Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
325 PM CST Wednesday Dec 17 2014

Synopsis...
issued at 323 PM CST Wednesday Dec 17 2014

12z upper air analysis has an 850mb ridge running from the Gulf Coast into
the northern plains. A storm system was over southern Canada with a
low level jet running from West Texas into the Central Plains. Satellite
trends through middle afternoon show clearing skies across the area
with clouds from the next system over the central and Southern
Plains.

18z surface analysis has high pressure over the northern plains and
upper Midwest. A weak low was in southeast Colorado with an implied
boundary running from the low into southwest Arkansas. Dew points
were in the single digits and teens across the northern plains and
upper Midwest with 20s and 30s from the Ohio Valley into the
Southern Plains.

&&

Short term...(this evening through thursday)
issued at 323 PM CST Wednesday Dec 17 2014

Quiet conditions with clear or clearing skies will be seen across
the area through late afternoon. Dry conditions will then be seen
tonight and into the pre-dawn hours with increasing clouds. Snow
will develop and spread northeast through Kansas and Missouri.
However...dry air across the area will prevent any snow from
reaching the extreme south prior to 6 am Thursday.

Lows tonight should drop quickly after sunset and then may become
steady or rise slightly as clouds move back into the area.

On Thursday...strong forcing will go into saturating the dry air at
the low levels and allow snow to move into the far southern areas.
The forcing rapidly collapses by late morning with minimal forcing
in the saturated atmosphere across the south third during the
afternoon.

Thus the opportunity for accumulating snow will occur Thursday
morning in a 4-6 hour window. The best chances for any accumulating
snow appear to be south of a kcnc...kfsw...to kbmi line or south of
a Chariton Iowa to Bloomington Illinois line. Accumulations will be
under an inch but amounts approaching an inch may be seen across the
extreme southern parts of Scotland/Clark counties in Missouri and
possibly the southwest corner of Hancock County Illinois.

Data does suggest the potential for a narrow band of snow in strong
forcing to set up near a Kirk to KUIN line. A northward shift of
this band would place it across the extreme southern parts of the
area with the potential for slightly higher amounts.

However...if the dry air is deeper than depicted in the models...
then the snow band would be much further south of the area and snow
amounts across the extreme south would be much less.

The Key Point is that there will be a very sharp gradient between no
snow and accumulating snow on Thursday.

Long term...(thursday night through wednesday)
issued at 323 PM CST Wednesday Dec 17 2014

Overview...zonal 500 mb flow will keep things quiet through Sunday.
Upper level disturbance associated with the subtropical jet will
eject out of northern Mexico and track across the southern U.S.
Through the weekend...remaining well south of Iowa. A large Pacific
jet streak will carve out a trough over the central U.S. Beginning
Monday and persisting through next week. Light rain and snow will be
possible on Monday across the area.

Friday and Saturday...very quiet period with high temperatures in the 30s
along with dry weather. A weakening trough axis will move through
Friday night into Saturday morning which has decreasing definition
form the middle-levels to the surface. Period of weak negative Omega is
forecast Saturday morning as DCVA moves through but layer just below
the lift at 600 mb is very dry during this time with relative humidity near 30%.

Sunday...surface winds will begin to increase out of the southeast in response
to a Lee-side surface low organizing over east Montana and the Dakotas.
Highs will once again be in the middle 30s to near 40 f.

Monday...period of rain and snow possible as low pressure center
remains northwest of the area across SW Minnesota and north Iowa...not a
favorable location for much accumulating snow to occur across the
dvn forecast area.

Timing and p-type issue exists between the GFS/ECMWF. European model (ecmwf) is
cooler and ~6-12 hours slower (than the gfs)...it has the 850 mb 0 c
isotherm to the south of the County Warning Area into the nighttime hours with
precipitation lingering past 00z/Tuesday. The GFS has 850 mb 0 c isotherm
over northern County Warning Area and has most precipitation over with by the afternoon. European model (ecmwf)
has low-level warm air advection raising 925 mb temperatures to +1 to +3 c by the evening
and both models forecast 2 meter maximum temperatures in the middle to upper 30s.

Forecast soundings have warmest temperatures in the lower portion of the
boundary layer which actually slowly increase through the day.
Therefore may have -ra/-sn mix or -sn transition to -ra through the
event with best chances for -sn across the north.

Tuesday and Wednesday...a strong low will likely form during this
time along a developing baroclinic zone over the southern U.S./Ohio
Valley and then move into the Great Lakes area. Model consensus for
the dvn County Warning Area is colder and breezy with scattered snow showers.

The European model (ecmwf) is showing a strong low pressure system developing across
Alabama and Tennessee Tuesday night and then lifting northward to
Lake Huron by Wednesday night with a central pressure down to 961
mb. The GFS brings it to the same area but is further southeast during the
cyclogenesis phase. It will be interesting to see the models work
out the polar-subtropical jet phasing issues over the next few days.

&&

Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1133 am CST Wednesday Dec 17 2014

MVFR ceilings will slowly clear across eastern Iowa and northern
Illinois through 00z/18 as high pressure builds into the Midwest.
After 00z/18 VFR conditions are expected through 12z/18. After 12z/18
snow will slowly move into southeast Iowa producing MVFR and
possibly IFR conditions.



&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&

$$

Synopsis...08
short term...08
long term...uttech
aviation...08

National Weather Service Glossary of Abbreviations