Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion...corrected 
National Weather Service quadrant cities Iowa Illinois
319 PM CDT Tuesday Apr 21 2015

issued at 319 PM CDT Tuesday Apr 21 2015

Another windy and cool afternoon across the region courtesy of the
tight pressure gradient around a slow moving...nearly stacked...low
pressure system over southeast Ontario. West winds were sustained
from 30 to 35 miles per hour with occasional gusts over 50 miles per hour this afternoon
along and north of I-80 with slightly lower speeds of 20 to 30 miles per hour
with gusts into the 30s over southeast Iowa...NE MO and west central Illinois. Cold
air aloft has resulted in scattered to broken strato cumulus...which has
been thickest across the north. Temperatures have reached only the
upper 40s north...while 50s covered eastern Iowa into west central
Illinois...with a few lower 60s in the far south. Area radars showed
elevated returns developing in western Iowa and NE Nebraska ahead of a weak
middle level impulse embedded in the tight cyclonic flow aloft.


Short term...(this evening through wednesday)
issued at 319 PM CDT Tuesday Apr 21 2015

Much like last night...strong winds will undergo a diurnal weakening
trend and there will be a potential for scattered showers over
especially the south early. Wednesday...windy and cool conditions
will resume...but winds should be considerably lighter than today as
the low to the northeast begins to weaken and shift eastward.

On-going strong winds will gradually diminish and will keep the
current Wind Advisory in place over the NE half of the forecast
area. Winds should drop below advisory criteria by the 00z
expiration time.

Tonight...will keep slight to low end chance
probability of precipitation going over the south as middle level forcing increases ahead of
the advancing middle level impulse and interacts with the daytime
heating/middle steep middle level lapse. These have the potential for
quick bursts of a few hundredths of an inch rainfall before
dissipating and moving out by middle evening. Overnight...dry air and
mixing in west to to northwest winds that should remain sustained form 10
to 15 miles per hour will likely prevent any significant frost...despite
temperatures dropping into the lower to middle 30s. A light freeze is
likely along the north of the Highway 30 corridor.

Wednesday...northwest winds will pickup increase to again to at
least 15 to 25 miles per hour with gusts into the 30s by late morning and
afternoon in deep mixing. Mixing to nearly 700 mb in the even
colder airmass aloft will be needed to warm temperatures into the

Long term...(wednesday night through tuesday)
issued at 319 PM CDT Tuesday Apr 21 2015

and possible frost will lead off the extended Wednesday night
followed by a slow moderating trend...into a wetter regime for late

Winds will subside under 15 miles per hour Wednesday night...but with cold
aloft and the high pressure ridge remaining just west of the
County Warning Area...we should not totally decouple through the night. In any
case...lows in the upper 20s northeast to middle 30s southwest look on
target...and with lighter winds than the past 2 nights...some patchy
frost does appear likely. An advisory is certainly possible...but
given our current advisory for wind...we will not issue one at this

Models in the extended remain focused on a slowly progressing deep
trough...over the Midwest from Friday through Sunday. While there had
been significant discrepancy of the northward extent on the 00z
model suite...the 12z suite is now in much better agreement in
brining rain and rain showers over at least the southern 2/3rds of
Iowa and Illinois Friday PM through Saturday night. Whether this
system lingers through Sunday is still a possibility...but most
solutions show the wave dropping east and southeast by that time.
The European model (ecmwf) remains the most bullish...with around 0.5 to 1.0 inch
of rain forecast. The GFS does show convection...and that does
seem unlikely given a large cool airmass over the Midwest and
Great Lakes. A stratiform rain is most likely. This stratiform
scenario does support higher probability of precipitation...and will introduce likely probability of precipitation
now for Friday night. Should the ecwmf be correct...Saturday also
would need widespread probability of precipitation.

With the rain potential growing for Friday and
Saturday...temperatures will be held to the lower to middle 50s for
highs. Should we get into a consistent all day rain...40s are not
out of the question with cool east winds. After this system
exits...milder air appears likely for Sunday through Tuesday...with
a low chance for rain early next week. This is due to a model
blend...including 00z European model (ecmwf)/12z GFS which show another wave towards



Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1244 PM CDT Tuesday Apr 21 2015

Strong winds around a slow moving low pressure system over the
Great Lakes will continue through the afternoon. West winds from
25 to 35 kts will gust as high as 48 kts through middle afternoon...
then begin to diminish. Much lighter west winds are expected for
the evening and overnight hours with conditions remaining VFR.
Winds will increase by middle morning Wednesday morning close to 15
to 25 kts.


Dvn watches/warnings/advisories...
Iowa...Wind Advisory until 7 PM CDT this evening for Benton-Buchanan-
Cedar-Clinton-Delaware-Des Moines-Dubuque-Iowa-Jackson-

Illinois...Wind Advisory until 7 PM CDT this evening for Bureau-Carroll-
Henderson-Henry Illinois-Jo Daviess-Mercer-Putnam-Rock Island-



short term...sheets
long term...Ervin

National Weather Service Glossary of Abbreviations