Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
555 PM CST sun Mar 1 2015

issued at 330 PM CST sun Mar 1 2015

Skies are clearing across the upper Midwest as another seasonally cold
airmass from the Dakotas filters in. Ejecting energy from California
should arrive in upper Midwest within 36 hours and override our current
cold airmass. Analysis confirms a fast and open wave to limit precipitation
amounts middle week.


Short term...(this evening through monday)
issued at 330 PM CST sun Mar 1 2015

Short term forecast confidence assessment...good or above average with
little sensible weather issues. Verification suggests clouds may a few
hours too fast arriving Monday afternoon. Area mins and maximum temperatures
most location should be within 3 degrees with most spots possibly a
degree or two too mild tonight with clear skies and decoupling toward
morning. Highs Monday with more sunshine may need to be raised by at
least a few degrees for later shifts.

Tonight...clear to mostly clear with northwest winds of 5 to 10+ kts
diminishing to around 5 kts toward sunrise. Areas where winds fall to
less than 5 kts in west 1/3 to 1/2 of area may decouple and result in
lows at least 3 degrees colder due to our snow cover. Isolated below
zero readings may occur there...along and north of I-80.

Monday...mostly sunny to sunny with increasing high clouds between 1
and 5 PM. Arrival of high clouds by a couple of hours will impact highs
for later shifts. Winds to be light becoming light south toward late
afternoon. Highs of 25 to 30 degrees good compromise which will still
be about 15 degrees below normal.

Long term...(monday night through next sunday)
issued at 330 PM CST sun Mar 1 2015

Monday night...assessing the 12z model runs handling the southwest
flow system arriving this period...seems with the open wave/trough
aloft along with Stout flow through the vertical column/jets at all
levels...the system will be very progressive. Still vigorous and fed
seasonably high/180-200 percent of normal/ precipitable waters of 0.60 to 0.80 of
an inch...agree with the previous shift that most of the County Warning Area will
still get from 0.20 to around a half inch of quantitative precipitation forecast despite the speed
of the system before it winds down later Tuesday afternoon and early
evening from west to east. The problem/challenge continues to be
what type of precipitation and it/S duration/evolution through the County Warning Area.
With saturation and dry air initially in place will keep Monday
evening before midnight mainly dry. But Stout southwesterly low level jet of
40-50 kts will bring a seasonably impressive warm moist conveyor up
across and then east of the County Warning Area roughly from 06z-18z Tuesday...precipitation
should really Blossom across the local forecast area after midnight and
into Tuesday morning. Assessing a plethora of forecast soundings and evolving
vertical thermal profiles...trying to hammer out a type of ensemble
to use as a forecast...the latest thinking suggests warm wedge aloft
above 850 mb mb to build quickly after midnight. A brief period of
snow/sleet after midnight along and south of a Fairfield Iowa to
Monmouth Illinois line...then transitioning to sleet and freezing rain in
these areas with up to a tenth of an inch of ice glaze possible by
sunrise before the surface layer warms enough to make for all rain
profiles. Mainly snow along and north of i80...but the i80 corridor
to become mixed with sleet and then freezing rain after 10z Tuesday.
Areas north of i80 may get from a half over 1.5 inches of
wet snow accumulate by 12z Tuesday morning. Surface temperatures steady or rising
overnight especially after 2 am...temperatures to or even above freezing
along and south of Highway 34 by 10-12z.

Tuesday...freezing rain with sleet plume to continue to migrate
northward through late Tuesday morning with up to a tenth of an inch of
glaze possible from i80 to just north of Highway 30 by middle morning
before those areas too transition to all rain with warming surface
layer and rising surface dewpoints. The Highway 20 corridor may hang on to a
wintry mix of snow...sleet and maybe some brief bouts of freezing
rain through midday. Will go with an additional half inch to 1.5
inches of wet snow along and north of Highway 30 from 12z-18z Tuesday. Thus
system snow totals right now look like from 1-3 inches north of i80
with the higher amounts generally north of Highway 30. The moderate rain
band may be exiting off to the east and northeast by 18-19z Tuesday
taking into account the system/S progression and arrival of a type
of dry slot from the southwest. Brisk southerly surface flow across the County Warning Area
just to the Lee of the passing surface cyclone may gust over 25 miles per hour Tuesday
morning...but drive surface temperatures into the upper 30s to around 40 for the
south half of the County Warning Area. May still have to watch out for some advection
fog over the snowpack in areas where the precipitation wanes or before it
begins late Monday night and on Tuesday. The cold front barrels it/S way
eastward through the County Warning Area Tuesday afternoon with a large surge of northwest
winds from middle to late afternoon again with a west-to-east
progression across the forecast area. Gusts could approach 40 miles per hour or
higher by Tuesday evening. Top-down cooling really accelerates on the
latest guidance...but again agree with the previous shift that bulk
of the precipitation will be exiting out of the County Warning Area or light/thanks also
to accelerated top-down drying/ for just minor additional snow
accumulation in the afternoon where the lingering precipitation switches
back to snow. Tuesday night...windy with increasing low level cold advection
but dry....lows down in the single digits by sunrise Wednesday. All in
all...headlines for a wintry mix late Monday night into the first
half of Tuesday will probably have to be finalized and issued in the
coming shifts.

Wednesday through Friday...Wednesday a raw windy day with highs only in
the teens even with some insolation...high clouds off overrunning
snows across MO/southern Illinois may blanket the southern County Warning Area on Wednesday. Clear
and very cold under Canadian high pressure Wednesday night into Thursday
morning...much of the area with potential of sub-zero lows and even
Wind Chill Advisory criteria north of i80 by Thursday morning if surface
winds can maintain 5-10 miles per hour. Broad northwesterlies flatten some but
still shunt the main clipper path to the north and east of the dvn
County Warning Area by weeks end. The net result is the region will look to get
engulfed by the warm draw side of any clipper low pushing into the
Great Lakes by Friday...highs may get into the upper 30s to around 40 by
late Friday afternoon.

Next Saturday and Sunday...the County Warning Area will still look to remain on the
milder and mainly dry side of the clipper track into Sunday. ..12..


Aviation...(for the 00z tafs through 00z Monday evening)
issued at 542 PM CST sun Mar 1 2015

Mainly VFR conditions next 24 hours as high pressure moves across the
region. There is a chance of patchy fog with MVFR to local
IFR visibilities later tonight and early Monday am if winds were to diminish
more than anticipated. Confidence not high enough to include in
tafs attim. West to northwest winds generally around 5-10 kts will
become light and variable middle am through midday before shifting to
the east/southeast 5-10 kts during the afternoon as the high shifts east. Looking
out just beyond the end of the taf cycle... conditions are expected
to deteriorate to MVFR and IFR late Monday night through Tuesday as
a storm system brings a wintry mix of precipitation to the region.


Dvn watches/warnings/advisories...


short term...Nichols
long term...12

National Weather Service Glossary of Abbreviations