Area Forecast Discussion
Issued by NWS Central Illinois
FXUS63 KILX 031908
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
208 PM CDT SAT OCT 3 2015
ISSUED AT 205 PM CDT SAT OCT 3 2015
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)
ISSUED AT 205 PM CDT SAT OCT 3 2015
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 seperate and
even retrogrades the upper low back into the Gulf of California by
the end of the week. On the otherhand 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 18Z TAFS through 18Z Sunday afternoon)
ISSUED AT 1239 PM CDT SAT OCT 3 2015
Low and mid level moisture advecting into the region this morning
from the systems to the east resulting in low clouds for SPI, DEC
and CMI. Although the low clouds are slower to reach BMI and PIA,
that will change before sunset. So far, majority of the low cigs
are at the MVFR/VFR break, but expected to drop to 1500 ft later
tonight. Mainly a stratus night, but cannot rule out a small vis
drop with dewpoints just to the east in the 40s and weak showers
forming to the east. Chance for light rain as well, but not enough
confidence to put it in any one terminal. MVFR in PIA, SPI, and
BMI for now... a drop into IFR for DEC and CMI overnight.