Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
628 PM CDT Thursday Mar 26 2015
Short term /tonight/...
issued at 325 PM CDT Thursday Mar 26 2015
Main forecast concern was with quick moving shortwave to
bring some light precipitation to the southwest corner of the forecast area
this evening. Models are in good agreement with location of the
precipitation...although the rap13 seems a little too far north and quicker
in bringing precipitation in the southwest...so leaned toward the a
hrrr/arwwrf/sref/nam12 blend. This resulted in increasing probability of precipitation
between 00-04z tonight over the far southwest. Csq forecast
soundings per GFS/NAM show a little Omega within the snow growth
region b/T the aforementioned time frame...but still shows a decent
dry layer below 850mb. However...there should be enough forcing
combined with saturation aloft to think some light snow/rain will
reach the surface this evening and break through the dry layer. Low
confidence with much snow accums but do have a few tenths mentioned
over the counties near Missouri in the southwest.
Temperatures will be cold across the north to northeast tonight
where should be cloudless even though winds stay strong enough to
hold off any significant radiational cooling. 850mb temperatures drop to
-10c to -12c by 12z Friday. Leaned toward the NAM for temperatures over the
north. Cloud cover should linger over the south but NAM still had a
decent handle of temperatures in this location.
Long term /Friday through Thursday/...
issued at 325 PM CDT Thursday Mar 26 2015
Main challenges this period will continue to be the extent of the light
snow event occurring Friday night across the area along with degree
of warming next week. A minor challenge right now...appears to be
the chances for convection by midweek next week.
Cold air still holding on for Friday with Arctic boundary over
northeast Iowa now...heading south with time tonight. That will keep
h850 temperatures well below normal for this time of the year...about -8c
to -10c through early Saturday. Latest guidance continues to
postpone and weaken the isentropic lift over the western forecast
area Friday night. Have trimmed back pop...qpf...and snowfall
amounts Friday night and early Saturday. With the stronger system
slowing down for Sunday...will see a slight warming trend with
increased chances for rain through the day. Included pop mainly
north northwest overnight and light if any quantitative precipitation forecast. Looking at
timing...little threat for any frozen precipitation and most guidance
shows liquid. Though mins may fall to near freezing far north the
column will probably take a while to saturate with little crossover
between precipitation and sub freezing temperatures. With light rain becoming
more likely on Sunday have lowered temperatures during the day. Would
like to go colder...but some uncertainty in temperature forecasts will
result in less emphasis on colder readings for now. By midday the
Euro cools the column to near 0c aloft at h850 across the north.
Highs north may struggle to reach the 50 f mark while the south
remains in the 50s. After the system departs...subsequent waves
will drive east as the amplitude of the h500 height field relaxes.
This will promote stronger adiabatic warming across the plains
heading east and kick off a Lee side cyclone by Sunday night and
again another deeper wave by Tuesday evening...both tracking east
into the northern plains. Warming will continue with most of the
forcing over the north third of Iowa. Tuesday night to Wednesday
the more vigorous wave still looks to impact the area...with
elevated convection possible. Some timing issues and placement of
the warm nose aloft will limit confidence on thunder for now...but
overall the more active weather is still prognosticated to be southeast
of the area by middle to late afternoon. Highs next week will hold in
the 60s to 70s. Zonal flow will result in at least a period of 3
to 4 days of milder conditions.
issued at 628 PM CDT Thursday Mar 26 2015
Although light rain/snow mix will drop down the immediate MO River
Valley this evening it is only expected to brush southwestern Iowa and not
affect taf sites. Otherwise expect VFR conditions with low end VFR
ceilings gradually giving way to mostly clear skies and scattered/broken high
cloudiness at worst.