Area Forecast Discussion
Issued by NWS Des Moines, IA

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806
FXUS63 KDMX 022354
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
554 PM CST Fri Dec 2 2016

.SHORT TERM.../Tonight through Saturday/
Issued at 317 PM CST Fri Dec 2 2016

A weak surface high will drift across the state through tonight
while an upper ridge tries to build into the region.  With this
pattern...a weak flow will continue and low level moisture will be
trapped so low clouds will be hanging tough across all but the far
Southwest. Some erosion on the Southern edge of the stratus is
expected but much of the forecast area will remain in cloud cover.
At least the cloud cover combined with surface winds becoming west
then south as the surface high shifts east will keep the temps from
bottoming out.

Saturday we should see some period of at least broken clouds
allowing for perhaps a peak at the sun at times. This will be short-
lived however as the upper ridge moves East of the area by mid to
late afternoon and a pretty decent shortwave enters the central
Plains and perhaps as far as Western Iowa. Model soundings
saturate pretty quickly in the late afternoon with the NAM much
quicker than the GFS. Only weak to moderate forcing approaches the
Western border by 00Z so while there is some potential for light
rain far West late in the afternoon the more likely scenario would
be a return of low stratus again. The main precip event will hold
off til Saturday night. Ahead of the system there will be a deep
west to southwest flow pushing across the state so temps will be
similar to or maybe even a couple degrees warmer than
today...especially if we get some sun.

.LONG TERM.../Saturday night through Friday/
Issued at 317 PM CST Fri Dec 2 2016

Confidence Medium...models continue to struggle with the amount
of cooling within the boundary layer which is resulting in some
uncertainty as to the amount of snowfall expected with the inverted
trough that is moving through the region Saturday night and early
Sunday. With cloud cover already in place by Saturday afternoon
highs should top out in the upper 30s to lower 40s. Given a few
hours of cooling during the onset of the precipitation...bufr
soundings suggest that there should be enough cooling for mainly
light snow across the west central and northwest areas an a
mixture of light rain/snow in the far south initially before
changing over to all snow during the night. Trends in the models
have been for higher qpf for the event overnight tomorrow night
into Sunday morning. Looking over winter parameters...have made
some minor adjustments to snow ratios and qpf which results in
slightly higher snowfall totals over the east/southeast as the
moisture transport and forcing increases overnight Saturday night
in those areas. Likely to be the first widespread light snowfall
accumulation of the season which will require cause some challenges
for area travelers. Certainly not a high snowfall...but given the
near freezing temperatures and crossover to freezing pavement in
some areas...headlines or at least an special weather statement
may be needed. Across central areas lighter amounts are expected
with an inch or so there. Lows Saturday night will only cool to
the lower 30s over the south and east. This may also result in
some snow compaction by early Sunday morning where snowfall is
higher.

/Sunday Night through Friday/...

Confidence medium to high: Following the inverted trough... expecting
some warming as downslope airmass moves quickly east as weak
ridge of high pressure builds into the region by Sunday evening.
Some sunshine will return toward late afternoon...but highs recover
only slowly to the upper 30s northeast to the lower 40s in the
south. Stronger warm air advection will arrive for Monday with
return southwest flow. This should melt much of the weekend snowfall
and allow for highs to warm into the mid to upper 40s over the
western half of the forecast area. Mid level clouds will accompany
the warm air advection...so partly sunny conditions are expected.
The long-wave pattern has "yet" to phase and once again by Monday
evening a significant southern stream storm will be affecting the
Gulf Coast and Ohio River Valley with a northern stream wave
moving through Minnesota. That wave will pull a cold front south
of the area by 12z Wednesday. A relatively weak wave from the
Rockies will track east along the boundary and brush the Central
Plains with light snow Wednesday afternoon and evening. Minor
accumulations appear to be in store for the area at this time
...though some uncertainty with the timing of the system vs. the
cool front exiting the region will determine the extent of upper
level forcing and snowfall. Following the boundary/wave...there
continues a consistent signal for much colder air to move into the
Central Plains...the coldest of the season so far. Thickness
values drop to 508 to 512 dm by Thursday morning with strong
northwest flow. H850 readings -12C to -16C. Overnight lows may be
near zero in the northwest with lows in the single digits to teens
elsewhere. The trend for temperatures continues down the
remainder of the day with flurries and clouds overspreading the
area. By nightfall -18C to - 20C at H850 with sfc temperatures in
the teens. As another wave approaches the plains a warm up will
ensue Friday into Saturday with afternoon readings rebounding into
the 20s.

&&

.AVIATION.../For the 00Z TAFS through 00Z Saturday evening/
Issued at 554 PM CST Fri Dec 2 2016

Widespread MVFR/VFR ceilings continue over all but the far
southwest corner of the state and are expected to persist through
the night. Confidence is low however as ceilings are drifting back
and forth +/- 3K ft. Patchy drizzle is also possible this evening
from KDSM toward KOTM, as is patchy fog SW removed from any TAF
sites. The likelihood of VFR conditions will increase into Sat
however as ceilings and any lingering fog begin to lift.

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

&&

$$

SHORT TERM...FAB
LONG TERM...REV
AVIATION...Small



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