Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 140933
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
333 AM CST Thu Dec 14 2017

.SHORT TERM.../Today through Tonight/
Issued at 325 AM CST Thu Dec 14 2017

Clouds still in place across much of the forecast area with some
clearing into the far northeast. For the most part some additional
clearing may come into the east/north this morning, but clouds will
then increase by late morning and remain in place this afternoon
into tonight. This is due to a shortwave trough currently dropping
through the western Dakotas/MT set to drop through IA by this
evening. Increasing low level moisture is expected, with thickening
low level clouds and potential for some very light snow flurries.
greater chances exist to the west of the forecast area, so have left
some slight chances pops just skirting the far west by late
afternoon/early evening. Otherwise went with scattered flurries for
the entire forecast area this evening into the overnight hours. Will
have some weak warm advection in place today, with another shot of
cold air expected tonight. However with the clouds in place, warming
will be limited with highs mainly in the upper 20s to mid 30s
expected. Tonight with the clouds in place, expect temps to drop
to around 20 to 25 across the forecast area.


.LONG TERM.../Friday through Wednesday/
Issued at 325 AM CST Thu Dec 14 2017

Bottom Line Up Front...
At this time, no hazardous weather conditions for the next
several days. After Friday, temperatures should be a solid 10 to
20 degrees above normal across Iowa, with the best widespread
precipitation chances occurring on Sunday.


Friday and Saturday...
An upper low will dip southeastward through the Great Lakes
region. As of now, it appears any (precipitation) impacts from
this low should remain east of Iowa. This low should bring an
elongated stratus deck with it, generally keeping Friday cloudy.
Winds will be slightly breezy Friday afternoon. But with a
pressure gradient of around 3mb/175 km, and given transport layer
winds, expecting wind values to top out in the 15 to 20 mph range,
with gusts to 25 mph.

For Saturday, 850mb temps make it into the +5 to +9C range across
Iowa. With the above-mentioned low pulling away to the northeastern
CONUS, am thinking a decent amount of sunshine can make it to the
ground during daytime heating hours. The record high for Saturday is
62 (1939)... which may be a little bit of a tall order to reach...
however, temperatures are trending upwards, so mid to possibly upper
50s seems very plausible.


Sunday Into Monday...
At this time, our best chances for widespread precipitation
appear to be on Sunday. For the past several days, models have
been showing an upper low pushing northeastward from west Texas
towards the Midwest and eventually the Ohio Valley region. The sfc
reflection for this low seems to be weak and becomes disorganized
as it makes it into the Midwest, with a MSLP of around 1015mb or
so. Our best precip shot comes from the baroclinic zone extending
off the NE quadrant of this low. For the past several runs, there
has been decent low-level frontogenetical forcing that has been
well-aligned with this baroclinic zone that has been positioned
more or less from the SW corner of Iowa through the NE corner of
Iowa at 12z Sun. A lack of low-level moisture is the biggest
mitigating factor...with the best moisture being relegated to SE
Iowa. Still some disagreement remains among the models, with the
GFS and now the NAM generally being faster and father east than
the ECMWF.

Ultimately, any impacts from wintry precipitation should effectively
be nil for central Iowa. Temperatures will be above freezing, and
with pavement temperatures likely warm from Saturday`s highs in the
40s and 50s, any snow should melt upon contact.



Tuesday and Beyond...
Somewhat strangely, the 00z Thu run of the long-range models show
better agreement for Tuesday (12/19) and beyond timeframe than
the Friday (12/15) through Monday (12/18) time period. Worth
noting that run-to-run consistency remains somewhat low, though
has begun to improve over the past 36 hours.

Basically, a "poor man`s" omega block, elevated latitudinally, is
suggested to set up by Tuesday into Wednesday. At this time, it
looks like Iowa will be in the warm and dry side, with a 1030mb
sfc high located near the center of the country. 850mb temps seem
to peak around the +3C to +7C range- slightly cooler than is
slated for this upcoming Saturday. Thus, think sfc temps in the
mid 40s to low 50s could be plausible if this setup holds. Also,
if this omega block does take shape, long-range models would be
too fast in kicking in colder air by the end of next
week...meaning "persistence" could play into next weekend.


&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1031 PM CST Wed Dec 13 2017

Main concerns will be clouds. Winds continue to diminish and will
finally drop below 12kts by 09-12z. Weak ridge aloft has allowed
cigs to lift over northeast sections and KMCW/KALO will see
improvement to VFR. Farther west at KFOD/KDSM/KOTM where MVFR
conditions will linger through 08-11z. As upstream wave propagates
toward Iowa from 18-00z...cigs will again lower to MVFR across
north sites/northwest areas...but lower confidence for KDSM/KOTM
where drier air may have a chance to dilute deck with time. /rev


&&

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

$$

SHORT TERM...Beerends
LONG TERM...Kotenberg
AVIATION...REV


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