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Area forecast discussion 
National Weather Service Des Moines Iowa
647 PM CDT Friday Mar 27 2015

Short term /tonight/...
issued at 311 PM CDT Friday Mar 27 2015

Weak middle-level shortwave and 500mb vorticity maximum will cut
across northwest to south-central Iowa this evening. Models are in
good agreement with overall location of probability of precipitation this evening with the
hrrr roughly 2 maybe 3 hours too fast with onset of precipitation compared
to other hires models. Leaned toward the rap/sref/hopwrf blend for

The shortwave embedded within the 500mb north to northwest flow will
enter the state by around 00z with onset of precipitation b/T 00z-02z over
western Iowa. It will take a little time for surface layer to become
saturated...if at all...this evening which should limit the snow
accumulations. Still...there is a brief stint when the forcing in
the dendritic layer will cause it to snow hard enough for it to
reach the surface and accumulate. Mainly a few tenths of inch
possible and no more than a half inch anticipated over the
southwest. Increased probability of precipitation with higher confidence in snow and only
some minor tweaks with the snow amounts. May see some fzdz on the
tale end of the event with the column losing ice introduction early
Saturday morning.

Long term /Saturday through Friday/...
issued at 311 PM CDT Friday Mar 27 2015

Little change to previous challenges coming up in the extended
package. By early Saturday the weak upper level shortwave digging
southeast on the backside of the h500 upper level trough will be
exiting southwest Iowa...taking any residual light snow away from
the region. With the high amplitude pattern
upstream wave is just entering the Pacific northwest from near
Washington/British Columbia areas. Water vapor shows the system nicely and
expected trajectory brings the wave into the northern plains by
06z Sunday. Just ahead of the upper level system a second system
over central Canada will be increasing southerly winds across the
plains...resulting in stronger warm air advection moving north as
well as an adiabatic warming component aiding in warming west of
the region. Some questions remain about the evolution of the
forcing/moisture across the Central Plains with this fast moving
system. The models remain split with the GFS/NAM drier than the
Gem/Euro though there has been a trend in the past run for better
consistency in amounts of precipitation...not so much on timing.
The latter will have significant implications on daytime highs on
Sunday with a spread of about 10 degrees between the two pairs of
models mentioned above. With cold air advection briefly returning
with the passage of the system Sunday...will continue to lean
toward the Euro/Gem thermal structure aloft and cooler daytime
highs. Winds may become an issue for Sunday as stronger northwest
flow will follow the cold front Sunday afternoon. Mixed layer top
winds approaching 40kt or higher at Kalo and kmcw and may need to
consider some headlines for wind in the next few shifts. By Monday
..another fast moving wave will move southeast...Tracking just
north of Iowa across central or northern Minnesota. This will keep all of
the forcing north of the area with the only impact to the forecast being
much warmer temperatures. Have raised highs on Monday to the upper
60s to lower 70s given the consensus h850 readings of 6c to 9c...
modest west southwesterly flow aloft...and some sunshine through
the afternoon. Both the GFS/Euro are coming into more agreement
for the Tuesday-Thursday period of next week with active weather pattern
to continue. Increasing moisture from the Southern Plains/Gulf
will be injected into the midweek system. The timing is critical
for any threat for thunder or severe potential. The 12z GFS is now
advertising a fair amount of instability on Wednesday...a change
from the past 2 days and the new Euro is also showing thunder for
the afternoon/evening hours. A quick look at both models have the
middle and upper level wind maxes continuing to lag the low level
h850 forcing and moisture transport. With features out of
phase...likely that any severe that might develop would not be
long lasting or extensive...and mainly a result of increasing
instability late day to early evening. This would be more likely
across the west third of the forecast area Tuesday afternoon. Will
trend toward blended pop for the day with thunder mainly in the
afternoon hours and evening hours. By Thursday night and Friday
some cooler air arrives once again with a shield of overrunning
precipitation and possible thunder. Pattern looking wetter with
southern stream becoming more active. Colder weather returns by
the weekend.


issued at 646 PM CDT Friday Mar 27 2015

Light snow will drop from the Dakotas/Minnesota border into western Iowa
tonight but will largely leave taf sites unaffected with nothing
more than lowering VFR cloudiness and potentially some virga at
kfod/kdsm/kotm. Thus entire period is VFR with nothing more than
middle cloud ceilings. Light and variable winds will increase slightly
and become southerly/southeasterly overnight into Sat.


Dmx watches/warnings/advisories...none.



Short term...podrazik
long term...rev

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