Area forecast discussion
National Weather Service Lincoln Illinois
222 PM CDT Monday may 20 2013
Discussion...
issued 222 PM CDT Monday may 20 2013
Severe weather chances continue to be the main focus...with rain
chances lingering through Thursday. A cooling trend will develop
middle week with below normal temperatures for late week and into the
weekend.
Short term...tonight through Wednesday night.
The atmosphere started out the afternoon a bit more stable than
model guidance had indicated. MUCAPE values are in around 500 j/kg
with some convective inhibition of 150-200 j/kg. Surface dewpoints and temperatures
are on the rise and increasing middle level jet winds to the
southwest should still to nose into central Illinois by evening.
The onset of severe storms may be slightly delayed due to the time
it will take for the ingredients for storms to develop. The
evolution should be for discreet supercells forming in Missouri
and advancing east along the axis of instability from SW to NE.
The directional shear in the plains will evolve to more
unidirectional shear across central Illinois. That will help the
supercells to evolve into more of a linear setup, with straight-
line and downburst winds more likely. Embedded supercells will
still be possible due to ambient vorticity near pre-existing
convective boundaries all over central Illinois.
The evening hours look to be our primary severe window...with a
downward trend in severe after midnight as the storms head into
our eastern counties. The 4km hrrr is depicting the northwest half of our
area to have the best chances for storms...with linear storm
segments developing after midnight in the southeast half...and a
more isolated severe threat but still present.
The severe potential on Tuesday will be tied to the advancement of
a 500 mb shortwave and resultant surface cyclogenesis from the
Southern Plains across central Illinois. The primary forcing for storms
looks to hold off until Tuesday evening...when the wave will moves
into western Illinois. Storms are expected to erupt across eastern MO
and SW Illinois in the evening and progress to the east-NE overnight. We
severe potential should be higher southeast of I-55...based on the
track of the surface low and location of the 850 mb low level jet.
Storms should be primary east of our area by sunrise
Wednesday...but additional showers and a few storms will be
possible as cold air advances into Illinois. Various shortwaves will be
rotating across Illinois as the upper trough axis moves into
western Illinois.
Long term...Thursday through Monday.
On Thursday...the slow movement of the upper level and surface
lows from the northern plains across the Great Lakes will cause
showers to linger across central Illinois. The axis of the elevated cold
pool aloft will pass across Illinois around middle-day Thursday. Lapse
rates appear favorable for some instability showers to develop in
the heat of the day...with a little help from channeled vorticity
in the Lee side of the upper trough.
Clearing skies Thursday night will allow lows to dip into the
upper 40s north of a line from Rushville to Lincoln to Champaign.
Despite sunshine on Friday...high temperatures will struggle to reach
around 70 in most of the area.
The 00z/20 European model (ecmwf) had a wet Holiday weekend forecast...with a dry
forecast from the 00z/GFS. They appear to have flip-flopped the
forecast with the 12z versions. The 12z GFS now showing rain
chances late Saturday through Memorial Day. The European model (ecmwf) still has
some rain late Friday night into Saturday...but dry conditions for
Sunday and Memorial Day. The allblend has basically kept some
mention of rain due to the present signal from at least one model
each day of the weekend. The main theme remains that a wave of
energy is forecast to exit the plains early in the weekend but
where the front stalls out for lingering rain remains to be seen.
Shimon
&&
Aviation...
issued 1246 PM CDT Monday may 20 2013
Cluster of thunderstorms currently crossing the Mississippi River
just south of Quincy will track east/NE into the area early this
afternoon. Airmass is beginning to destabilize in advance of this
convection...so confidence is growing that it will hold together
and arrive at kspi between 19z and 21z. Its evolution beyond that
still remains in question...so will only carry thunderstorms in the vicinity after 21z at
the remaining terminals. Outside of any convection that develops
this afternoon...strong southerly winds will gust to between 25
and 30kt. Models disagree about the timing/placement of additional
convection later today and tonight...however the hrrr continues to
focus the most widespread storms during the evening into the
overnight hours. Will therefore keep tempo groups for thunder
between 02z and 06z at kpia and between 04z and 08z further east
at kcmi. After that...mid-level drying in the wake of the
overnight convection will lead to a dry Tuesday morning.
Barnes
&&
Ilx watches/warnings/advisories...
none.
&&
$$
