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Area forecast discussion 
National Weather Service Des Moines Iowa
551 am CST Monday Nov 30 2015

Short term /today/...
updated at 550 am CST Monday Nov 30 2015

Well-advertised winter storm still on course to impact primarily
western and northwest Iowa today. The state remains situated in
broad diffluent upper level flow out ahead of a 300 mb low
centered across Wyoming this morning with a 110 knots jet beginning
to eject to the northeast from the base of the trough. Moisture
has streamed northward this evening along the 280-300 k isentropic
surface...with scattered rain/freezing rain showers occurring throughout the
night. Ice accrual so far has been low with the biggest impact to
roads noted across northern Iowa. Surface low developing over the
Texas will rapidly translate northeastward and reside in southwest
Iowa by 00z this evening.

While there is good confidence in the synoptic track and quantitative precipitation forecast amounts
associated with this storm...p-types and snow/ice amounts across the
northwest half of the of the state will be walking a very fine line.
Surface temperatures...the strength of the elevated warm
nose...and moisture depth in the dgz will all play a huge role.
Surface temperatures/wetbulb values overnight have remained steady in the
32 to 34 degree range southeast of a line from Waterloo to
Atlantic with neutral to even slight warm air advection expected to take place
during the day. Road temperatures for a ways northwest of this line are
in the 32 to 34 degree range...which may limit ice accretion to
elevated surfaces and power lines. The chance of accumulating
freezing rain southeast of this line looks to be fading quickly.
The current ice grids model this setup well...but hinge on warmer
air not working further north.

The bulk of the snowfall/ice amounts are tied into a strong and deep
region of dgz lift that is generating showers across southeast
Nebraska and central Kansas. This precipitation looks to reach
northwest Iowa around 15-16z and last through 20-21z. NAM/RUC
bufr soundings for fod/frm/slb show an impressive setup for
effective snow production in this window with snowfall rates of 1
to 2 inches an hour not out of the question. Dry slot already
manifesting itself well in water vapor imagery over the Texas
Panhandle will rapidly wrap into the state on it's heels...with
ice introduction quickly being lost and any lingering precipitation
transitioning to freezing rain/drizzle. Should the timing of
either the precipitation shield or dry slot vary from the going
forecast...or maximum temperatures in the elevated inversion be even slightly
higher than forecast...winter precipitation amounts across the northwest
will likely be significantly affected.

Overall though...despite these uncertainties...the placement of the
ongoing headlines line up well with the best probabilities of
receiving hazardous weather...regardless of the type. The
southern/southeast portions of the advisory might be able to be
cancelled earlier than their expiration times should the currently
forecast rain/fzra/sn line hold true.

Long term /tonight through Sunday/...
issued at 358 am CST Monday Nov 30 2015

All of the sensible weather concerns in this period will be
early...through Tuesday night. Although the models have come into
a decent consensus with overall track and forcing...confidence is
still diminished tonight due to middle level moisture variability.
This will leave ice introduction and precipitation type problematic and
in question until early Tuesday when much of the forcing will have
weakened over Iowa and pushed off to the north and east. A deep
strong rise/fall forcing organizing in the base of
The Rockies long wave trough...will be pushing through Iowa this
evening. Both kinematic and thermodynamic contributions will be
strong during the evening in the right exit region of the strong
Southern Plains jet...however the passage of the cold front aloft will
sharply drop Theta-E advection by midnight with mainly middle level
kinematic forcing associated with the upper low left behind. Model
middle level relative humidity fields have little consistency in magnitude or
location with the dry slot progression so a low confidence
forecast would suggest a snow and rain/freezing rain mix...with
the likelihood of snow somewhat higher north. This variability has
resulted in lower snow ratios tonight and another one or two of
additional accumulation north.

By Tuesday middle level moisture concerns will abate with deeper
moisture and higher confidence in precipitation type /snow/ but the forcing
is spotty at best so do not expect much in the way of additional
accumulations into the more than an inch or so north over
that 24 hour period...maybe near two toward the Minnesota border. The
main themes for the remainder of the forecast will be fair weather
and temperatures moderating back to just above normal. After this
system exits through the Great Lakes Wednesday upper level ridging and
seasonally weak flow will follow behind. Although not included in
the forecast at this range...there may be periods of fog and/or
stratus with trapped low level moisture and relatively weak
surface wind. Previous model runs were suggesting a brief minor precipitation
window around next weekend...but the trends have been toward less
phasing and Iowa left between a Southern Plains cutoff and forcing with
westerlies well to the north. The next precipitation chances may be
beyond the current period into early next week as the
aforementioned southern closed low eventually moves through the lower
MO and MS valleys.


issued at 550 am CST Monday Nov 30 2015

IFR to LIFR ceilings will spread northeastward across the state today
and remain in place for much of the taf period. Snow will
overspread northwest Iowa by late morning and persist through much
of the afternoon...with a mixture of rain and freezing rain along
a line from Kaio to kmcw and predominantly rain further to the


Dmx watches/warnings/advisories...
Winter Weather Advisory until 6 PM CST this evening for Adams-

Winter Storm Warning until 6 PM CST this evening for Adair-

Winter Storm Warning until 6 PM CST Tuesday for Boone-Calhoun-
Carroll-Cerro Gordo-Crawford-Emmet-Franklin-Greene-Hamilton-
Hancock-Hardin-Humboldt-Kossuth-Palo Alto-Pocahontas-SAC-Webster-

Winter Weather Advisory until 6 PM CST Tuesday for Black Hawk-



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

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