Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
328 PM CST Sat Dec 20 2014
Short term /tonight/...
issued at 327 PM CST Sat Dec 20 2014
Situation has changed little over the past 24 hours with similar
regime anticipated into tonight. Expect persistent stratus and fog
to continue with drizzle developing again into the night.
925/850mb layer relative humidity and Omega suggests upward vertical motion will
increase by the early morning hours in strengthening southwesterly flow.
Surface wet bulb zero line extends roughly along a SAC City- Ames-
Grinnell line at 21z so any freezing potential would be relegated
to locations north of this line with steady southerly winds overnight.
Only question is whether ice aloft will induce some seeder/feeder
mechanism and Covert some precipitation to frozen too as weak long wave
through just upstream passes. Middle level lift appears to be fairly
week and soundings depict dry layer to 5kft so have left precipitation
all liquid through the period...but this will need to be
monitored. Any precipitation amounts will be quite light...but as shown
this morning it does not take much to produce scattered slick
spots. Coolest Road surface temperatures are over far northern Iowa so these
locations would seem most prone to any token ice accums and travel
issues...likely a tier or two of counties north of actually
surface freezing line.
Long term /Sunday through Saturday/...
issued at 327 PM CST Sat Dec 20 2014
Main forecast concerns dealt with lingering fzdz/dz and fog
Sunday morning...then multiple periods of precipitation chances Sunday
night through Christmas evening. Continued leaning towards an
European model (ecmwf)/NAM/sref blend Sunday into Tuesday. However...went with a
GFS/European model (ecmwf) blend the remainder of the forecast time frame as the
20.12z European model (ecmwf) is more in line with the 20.12z GFS solution late in
Sunday...low level moisture remains across much of central Iowa
Sunday morning into the afternoon hours. Questionable how long the
fzdz will persist during the morning hours Sunday over the over
the north-northeast County Warning Area. Surface temperatures look to rise above freezing
b/T 15-18z Sunday across northern Iowa with the last fzdz
lingering in the far northwest corner by 18z. Not mention of snow
at this time as the dry air aloft might be deep enough to mitigate any
seeder-feeder effects. Regardless...potential for some additional
travels issues Sunday morning over northern Iowa due to the very
light ice accrual expected.
Sunday night into Tuesday...a second stronger shortwave ejects
out of the northern rockies and travels east through the Dakotas
Sunday night into Monday. The parent surface low tracks across
Minnesota placing Iowa well in the warm sector Monday and have
all rain mentioned during this time frame across the forecast
area. A good amount of middle-level dry air advects into the state
late Monday afternoon into the evening and puts much of the
forecast area in the dry slot. Thus...trended towards a much drier
forecast over west-central to southern sections of the forecast
area Monday afternoon into Tuesday morning. Light snow potential
over the north past 06z Tuesday with cold air advection into the state.
However...there is not much of the way of forcing within the
dendritic layer into midday Tuesday and so snow amounts look
minimal at this time.
Wednesday through Saturday...the 500mb trough continues to deepen
Tuesday as another stronger vorticity maximum ejects out of the southern
rockies. This vorticity maximum quickly moves east into the middle-Mississippi
Valley by midday Wednesday and helps strengthen the surface low
as it moves east over the Great Lakes by Wednesday
afternoon/evening. The GFS remains the outlier as it seems too
slow and further west with the merging systems as the
Canadian/European model (ecmwf) push the system further east by 12z Wednesday.
Either way...it should be fairly windy on Wednesday as surface
pressure gradients tightens and mixing increases through the day.
The windy signal not as potent as previous model runs...but still
could see some light falling snow with wind gusts around 40mph on
Quiet weather in store for Christmas day before the European model (ecmwf)/GFS
brings in a fairly strong wave across the region Friday night
into Saturday. European model (ecmwf) now suggest more of an open wave like the GFS
has been suggesting during this time. Still...extended models in
okay agreement with timing of the system Friday...but GFS is
slightly further north. This potentially looks to be the most
problematic system for central Iowa in the extended period.
issued at 1201 PM CST Sat Dec 20 2014
Ceiling and visibility trends will remain primary concern through the
period. Widespread stratus and fog remain in place with variable
conditions from LIFR to MVFR. Conditions are expected to change
little into sun with slowly deteriorating conditions...patchy
dz/fzdz development overnight...and only minor improvement into