Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 111748
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1148 AM CST Sat Nov 11 2017

.SHORT TERM.../Today through Tonight/
Issued at 404 AM CST Sat Nov 11 2017

Surface high pressure will shift to the east today allowing a low
over the Southern Plains to shift east northeast across Missouri.
Associated with the low is a weak boundary/inverted trough that will
lift up into southwest Iowa by mid morning then across the state
through tonight.  Moisture with this system will gradually increase
but initially there is relatively weak forcing, the result of which
will be light rain or drizzle moving up from the southwest from mid
morning into the afternoon.  Overall though the precip chances will
be better later in the day vs earlier.  Best window of opportunity
for precip looks to be from 20Z through 06Z then the system shifts
to the east leaving lingering precip over locations east of I-35.

Adding to the forecast difficulty will be a shortwave dropping
through the state tonight bringing in colder air with it and a
considerable amount of forcing.  The airmass will be cold enough by
late tonight for the precip to change over to snow however the best
forcing is well above the dendritic layer and there is some question
as to how fast the precip will change over.  We may get a brief
period of a wintery mix followed by a change over to snow...mainly
along the highway 20 corridor...with snowfall amounts under a half
inch.

Abundant cloud cover will have a detrimental impact on temps with
little diurnal change expected today.  Tonight, temps drop into the
mid to upper 20s north with mid to upper 30s elsewhere.

.LONG TERM.../Sunday through Friday/
Issued at 404 AM CST Sat Nov 11 2017

Precipitation likely will linger over the east early Sunday though
much drier air will be punching into central Iowa from the west and
should bring an end to the precipitation by mid to late morning. The
NAM continues to be the most robust regarding snow potential though
at this time appears a bit overdone. The GFS and SREF profiles
suggest potential for a brief transition to snow before the dry air
arrives and ends the precipitation. Have followed this consensus for
precipitation type.

A transition to zonal flow will commence Sunday night into Tuesday
and will lead to warmer temperatures arriving to Iowa. High
temperatures will be in the 40s and 50s on Monday and the 50s on
Tuesday. A strong short wave and associated boundary will pass
through the state Tuesday night. A few thunderstorms may impact
portions of central and southeast Iowa as this system passes through
as the low level jet focuses into the state. This will lead to
steepening mid level lapse rates and MUCAPES of 200-500 J/kg. The
mild temperatures will persist Wednesday and Thursday.

Another significant shortwave will arrive towards the end of the
week though still temporal and spatial issues exist which leads to
uncertainty towards the end of the week. Good consensus on strong
lee side cyclogenesis commencing as the short wave crosses the
Rockies then the low quickly deepening as it passes east of Iowa.
Strong winds appear to be the most likely impacts for Iowa with this
system. Precipitation type will depend on storm track, which will
undergo multiply iterations through the forecast period and could
range from light snow with a more southern track to even
thunderstorms should the system move north. The current scenario
would seem to favor the more southern solutions.

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1140 AM CST Sat Nov 11 2017

MVFR or lower ceilings are the primary forecast challenge through
the current TAF period. As of 18z cigs have fallen into MVFR
category across western into north central Iowa. The lower cigs
will expand east through the afternoon along with chances for
light rain and/or drizzle. Lower cigs are likely overnight. Model
guidance in sound agreement with IFR cigs at most TAF sites,
and potentially into LIFR during the early morning hours at DSM.
The light rain may change over to rain/snow mix or all snow
tonight, mainly north of Hwy 30. Not enough confidence in the
current forecast to include mention of -SN in the FOD or ALO TAF
at this time. If wintry precip occurs it should not create any
further aviation disruptions.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...FAB
LONG TERM...Donavon
AVIATION...Martin



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