Area forecast discussion
National Weather Service quadrant cities Iowa Illinois
643 PM CDT Wednesday may 22 2013
Synopsis...
issued at 246 PM CDT Wednesday may 22 2013
Latest surface analysis was indicating an occluded low just west of
mke WI...with secondary feature draping back toward Waterloo. Surface
pressure rises increasing across northwestern Iowa...with a falls maximum
pushing across The Heart of the Great Lakes. Aloft...water vapor loop
was indicating close upper low across north central Iowa at this time with a
slow gyration to the east-southeast and wanting to open up. Low level
cold conveyor evident on western flank of this feature with temperatures
in the 50s across much of the western half of Iowa.
&&
Short term...(this evening through thursday)
issued at 246 PM CDT Wednesday may 22 2013
Tonight...expect sctrd showers and possibly an isolated thunderstorm
to increase from the west through early evening...but showers to
not become more widespread across the northern/northwestern third of
the dvn County Warning Area out of Minnesota/WI not until after 03z as upper level wave
pivots around the parent trough and then down across the County Warning Area as the
night progresses. Forcing and column saturation suggest a few
areas north of i80 may get up to 0.15 of an inch of rainfall by
12z Thursday...otherwise the majority of areas to receive under a
tenth. Top-down drying from the northwest not as aggressive looking
on current 12z forecast sounding forecasts as yesterday at this
time...thus will keep all shower wording or a hybrid of shower-
light rain late tonight as opposed to drizzle. But there may still
be light precipitation similar to drizzle or sprinkles along Highway 20
toward daybreak. Lows in the upper 40s to lower 50s in the
northwestern third...to the middle 50s in the southeast.
Thursday...as upper wave passes off to the southeast and top-down
column drying really takes grip of the region...lingering light
rain/showers to push off to the southeast/decay as well by late
morning...with clouds clearing from north to south during midday and
in the afternoon. Seasonably impressive Canadian back door high to
dump down the western Great Lakes and upper MS River Valley Thursday
afternoon...with incoming gradient and mixing causing veering wind
profiles to the north-northeast to mix to 10 to 20 miles per hour during
midday and much of the afternoon before wind drop off and decouple
commences toward Thursday evening. Some higher gusts may be possible
Thursday afternoon as well. Incoming cool air mass supports highs only
in the low to middle 60s for Thursday...but expect afternoon insolation
and deeper mixing to produce some upper 60s or even a 70 in a few
spots. ..12..
Long term...(thursday night through wednesday)
issued at 246 PM CDT Wednesday may 22 2013
The extended period will begin with a benign and cool pattern...and
transition to an active and gradually warmer weather regime from the
weekend through next Wednesday. High pressure will continue to build
in Thursday night...with clear skies and light winds. This will
provide an excellent radiational cooling night...and lows in the
upper 30s northeast to lower 40s central and southwest are well
justified by model data and analysis.
This cool high pressure remains over the Midwest Friday...and this
should provide a stellar Friday. Cool air aloft...light winds...and
sunshine is expected...thus the intense sunshine of late may should
struggle to boost temperatures through the middle to upper 60s.
Friday night through the weekend...there are positional differences
in the elevated boundary placement between the southern European model (ecmwf)...and
the more northerly GFS/Canadian/NAM. This seems to Stem from how
the European model (ecmwf) differs in handling how much energy is spinning around the
East Coast low pressure. In the wake of the super energetic East
Coast low...high pressure is stronger in the upper Midwest. Thus...a
more southerly boundary placement. So it becomes a question on how
much rain potential we have...may be answered more directly by
finding how much of a handle models have on the evolution of the
East Coast low pressure. Toss in the likelihood that models are
overdone initially on moisture in the plains due to lower than
normal crop progress...and it seems to ME we have less cape at low
levels than any model suggests to our south...and there is good
potential for a dominance of elevated showers and storms. Thus...a
solid moderate to high chance for showers and elevated storms
throughout Iowa and Illinois Friday night after midnight into at
least Tuesday morning. Will keep probability of precipitation mainly below likely ranges at
this point due to the aforementioned uncertainties.
Looking at temperatures...we should only be restricted to the 50s
Saturday and Sunday if it is a total washout. This seems
unlikely...and will go lower 60s to middle 60s Saturday...and middle 60s
to around 70s south Sunday. After this...will side closer to the
more northerly GFS/Gem and go milder Monday through Wednesday...with
highs rising through the 70s to near 80 by Wednesday. Lows will be
mainly in the 60s.
Ervin
&&
Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 637 PM CDT Wednesday may 22 2013
Low pressure system over southern Wisconsin has spread low end VFR
to MVFR clouds over the terminals with scattered showers with MVFR
visibilities in light rain. Expect these scattered showers to diminish in
coverage and should be gone by 02z. However...a main band of light
rain will rotate across the area between 05z and 15z with a band
of widespread IFR ceilings and visibilities. After 15z we should return to
MVFR ceilings and VFR visibilities as the light rain moves out of the area to
the south. After 05z tonight winds to increase out of the north
sustained speeds around 15kts and gusts to 20kts...continuing
until about 15z before falling off Thursday afternoon. Le
&&
Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&
$$
Synopsis...12
short term...12
long term...Ervin
aviation...le
