Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 091149
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
545 AM CST Mon Jan 9 2017

.SHORT TERM.../Today through Tonight/
Issued at 345 AM CST Mon Jan 9 2017

There will be several things to address in the short term.  For
today, there will be a short wave passing mainly across MN today and
like yesterday...moisture is of concern because when we have better
mid level moisture our low level moisture is lacking and when lo w
level moisture catches up then we start to lose our mid level
moisture. Forcing is a little better with this system but I am a
little gun shy after yesterday. Clearly the moisture is spotty at
best and this will likely again play a bigger role than the weak
forcing we have in place. I stayed close to model blends in
keeping just slight chances across the far North.

That system passes east late afternoon and early evening then
another low develops and deepens over the Plains in response to a
deeper trough pushing across the Rockies. This system will pass
through Iowa tonight into Tuesday. For tonight...the surface low
moves across Nebraska/Southeast South Dakota into Northern IA and
a push of warmer air rides up into the state. We still have
moisture issues in that forecast soundings are either sporadic
with deeper moisture or void of it and we are dealing with shallow
moisture but again some decent forcing. Temperatures at the
surface...especially North will be below freezing while South they
will be at or just above freezing. Ptype concerns will impact the
forecast tonight. At this point we could see some freezing
drizzle or light freezing rain across the North then late in the
overnight it`s possible that deeper moisture will finally develop
over Eastern Iowa and we could see rain across the far East for
locations South of highway 30 and a mixed precip event North of
highway 30 with freezing rain or drizzle. Tomorrow night will
likely be messy North but since moisture is such a concern I am
not considering headlines. The situation will need to be monitored
today to see how the moisture develops.

.LONG TERM.../Tuesday through Sunday/
Issued at 345 AM CST Mon Jan 9 2017

The weather will be active through the entire period but confidence
in any particular event is low. At onset the system currently
bringing much needed precip to CA will be moving through the
immediate MO Valley by Tue morning. Forcing will be fairly deep
but moisture depth will be spotty leading to questionable ice
introduction at times and snow vs liquid concerns. Rain vs
freezing rain is also questionable with persistent southerly winds
finally boosting the freezing line to somewhere near an Atlantic
to Iowa Falls line by 15z. Thus light snow, light freezing rain
and light rain are all possible over the northwest half of the
forecast area in the morning. There will be higher confidence in
just rain southeast however. The lack of moisture and the passing
of the sharp lift couplet will keep precip amounts low however so
any snow or icing accums would be minimal, if at all.

The upper level jet and steady stream of weak short waves will then
induce cyclogenesis from the Rockies into the High Plains Tue Night.
This will certainly increase moisture with a noted surge of
thermodynamic and frontogenetic forcing, but moisture will be
limited keeping PoPs to the north. The models are depicting big
differences with the speed of the resultant frontal passage Tue
Night into Wed which leads to a large spread in temperature guidance
and more of the aforementioned low confidence. The NAM is notably
slower/farther northwest leading to much warmer temperatures. The
NAM MOS is an eye opening 15-20 degrees warmer than the GFS MOS
at times. The 00z ECMWF suggests much closer to the cooler GFS so,
while not going all in, have trended in that cooler direction for
Tue Night lows and Wed highs. Spotty moisture will persist
through Wed but a sharp H85/H7 wave and lift couplet will pass
during the afternoon likely initiating light precip. Ice
introduction is spotty so the precip could be snow or liquid
leading to possibility of light snow or ice accums into the
evening, mainly east. Spotty forcing continues into Thu however
with the parent long wave trough still to our west so low end PoPs
linger into Thu morning.

There should be a brief break from precip to end the weekend with
the long wave trough passage through the Great Lakes, but the
chances return again for the weekend as we never really exit the
influence of SW flow aloft. The remnants of the closed upper low
currently well off the Pacific NW coast will eventually drop
through the SW CONUS and into the Plains. The first wave arrives
Fri Night into Sat with decent mid level lift and moisture finally
raising confidence in ice introduction and chances for light
snow. The parent wave will eventually exit into the Plains over
the weekend or early next week however spreading the inverted
trough and surface low track somewhere through our south and east.
Persist southwest flow above this frontal boundary may bring a
warm layer north into IA creating yet another precip type
headache. The present forecast has a wintry mix from freezing rain
and sleet SE to all snow NW, but do not get too caught up in
precip type details for any particular location at this point due
to uncertainties with the NW-SE baroclinic zone placement. Do
continue to monitor the forecast for the potential of a more
significant snow or wintry mix event than what we have experienced
thus far this winter. Low confidence needs to be emphasized again
however as recent model runs have been inconsistent ranging from
a cooler, snowier low track through the immediate OH Valley to a
warmer more `mixy` track through MO and IL. GEFS timing is slower
than its deterministic run as well and the ECMWF ensemble hints
at a little slower, farther north so the system could even hold
off until beyond the valid period.

&&

.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 545 AM CST Mon Jan 9 2017

Stratus will push down across mainly Northern TAF sites producing
MVFR cigs.  We could also see some light snow across the Norther but
confidence isn`t high enough to include in TAF`s at this time. Lower
stratus will return tonight with better precip chances aft 09Z for a
wintery mix North and rain South.  Icing will be possible at KFOD,
KMCW and KALO aft 06Z.

&&

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

&&

$$

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



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