Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Des Moines Iowa
534 am CST Sat Nov 28 2015

Short term /today/...
issued at 409 am CST Sat Nov 28 2015

Surface high pressure centered over the siouxland will move little
today...placing much of the County Warning Area in light northerly flow. Upper level
pattern has changed very little in the last 24 hours. The region
remains under broad southwesterly flow with a 100-110 knots jet tapping
into tropical moisture from the eastern Pacific. A large cutoff low
remains anchored over Nevada...ejecting intermittent weak
disturbances into the central US. One such wave is currently over
western Kansas on water vapor imagery and has generated light snow
showers over southeast Nebraska early this morning. Southern Iowa
sounding profiles are quite dry below 800 mb...which combined with
deep subsidence coincident with a 500 mb ridge...should lead to
general weakening of these showers. Thus expecting only a very
brief period of snow or freezing rain as these showers move
through southern Iowa this morning. Otherwise...look for clouds to
hang on in the south through the day with clearer skies closer to
the high.

Long term /tonight through Friday/...
issued at 409 am CST Sat Nov 28 2015

Primary concern will be precipitation influences gradually increasing
through the weekend...peaking around Monday as current Nevada/Utah upper
low finally ejects into the MO valley. Patchy forcing will already
be passing through the region in SW flow at onset...building into
Sunday. Examination of various isallobaric layers suggests this
forcing will mainly be kinematic and somewhat lower based than
what typically occurs with this type...more in the 850 mb/700 mb layer.
Forecast soundings show there is little moisture on either side of
this layer precluding any ice introduction for the time
being...and also limiting precipitation that may be reaching the surface.
It is difficult to quantify how much of this could potentially
reach the ground...if any...but have leaned toward a quantitative precipitation forecast based
consensus suggesting southern locations would be most susceptible. This
would mainly be sprinkles rather than drizzle with ceilings still
fairly high based...and could be freezing in some spots depending
on surface temperatures. And speaking of surface temperatures...MOS temperature range
seems to extreme...especially for mins. Clouds...precipitation and a
period of Ely winds should limit radiational cooling so little
diurnal curve expected into Monday with raw model temperatures more favored.

Forcing will then increase sharply SW-NE later Sun night and
especially into Monday. Details are still yet to be determined with
somewhat diminished confidence. The 12z European model (ecmwf) trended farther north
and 00z GFS farther south...away from each other. However the 00z
European model (ecmwf) is now going a bit farther south toward the GFS...but the GFS
still seems like somewhat of an outlier and farther south than
consensus. This may be a moot point however as 00z European model (ecmwf) quantitative precipitation forecast is
still quite is the NAM...and based more on forcing
ahead of the low rather than duration of deformation zone precipitation
like the GFS. Most scenarios have a pronounced intense synoptic
rise/fall couplet moving through Iowa Monday with deep and intense
mainly kinematic forcing. Cannot find significant thermodynamic or
frontogenetic contributions. This still results in moderate to
heavy snowfall over the northwest half of the forecast area...with
preliminary and somewhat conservative amounts for now eclipsing
seven inches Denison to Estherville...and a several County band of
a few hundredths of freezing rain in the mixed precipitation transition
zone farther south and west. The snow ratios with this event are
quite more than climatology /13 to 1/ with the GFS with the
dendritic lift zone /dgz/ no too deep and potentially not often
phased with peak Omega or adequate moisture. The NAM is quite
higher with maximum lift coincident with the dgz. European model (ecmwf) Omega is not
available and unknown but its dgz relative humidity suggests the favored
residency time would be less than the NAM or GFS. Its quantitative precipitation forecast is still
fairly high however suggesting a shorter but more intense burst of
precipitation Monday.

Some things to watch for in this storm will be lower static
stability noted by the NAM...which actually shows some token
MUCAPES and potentially thunder...and the GFS which has an
isothermal layer just warmer than -5c in the lowest 1.5km which
could cause the snow to be somewhat wet and sticky. Increasing
winds Tuesday could potentially produce some minor tree issues in this
case. However it needs to be emphasized that all these details
/track...associated quantitative precipitation forecast...dgz Omega phasing...etc/ could cause big
forecast variances moving forward with the short wave that will
kick the Nevada/Utah closed low out still well off the California coast this
morning. The models may not have a better handle on things until
sun runs when it comes on shore. Thus no watch as of yet...but
will continue to highlight this potentially significant storm in
the severe weather potential statement and other products.

The remainder of the forecast is on the other end of the uneventful as it is active early next week. As the
aforementioned system pulls away Tuesday into Wednesday...the flow will become
relatively weak and inactive with the westerlies well to our north.
Winds will be seasonally light with temperatures near normal. The GFS and
European model (ecmwf) both suggest another iteration of the SW cycle beyond the
forecast period however with the next system around a week after
this one.


issued at 532 am CST Sat Nov 28 2015

Light northerly winds will persist throughout the taf period with
VFR conditions in the north. MVFR ceilings will linger across the
south through the morning and gradually erode from the north. A
few light snow showers will track across southern Iowa as well
this morning.


Dmx watches/warnings/advisories...


Short term...skow
long term...small

National Weather Service Glossary of Abbreviations