...(Tonight)

The long time influence of high pressure has lost ground to the
upper low over the southeastern US, and clouds and low level
moisture are edging into Central Illinois throughout today.  The
same trend will continue overnight with northeasterly winds.  Some
of the gusts will dwindle with decoupling, but the increasing
pressure gradient will keep the sustained winds up.  Persistent
cloud cover tonight will keep the temps up a degree or two above
last nights lows.  In addition, the enhanced llvl moisture could
result in some light rain, particularly in the extreme east.  Not
particularly impressed with the models, as the NAM is even drying
out, but the sprinkles and light rain from this morning were not
previously handled well either in a previously drier air mass.

.LONG TERM...(Sunday through Saturday)

Illinois remains in the col between the weakening southeast upper
low and the northern stream for a few more days at mid-levels.
Influence of the low and related strong surface gradient should
diminish early in the week with shortwave ridging developing and
warming temps as 850 temps climb into the double digits C by
Sunday evening and then remain there through at least mid-week.
Models are in reasonable agreement with weakening flow throughout
the atmosphere through Tuesday and should provide generally dry
and slightly above normal temps.

Differences become apparent by Wednesday with the focus on the
amount of phasing between deepening cutoff over desert southwest and
the faster northern stream. ECMWF (00Z) kept energy separate and
even retrogrades the upper low back into the Gulf of California by
the end of the week. On the other hand the GFS (06Z) kept the
energy moving eastward until it is absorbed back into the northern
flow through a strong shortwave toward the end of the week.
Although there is some spread in the GEFS with the upper low
movement, most members are east of the ECMWF position. The 12z
version of both models are trending toward middle ground. Even
though GFS still has more phasing the overall pattern is much more
in sync.

Implications of the difference are the speed and strength of
northern stream waves - the first of which begins to impact the area
Wednesday Night and Thursday.  With more phasing, the GFS is slower
and stronger with this initial wave while the ECMWF progressively
moves it to the mid-atlantic region by Friday.

Given the better performance of the ECMWF with the current southeast
U.S. upper low and the propensity of the atmosphere to cutoff
recently will lean toward the ECMWF solution this package. This
faster and weaker evolution of the northern stream will likely lead
to weaker WAA ahead of the mid-week system and will be undercutting
MEX guidance a tad.

Even though the ECMWF does bring in next wave on Saturday, moisture
appears to be mostly shunted east along the Atlantic Seaboard.  The
GFS is strong enough with this next wave to tap some Gulf moisture.
Given the differences evident in model forecasts, will leave
mentionable PoPs out of Saturday for now.


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

Satellite and radar trends confirm a persistent push of moisture
and lift into Illinois from the east this evening, with MVFR
clouds poised to overspread all TAF sites by 06z-07z. Spotty
showers or drizzle will accompany the decreasing cloud ceilings,
as the warm conveyor belt funnels a stream of moisture up the
isentropic surfaces. Precip chances appear to mainly confined to
our eastern terminals of BMI, DEC, and CMI, based on the projected
westward progression of the upper trough. Therefore, have only
included vcsh/-dz in those TAFs. IFR conditions will be possible
after 06z through a couple hours after sunrise as the low levels
saturate and LCLs dip below 1k ft. Again, the eastern TAF sites
will have the best chances of IFR conditions later tonight.

Northeast wind gusts will diminish by 03z, but sustained speeds
of 10-15kt will continue overnight. A relaxed surface pressure
gradient will finally develop tomorrow, helping to calm down the
very breezy conditions of the last 4-5 days. Wind direction will
remain northeast, but speeds should remain in the 9-13kt range.




LONG TERM...Barker
AVIATION...Shimon is the U.S. government's official web portal to all federal, state and local government web resources and services.