Area forecast discussion
National Weather Service Lincoln Illinois
606 PM CDT Wednesday may 22 2013
Discussion...
issued 243 PM CDT Wednesday may 22 2013
Plenty of challenges with this forecast package...particularly in
the extended when a stationary frontal boundary brings unsettled
weather to the region over the upcoming Memorial Day weekend.
19z/2pm surface analysis shows low pressure centered over southern
Wisconsin...with cold front trailing southeastward along the
Illinois/Indiana border. Further upstream...a trough axis/wind
shift is noted over central Iowa. Aloft...main feature of interest
is a closed 500mb low spinning over Iowa.
Short term...tonight through Saturday
Iowa upper low will progress slowly eastward over the next 24
hours...providing plenty of cloud cover and perhaps a few showers
to central Illinois tonight into Thursday. Forecast soundings Show
Low overcast currently in place beneath this feature shifting
eastward into the kilx County Warning Area on Thursday. Good upper forcing
combined with convergence along/near the approaching surface
trough will trigger scattered light showers across the
area...especially later tonight into Thursday morning. Once upper
wave shifts further east...showers will come to an end from west
to east by Thursday afternoon. Due to the clouds and
showers...temperatures will remain well below normal for this time
of year...with highs only reaching the lower to middle 60s.
High pressure will build into the Midwest for the end of the
week...bringing a period of dry but cool weather. Clear skies and
light winds will allow low temperatures Thursday night to drop into the
lower to middle 40s...with a few favored low-lying and rural spots
potentially dipping into the upper 30s. Sunny skies will prevail
beneath the ridge axis on Friday...with high temperatures remaining in
the 60s.
Next chance for rain comes into the picture on Saturday...as
surface ridge slides off to the east and a warm front develops
across the plains into the middle-Mississippi River valley. 12z 22
may models have slowed the initial approach of precipitation into the
mean ridge axis Friday night. As a result...have dropped probability of precipitation
entirely...except for just a slight chance after midnight
along/west of the Illinois River. Once front sharpens and
high pressure shifts further away...scattered showers/thunder
will spread across the western half of the kilx County Warning Area on Saturday.
With high pressure/drier airmass not that far off to the
east...have also cut probability of precipitation on Saturday. Opted to keep east-central
and southeast Illinois dry...with chance probability of precipitation mainly west of I-57.
Long term...Sunday through Wednesday
Big model differences still exist in the extended...however the
general synoptic picture indicates weak upper ridging over the
Midwest with a nearly stationary frontal boundary somewhere in the
vicinity. Exact placement of front will determine where the
greatest probability of precipitation will exist through the period. Latest GFS continues
to develop widespread showers/thunder along and north of the front
across mainly the northern half of the County Warning Area Saturday night into
Sunday. GFS then keeps boundary across the north through
Monday...before lifting it further northward by Tuesday in
response to digging upper trough over The Rockies. This solution
would focus highest rain chances north of I-70 Saturday night
through Monday night...with much lower probability of precipitation further south.
Meanwhile...European model (ecmwf) is quite a bit further south/southwest with
the boundary. It also develops widespread convection Saturday
night into Sunday...but generally keeps it focused across the west/SW
County Warning Area. It also maintains this frontal position longer...before
finally lifting it northward by Wednesday. With such a wide spread
in model guidance...confidence remains low in the extended. With
plenty of convection expected to develop along/north of the front
nocturnally...think the overall position of the synoptic boundary
may tend to be suppressed further south than the GFS is
advertising. Will therefore trend the forecast toward the European model (ecmwf).
As such...have probability of precipitation mentioned Saturday night through Tuesday.
After that...digging trough over The Rockies/plains will help build
the downstream upper ridge over the Midwest by the middle of the
week. This will finally give the surface boundary a good push
northward...resulting in warmer/drier weather by Tuesday night and
Wednesday.
&&
Aviation...
issued 600 PM CDT Wednesday may 22 2013
Coverage of MVFR and possibly some IFR ceilings late tonight into
Thursday morning the main forecast concern.
Secondary boundary to our northwest forecast to settle southeast
overnight tonight bringing lower ceilings and the possibility for some
isolated showers especially aftr 06z. The current instability showers
east of I-55 will affect mainly Champaign over the next hour. Current
forecast trends look reasonable at this time so am not planning on
any sig changes...other than pulling back the timing of the MVFR
ceilings into the area later tonight rather than this evening based on
current satellite and surface observations to our west. Once the
ceilings lower later this evening...forecast models suggest they may
hold over the area until aftr 19z Thursday. The gusty west winds
will diminish quickly aftr sunset this evening and range from 7 to
12 kts overnight...and then gradually shift into the north at 10
to 15 kts Thursday morning.
Smith
&&
Ilx watches/warnings/advisories...
none.
&&
$$
