Area forecast discussion
National Weather Service Lincoln Illinois
1134 PM CDT Tuesday may 21 2013
Discussion...
issued 830 PM CDT Tuesday may 21 2013
Latest surface analysis indicates a weak cold front located over
parts of west central Illinois early this evening. Had some
isolated showers develop just to the east of the boundary which
have affected mainly our northern counties. Meanwhile to the
south...several large thunderstorm complexes were tracking east
along and south of the Ohio River with the northern fringes of
the storms edging into our southeast counties.
A shortwave trough noted on the water vapo loop over Oklahoma this
evening is forecast to rotate NE towards our area by Wednesday
morning spreading showers and isolated thunderstorms northward
into most of our area. Wondering how much of an affect...if
any...the mesoscale convective system activity to our south will hold back the moisture
feed north into our area ahead of the Southern Plains upper wave.
Looks as if the last few runs of the hi-res rapid refresh is
pulling back some from its previous forecasts in bringing in a
large area of showers and storms...at least until the upper wave
moves into our area Wednesday morning...about 6 hours slower than
what we were seeing earlier today.
Have made some minor adjustments to the grids with respect to the
two areas of rain currently affecting our north and south...but
overall...the forecast looks on track for the overnight hours into
Wednesday morning. Should have the updated zones out by 900 PM.
&&
Aviation...
issued 1120 PM CDT Tuesday may 21 2013
Main concern this forecast period will be the coverage of showers
and isolated thunderstorms and rain overnight and into the day on Wednesday as an
upper level wave drifts across the region. Other than some isolated
showers that moved across the area over the past few hours that has
been about it rain-wise. VFR conditions continue and are expected
to hold into at least Wednesday morning before a slow moving cold
front tracks through the taf sites bringing some scattered showers
and ceilings at or just above MVFR Cat. Satellite and surface observation to the
northwest of the surface low currently over Iowa and Wisconsin showing
a large area of MVFR ceilings and it appears as the low shifts off to
our east by later tomorrow...some of those lower ceilings will shift
southeast into our area...especially tomorrow night. For now...will keep
the ceilings at or just above MVFR for now but most sites may see
the lower ceilings move in aftr 00z Thursday. Surface winds will remain light
south-southwest tonight at 10 kts or less...and then veer more into a westerly
direction on Wednesday at 10 to 15 kts.
Smith
&&
Previous discussion...
issued 300 PM CDT Tuesday may 21 2013
After several warm and humid days...we are headed toward an
extended period of at or below normal temperatures as the occluded low
over the Minnesota/South Dakota border finally becomes progressive and departs
eastward through the Great Lakes in the next couple days.
Short term...tonight through Friday.
The latest surface analysis shows the synoptic cold front entering
western Illinois at 19z. Satellite loops show a train of 3 storm
complexes from Oklahoma to Tennessee moving east. Our area will
see one more round of showers and storms tonight...but no severe
storms are expected. The noon update to the day one outlook from
Storm Prediction Center pushed the northern edge of the slight risk area south into
the Ohio River valley. That seems to be in response to marginal
instability and wind shear north of the storm complexes to the
south. Also...those systems will likely cut limit our supply of
moisture on the southerly low level jet.
We still kept categorical probability of precipitation in the majority of our counties
tonight as a wave of low pressure lifts northeast along the cold
front overnight...with its path set to be roughly up the I-55
corridor. East of that track we may see some storms develop...with
gusty winds, small hail and locally heavy downpours. The 4km hrrr
shows steady rains mainly southeast of I-55 tonight...while the
NCEP 4km-WRF-nmm shows widespread rains developing by 06z all
across our counties and lingering through Wednesday morning in the east.
Our regular deterministic models were generally in agreement with
that progression as well.
As the cold front finally departs into Indiana on
Wednesday...showers will still linger in central Illinois. An isolated
thunderstorm may develop in the east...but dry and stable air
should begin to shut down convective updrafts in the SW half of
our area by Wednesday evening. Shower chances will continue on
Thursday as the upper cold pool slides across Illinois. Steep lapse
rates will help fuel some diurnal showers...with help from a 500
mb shortwave rounding the base of the upper trough.
Dry and cool conditions should return to the area for Thursday
night and Friday. Lows Friday morning will dip into the low 40s in
much of the area...with afternoon highs in the upper 60s.
Long term...Saturday through Tuesday.
Saturdays temperatures will remain below normal in the upper 60s to
around 70...as clouds and showers return to central Illinois...according
to the European model (ecmwf). The GFS keeps the Holiday weekend dry until late
Saturday night into Sunday morning then keeps rain through
Memorial Day. The European model (ecmwf) on the other hand shows the opposite
trend. It has wet conditions Friday night through Saturday
night...then dry Sunday through Monday as it pushes the cold front
south of Illinois. The allblend has been influenced by both
solutions...so the extended is full of chance probability of precipitation. The Canadian
is more closely following the GFS solution...so there appears to
be more momentum toward a wet end to the Holiday weekend.
&&
Ilx watches/warnings/advisories...
none.
&&
$$
