Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 151759

Area Forecast Discussion
National Weather Service Des Moines IA
1259 PM CDT Thu Mar 15 2018

...Updated Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 331 AM CDT Thu Mar 15 2018

Early morning GOES-East imagery shows high clouds over eastern
Nebraska slipping into western Iowa with clear skies over central
and eastern Iowa. A few light fog patches are over southern
Minnesotan and northern Iowa per observations with Mason City,
Algona, and Estherville being the Iowa stations reporting slightly
reduced visibilities at 3am. Over the last half hour since 3am, GOES-
East Nighttime Microphysics and Night Fog Difference has shown fog
developing southwest of Albert Lea. There is a weak cold front that
has slipped to near the Missouri border at 3am as well with weak
northerly flow over much of the state.

Overall, quiet weather for the daylight hours today as the light
northerly winds increase a bit and become from the east in the
afternoon. High clouds will begin to arrive by later this morning
and with 850mb temperatures a few degrees lower compared to
yesterday, will see high temperatures 5 to 10 degrees lower. Over
the northern third of Iowa, this will give highs near 40 degrees to
the middle 40s and near normal for the Ides of March. For central
and southern Iowa, highs in the 50s will be about 5 to 10 degrees
above normal.

Otherwise today, our attention will be on western US trough
and a vorticity maximum rounding its base and moving over the
southern Rockies as it becomes negatively tilted. Resultant
cyclogenesis is expected over eastern Colorado with low pressure
moving into Kansas later today into tonight. With high pressure over
Ontario and the approaching low pressure, our easterly winds will
become breezy overnight as the pressure gradient tightens. Clouds
will increase tonight as well, especially over southwestern Iowa.
Cross sections and soundings show that there will be dry air to
overcome before precipitation will be able to reach the surface.
While the GFS saturates the mid-levels faster than the NAM, both
maintain dry air in the lowest levels. Therefore, slowed down onset
of precipitation by a few hours. The dry air at the surface is also
of concern as dewpoints will be in the 20s with temperatures in the
low to middle 30s. This raises concern for temperatures to fall to
the wet bulb temperature. With both NAM and GFS soundings showing a
wedge of warm air aloft from 1 to 2 km, will likely see a narrow
band of freezing rain or perhaps ice pellets on the leading edge
initially around to 2 to 4am. This area will likely expand as the
wedge of warm air aloft remains and more areas have their
temperature fall to the wet bulb. While icing amounts prior to 6am
will be a glaze to a few hundredths.

.LONG TERM.../Friday through Wednesday/
Issued at 331 AM CDT Thu Mar 15 2018

Relatively strong isentropic lift will be underway across the
southwest third of the forecast area at the beginning of the
period. This will be associated with precipitation that will
expand northeast through the day. Main concern early on will be
precipitation type as strong dry east surface flow will aid in
near surface evaporative cooling. This is expected to keep surface
temperatures near to below freezing initially favoring freezing
rain in portions of west central into central Iowa. Several things
to keep in mind about potential ice accumulations. Surface
temperatures will be near freezing and any ice accrual will lead
to latent heating at the surface and try to push temperatures
upward. Daytime heating will be increasing as much of the
precipitation will pick up after sunrise. In addition, road
surface temperatures are likely to be at freezing or even slightly
above given the recent warmth. Therefore, somewhat dubious as to
how much ice will form on roadways and therefore potential
impacts. Have still gone with some icing but undercut values given
the above consideration. Headlines may still need to be
considered as well but at this point, confidence on overall icing
remains low.

Otherwise, isentropic lift will push northward during the day
along with the band of precipitation. Southern and central areas
should transition to all rain but farther north, it still remains
uncertain as to eventual precipitation types. Evaporative cooling
will remain strong through the day/evening with strong east flow
bringing in drier air. Soundings are suggesting the possibility of
more snow in the far north and now have 1-2" amounts north of
Highway 3 in northern Iowa. The forcing will linger across the
state into Friday evening although mid level saturation is lost
across central into southern Iowa. As temperatures cool, some
transition back into freezing drizzle will be possible in central
Iowa overnight but amounts should remain light.

Precipitation will end as forcing subsides on Saturday morning
with much of the rest of the weekend remaining dry. The next
system ejecting from the west will begin to influence the weather
in Iowa by late Sunday. However, the 00Z runs of both the GFS and
Euro ram the shortwave more due east well south of Iowa with the
precipitation threat not nearly as robust as before. While pops
remain somewhat robust into early next week, if the trend to a
more southerly track continues in the models, then some
reevaluation will be needed.


.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1259 PM CDT Thu Mar 15 2018

Kept with VFR conditions throughout the forecast period with the
main concern focused on winds increasing late tonight into Friday.
The precipitation onset looks to be between 12-18z Friday and
likely only impacting OTM and DSM. For now, only mention of VCSH
as timing continues to be a bit slower.





SHORT TERM...Ansorge
AVIATION...Podrazik is the U.S. government's official web portal to all federal, state and local government web resources and services.