Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
524 am CST Monday Jan 26 2015

issued at 328 am CST Monday Jan 26 2015

Northwest flow aloft...with another clipper poised in northern
Minnesota is in place early this morning. At the surface...a very
narrow surface ridge is right over our County Warning Area...with the only calm
winds in the Midwest found from southeast Iowa through southeast
Wisconsin. Under a narrow sliver of clear skies between the cirrus
to the southeast...and extensive stratus to the west...we have
sharply radiated out over the fresh snow from the quadrant cities to
Dubuque. As of 2 am...likely near our lows...readings as cold as the
single digits are taking place along this axis...while temperatures
just to the west in the stratus are holding near 30. The stratus is
found along a extremely sharp warm front just to our west enhanced
by down slope conditions in the Western Plains.


Short term...(today and tonight)
issued at 328 am CST Monday Jan 26 2015

The clipper upstream will be acting on this strong frontal
boundary...and with stratus in place...and strong lift arriving in a
narrow axis on the cold side of the boundary today...we will likely
see snow breaking out just on the cold side of the boundary. That
means snow for eastern Iowa and northwest Illinois this morning. It
should not last long...but with strong lift...some light to moderate
snow may fall for around 1 to 2 hours for location along and east of
Interstate 380. The snow may fall moderately and dendritic based on
model data...but the short duration should make most of the day dry
for nearly all locations. That said...with the morning start of the
band of should accumulate...and a half to 1 inch seems
possible. We plan to handle this with Special Weather Statement products and social media
posts given the short fused impacts this morning.

After the wave moves through in the west...we will see significant
warming today. Highs will be a challenge...but will conservatively
go with lower to middle 40s west...and hold the east in the upper 20s.
Having a tight boundary with clouds over head offers up a huge bust
potential by follow short term trends of the front
locations today for changes in our forecast.

Tonight...lift in the far northeast should end by 9 PM...with
flurries or freezing drizzle at the tail end of the event as we lose
ice introduction into the lower clouds. This sets up a mainly dry
night...with mixing continuing through the night and mainly cloudy
skies east. Thus...lows should only fall to the middle 20s east to
lower 30s west overnight.

Long term...(tuesday through next sunday)
issued at 328 am CST Monday Jan 26 2015

Tuesday and Wednesday...latest suite of 00z run medium range model
solutions surface ridge to pass east across the area Tuesday into Tuesday
evening...with stratocu field along and east of the MS river also
making headway east or decreasing some diurnally to allow for some
insolation. But forecast soundings suggest weak flow/mixing
scenario...and along with lingering snow cover will side with cooler
temperature guidance across the east/northeast...but will go closer to the
warmer European model (ecmwf) MOS in the west and south where some afternoon return
flow may have an effect. Backing low to middle level flow really starts
to tap into plains thermal ridge and warm air advection Tuesday night making for a
potentially broad warm wedge/inversion aloft in the h9-h65 mb layer
into Wednesday morning. Will keep the forecast dry...but signs of at least a
warm air advection Wing of middle deck clouds moving across the region
ahead of the warm push aloft.

Western ridge-riding wave will look to dig down across the plains late
Tuesday night into Wednesday...trying to phase with southern stream energy. Of
course the various 00z model runs all have their differences in
handling of this process...but the general idea is that what ever
wave develops and propagates across the MO River will induce
a strengthening low level warm air advection Mode across the local area by
Wednesday. For now will side with the more modest of warm up potential. But
this still suggests widespread upper 30s to middle 40s across the
northeastern half of the County Warning Area...with the far southwest breaching the
50 degree mark. The 00z European model (ecmwf) takes the developing surface low associated
with the approaching upper trough right across central Iowa into northern
Illinois...with the 00z GFS a bit further south along the Iowa/MO border into
central Illinois Wednesday into Thursday morning. Models still having a time picking
up on eventual moisture return and saturation that this wave will be
able to utilize...but currently indicated lift/strength sweeping across
the area suggests that even modest moisture return should be able to
produce light to pockets of moderate precipitation. Timing looks to be
mainly Wednesday night with mainly rain in warm air advection pattern for first half of
the night. Then depending on wave passage timing...low level cold
conveyor wrap in behind the well as dynamical top-down
cooling for a potential switch over to a wintry mix in the
northwestern third to half of the dvn County Warning Area toward Thursday morning. Will
keep low to moderate chance probability of precipitation this period to mark the window of
precipitation potential. Current system timing suggests lows Wednesday night will
occur at dawn...after holding up well above freezing for most of
the night.

Thursday and Friday...Thursday could be a raw day with sctrd snow showers
and flurries in the wake of the passing low pressure system and in
increasing low level cold air advection pattern. Tight pressure gradient
behind the departing wave may also make for a blustery day with
brisk northwest surface winds gusting well over 30 miles per hour. Morning highs middle to
upper 30s...then falling as the day progresses. Then it appears a
passing bout of both surface and upper ridging to make for dry and
seasonable conditions for the end of the week.

Next Saturday and Sunday...longer range indications/ensemble trends
suggest West Coast upper ridge amplification and almost a type of
temporary Rex block development to carve out a cold core l/west trough
across the Great Lakes and into the northeastern Continental U.S.. latest runs
suggest brunt of potential Arctic air surge to the northeast of the
region or just clipping the middle/upper MS River Valley over the
weekend...with a clipper type system rippling along the process
somewhere across the upper Midwest. Timing problematic at this point
but the latest solutions suggest a potential snow chance in or
near the local forecast area either Sat or Sat night. Another one Sunday
night into Monday...then signs of another Arctic surge further west
and south which may target the region early next week...the latest
European model (ecmwf) has -22 to -26c by Monday evening...the 00z GFS not quite as
cold but still targets Monday into next Tuesday with a Canadian air mass
dump. ..12..


Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 517 am CST Monday Jan 26 2015

A fast moving round of snow will spread through all terminals this
morning. While MVFR ceilings will persist through the period between
1200 and 2000 feet...this short lived 2 to 3 hour event should bring
IFR conditions with snow between 13z and 17z...earliest in
northeast Iowa and latest in southeast Iowa. After the snow
ends...there may be some light freezing drizzle in far northern
Illinois through 03z/27. All precipitation should be done in
terminals by 18z today. Once again...MVFR stratus should linger
through the remainder of the period after the snow ends.


Dvn watches/warnings/advisories...


short term...Ervin
long term...12

National Weather Service Glossary of Abbreviations