Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 262104

Area Forecast Discussion
National Weather Service Des Moines IA
304 PM CST Sun Feb 26 2017

.SHORT TERM.../Tonight through Monday/
Issued at 304 PM CST Sun Feb 26 2017

Confidence: Medium to High. A couple of minor concerns this
period with snowpack over the north and weak trough traversing the
south at this time. Subjective sfc analysis shows weak
convergence are over the southeast this afternoon with cu field
developing over the far southeast. Models...especially
Hires...indicate the potential for a brief shower/sprinkle this
afternoon and early evening there mainly assisted by daytime
heating. Clouds should dissipate quickly in the evening hours
there. Over the north we continue with decent snow field.
Temperatures have slowed to warm expected... though
over the south afternoon highs eclipsed earlier projected highs.
Tonight will see quick cooling over the northern areas. Though
there is a weak cold front across southern MN now...sfc wave in
the Dakotas will move east and keep boundary from any further
migration south tonight. Hires models have been promoting fog
development overnight but consensus rather weak. Preferred
location would be snowpack north and perhaps climatologically
favored areas east/southeast. With light south winds overnight and
near steady state H850 temps...fog may be limited to patchy 2 to
5 mile at this point. Other than Hires...guidance has been void of
any hint of widespread fog overnight. Given some
uncertainty...will continue patchy fog across most of the region
and let evening shift update if needed. Overnight mins will again
drop into the teens north with mid to upper 20s over the snow free
areas south. Monday looks similar to today with quite a range of
afternoon highs on the way. Though snow will continue to melt
north...enough will be around to hinder warming over the far
north. In the south with increasing warm air advection through the
column...afternoon readings will easily reach the lower to mid
50s. Cloud cover should prevent any lower 60s over the south
though a few areas along the border may end up close to 60 by mid
to late afternoon. Winds Monday will once again be breezy with
south winds 12 to 25 mph.

.LONG TERM.../Monday night through Sunday/
Issued at 304 PM CST Sun Feb 26 2017

A prolonged light precipitation event will unfold in several
rounds from Monday night into Wednesday. From Wednesday through
Friday the forecast is mostly dry and with temperatures a bit
cooler, but still around or a little above normal for the season.
Temperatures will then rebound significantly for next weekend.

By Monday evening steering flow aloft will have come around to
southwesterly, with a surface low developing in response along the
lee of the Colorado Rockies. As this low develops Monday night it
will extend an inverted trough northeastward across Iowa and our
forecast area. Within the resulting zone of low-level warm air and
moisture advection beneath modest forcing aloft, precipitation may
become fairly widespread at times until the system finally kicks
out. Initially, it appears that a subtle shortwave will coincide
with the leading surge of warm air/moisture advection and result
in a batch of light showers crossing our forecast area from
southwest to northeast on Monday evening and early Monday night.
There is some meager instability aloft and a few thunderstorms
will be possible in our eastern counties roughly around midnight.
In wake of this initial activity forcing will peter out for a
while but low-level saturation will result in sprinkles/drizzle
late Monday night into Tuesday morning, along with a possibility
of fog.

During the day on Tuesday the aforementioned surface low will
eject east northeastward across Iowa, pushing a cool front across
the state during the afternoon and evening, with the boundary
likely stalling somewhere over Missouri. Meanwhile a deeper mid-
level trough will be moving across the Rockies, and as it
progresses eastward across the Midwest on Tuesday night and
Wednesday it will generate additional precipitation along and
north of the old frontal boundary before finally clearing
everything out to the east on Wednesday. Within our area this will
result in precipitation being likeliest across our southern and
southeastern counties, probably in the form of showers initially
and then changing over to light snow around Wednesday morning just
before the precipitation ends. As of now it appears this will be
of little impact, but it is worth noting that the latest NAM runs
stall the initial frontal boundary further north across Missouri
and thereafter indicate much more substantial atmospheric moisture
content over southern Iowa, resulting in significantly higher QPF
Tuesday night into Wednesday than any other solutions. For now
will treat this as an outlier, especially given the penchant the
NAM has shown for overestimating precipitation this winter.

Once the longwave trough moves overhead on Wednesday any remaining
precipitation will clear out quickly to the east, and steering
flow aloft will turn more northwesterly. This will keep us cooler
for two or three days, though temperatures will still remain
perhaps a couple of degrees above normal. Two clipper systems will
move through the northwesterly flow and may produce light
precipitation, probably flurries or light snow showers, across
about the northeastern half of Iowa around Thursday and Friday.
However, most of the forcing associated with these features will
remain over Minnesota and Wisconsin, and available moisture will
be quite limited. Have maintained some low POPs on Thursday and a
dry forecast on Friday. Next weekend a deep layer ridge will move
across the region, coincident with increasingly southerly low-
level flow. This should allow temperatures to rebound
considerably, probably into the 60s at least across southern Iowa
both days.


.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1132 AM CST Sun Feb 26 2017

Warm air continues to stream across the area...with stronger WSW
winds over the south through 00z. Main concern overnight will be
fog potential. Hires models show a signal for areas fog...but
there is little consensus on location. Climatology favors northern
areas nearer snowpack...though light warm air advection overnight
may tend to inhibit widespread fog. For now have included 2sm
visby across north sites KMCW and at KOTM and 3-5sm at KALO/KFOD
through 13-14z with no mention at KDSM. Remainder of period will
see warming and VFR conditions. /rev





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