Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
328 PM CST Friday Dec 19 2014
Short term /tonight/...
issued at 328 PM CST Friday Dec 19 2014
Upper level trough currently rotating southeastward through the
western Dakotas on water vapor...and also another little weak
impulse of energy lifting eastward from the High Plains of co/NE.
Both of these features to merge and push across the state
overnight with weak warm air advection and southerly flow. Weak isentropic lift
also across the County Warning Area so potential for very light freezing drizzle
or flurries in place mainly late tonight. Soundings indicate more
of a freezing drizzle potential...but try to show some saturation
aloft which could lead to a seeder feeder situation changing
precipitation to flurries. With the weak warm air advection and increasing low level
moisture...temperatures will stay relatively steady overnight maybe only
falling a few degrees at most. Precipitation should remain very light
with only trace amounts expected...however the NAM keeps trying to
show some enhancement in a band of precipitation associated with the weak
impulse of energy lifting from the High Plains and merging with
the upper low dropping southeast. Therefore will have to watch for
some patches of steadier precipitation at times late tonight as that wave
Long term /Saturday through Friday/...
issued at 328 PM CST Friday Dec 19 2014
Several forecast concerns leading to some potentially significant
travel issues this weekend into early to middle next week. Leaned
toward the nam12/sref/ECMWF through Monday and then towards the
European model (ecmwf) as GFS seems to be the outlier late in the period.
Saturday through Monday...weak warm air advection persists over the weekend and
into Monday with a weak middle-level shortwave to cut across southern
Minnesota and northern Iowa late Saturday night into Sunday
morning. The best forcing remains confined in the aforementioned
area from 06z to 18z Sunday. In this area...higher confidence with
freezing drizzle over the north to northeast as surface temperatures look
to remain below freezing into Sunday morning. Kept patchy fzdz
over the west-northwest b/T 06-12z Sunday before surface temperatures
warm to above freezing after sunrise. There is a potential for
some travel issues over the northeast early Sunday morning and may
warrant a short-fused advisory.
The warm air advection continues throughout the day Sunday into Monday afternoon
and have rain/drizzle going over the entire forecast area through
18z Monday. There is some decent forcing within the dendritic layer
early Monday morning but with the warm air advection below 850mb...this should be
strong enough to keep precipitation all rain Monday and took out any
mention of sleet.
Monday night into Wednesday...surface low begins to track east
across southern Minnesota and northern Iowa Monday evening into
early Tuesday and both the GFS/European model (ecmwf) place the forecast area in
the dry slot through 18z Tuesday. Thus...trended toward lowering
probability of precipitation across the forecast area to reflect this change Monday
evening and Tuesday. The cold air advection is slow to develop Monday night and
the transition to rain to snow over the west-northwest looks to be
from 00-03z Tuesday and further south and east in the 03-09z
Tuesday time frame. However...with the lack of deep moisture
present and any significant forcing within the dendritic
layer...low confidence with measurable snowfall until Tuesday
afternoon/evening when wrap-around moisture pushes into the
Between 00z to 12z Wednesday...the deep moisture pushes back into
northern Iowa and the northwest winds increase substantially towards
Wednesday morning. Winds atop the mixed layer increase to around 55
knots at mcw by 18z Wednesday and forecast soundings still remain
saturated in the snow growth region during the day Wednesday.
Potential for some blowing snow/visibility restrictions over
northern Iowa even though not expecting the good dendritic type
snow. Regardless...increased winds on Christmas evening and may need
to add blowing snow if models continue with this scenario.
Christmas day and Friday...weak surface high looks to build into the
region on Christmas appears to be the only 'quiet' weather day in
the extended. European model (ecmwf) has been fairly consistent bringing a strong
deep low to punch across the region Friday into Saturday. GFS
continues to bounce back and forth between initializations...but
something to continue to monitor as we get closer.
issued at 1143 am CST Friday Dec 19 2014
Low ceilings continue to be an issue with several locations reporting
IFR ceilings and MVFR visibilities across the area. Low ceilings may lift some
this afternoon to MVFR...but are expected to drop again overnight
tonight. Kotm remains VFR...but may lower throughout the period.
Winds will remain light out of the southeast. Other issue is the
potential for very light fzdz or flurries tonight into early
Saturday. Still some uncertainty...but given the conditions
produced some light precipitation this morning and look even better
tonight will likely have to add a mention in in subsequent taf