Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Des Moines Iowa
413 am CST Monday Dec 22 2014

Short term /today/...
issued at 324 am CST Monday Dec 22 2014

Water vapor imagery this morning showing an intense upper level jet
moving across the far Pacific northwest and down along The Rockies
into Colorado. This flow has numerous embedded shortwaves and
intense pv anomalies...the strongest of which is moving into Colorado
now and will lift northeast into southern Missouri by late this
afternoon. This flow will cause the middle level trough over the
central Continental U.S. To deepen through the day with a closed low developing
by late afternoon over South Dakota and Nebraska. The surface
response will be an intensifying low pressure system over northern
Missouri this afternoon.

Weak kinematic and thermodynamic forcing is currently ongoing across
Iowa early this morning with scattered showers developing across
Iowa with a more focused region of precipitation over the middle
Missouri River valley. Possible a mix of graupel is accompanying the
more intense showers with several site reporting unknown
precipitation the past few hours. Static stabilities remain low
through the day thus convective showers remain possible through the
day with the potential for graupel at times.

Two regions of precipitation to focus on today. Expect the area of
precipitation over far western Iowa to refocus and become more
aligned near and just north of the developing low pressure over
northern Missouri while another band developing through the
Mississippi Valley near the area of stronger Theta-E advection.
Therefore precipitation will not be steady all day and there will be
breaks across central Iowa as it will be between the two primary
areas. By late this afternoon...the beginning stages of a
deformation band of precipitation will be moving into southwest and
south central Iowa as an extension of the warm conveyor belt wraps
around the developing low and feeds moisture into the area. Other
than the previous mentioned graupel...the precipitation type for
much of the day will be rain. Temperatures above 1500 feet aloft will
begin to cool to near or slightly below 0c over the far northeast.
That means any hydrometeors falling into the lowest 1500 feet will be
arriving as snow. Average wet bulb temperatures in the lowest 1500
feet will be around 33-34 north of Highway 20 which will not
completely melt everything to rain however near surface wetbulbs
above 35 degrees should do it. Locations with surface wetbulb
temperatures less than 35 with have the chance for some snow by late
afternoon however no accumulations through 00z.

Long term /tonight through Sunday/...
issued at 324 am CST Monday Dec 22 2014

Numerous changes made to beginning of period due to changes in
most recent model runs with placement of low pressure system to
affect the area for early in the week. Past several model runs
have seen a shift in the path of the low much further
south...pulling low into northern Missouri. Given current run to
run consistency with this track...and that GFS/NAM/ECMWF/Gem all
have similar evolution of system...have adjusted forecast
accordingly.

With further south track...precipitation will be widespread
across central Iowa...with dry slot no longer expected to limit
precipitation. In fact...best precipitation chances will likely
come across southern Iowa for beginning of period. As low begins
to lift back northeast across Iowa...axis of best precipitation
will also shift back to the north through the overnight tonight.
Forcing diminishes somewhat through the overnight...however will
remain sufficient to see widespread precipitation...though may
become more scattered. Precipitation will persist into
Tuesday...though less certainty as low pulls quickly northeast and
forcing weakens. However...given amount of moisture light
precipitation will remain possible and have kept slight chance and
chance probability of precipitation Tuesday. Secondary low expected to track across south
Central Plains and lift north across Missouri Wednesday into
Thursday. This may allow for some additional precipitation in the
far southeast as system skirts Iowa. However...have kept forecast
mostly dry for Wednesday given current track of this second
system.

With surface temperatures generally remaining near
freezing...main concern with forecast for beginning of period will
be precipitation type. Temperatures aloft will be sufficiently
cold with saturation into the dendritic growth zone...therefore
main concern with precipitation type will be with transition
timing from rain to snow and also snow accumulations. As
mentioned...ice introduction is expected...with main concern
becoming if near surface temperatures will be warm enough to melt
back into rain...or will keep a mix or all snow. For beginning of
period near surface temperatures will keep precipitation primarily
rain...though soundings indicate a possible transition to mix or
all snow near 06z across much of area...having fully switched to
snow near 12z across all but far southeastern County Warning Area. Expect snow to
persist into middle day Tuesday. While forcing is somewhat limited in
the lower levels...it is strong throughout dgz and therefore may
aid in a quicker transition to snow. Therefore have highest
confidence in snowfall totals across northern County Warning Area where believe
timing on transition to snow is more certain...and much less
confidence across central and southern Iowa. In
addition...temperatures may warm sufficiently during the day
Tuesday to see precipitation change back to rain...though cold air advection may
keep temperatures cool enough for snow. Therefore...generally 1-2
inches expected across most of the County Warning Area...with higher
amounts...which may near 3 inches...in the north.

As system begins to depart...northwesterly winds will increase
Tuesday night into Wednesday. Gusts may be as high as
25kts...which may be sufficient to limit visibilities with falling
snow or fresh fallen snow. However...given sounding profiles near
freezing throughout much the column across most of the
County Warning Area...anticipate any snowfall to be generally wet...which may help
to limit accumulations and blowing potential.

Very weak ridge builds in for late Wednesday and Thursday. This
will keep the Holiday dry...though precipitation chances return
late in period with secondary system set to work into the Midwest.
Models continue to have very poor consistency with handling of
this system. Current solutions are in decent agreement with
handling of system at this time. Current solution is for surface low
under upper trough to lift northeast into central Iowa from
Nebraska. However...this is further north than previous
solutions...which keep precipitation Thursday further north into
Minnesota and main precipitation chance for central Iowa limited
to Friday as system pulls east. However...European model (ecmwf) and Gem are faster
than GFS...bringing precipitation into Iowa beginning Friday
morning. Given such uncertainty and continued model
variances...have cut back on probability of precipitation for Thursday...keeping
precipitation confined to Friday. Temperatures will be cold
enough for snow...and winds will be strong behind system.
Therefore...even though system will be short lived could
potentially lead to travel problems for late in the week and will
need to continue to be monitored. Behind this system...conditions
will be dry for end of period.

&&

Aviation...22/06z
issued at 1141 PM CST sun Dec 21 2014

Widespread mainly IFR conditions expected through the forecast
period primarily due to ceilings with periods of drizzle or light
rain. Precipitation will become more widespread as rain after 22/10z and
overspread southern taf locations through the morning. After 23/02z
precipitation should shift into northern Iowa. Visibilities may drop to 1sm at
times due to a combo of precipitation and fog with drizzle at times. Surface
flow will be generally srly but become more easterly after 23/03z.



&&

Dmx watches/warnings/advisories...
none.

&&

$$

Short term...donavon
long term...awb
aviation...fab

National Weather Service Glossary of Abbreviations