Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 101831
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1231 PM CST Tue Jan 10 2017

.UPDATE...
Issued at 958 AM CST Tue Jan 10 2017

Were getting icing reports from areas where road and surface temps
were above freezing, yet sub-surface road temps remained around
28 to 30 degrees. This was essentially causing icing on roads
from the bottom up. Thus added many counties along I-80 and Hwy 30
and up toward Waterloo into the advisory. Will be watching as
precipitation ends over the next few hours as many of those areas
will likely get removed from the winter weather advisory.

Strong and gusty northwest winds expected to pick up on the
backside of this system as the sfc low shoves quickly off to the
northeast. Therefore have coordinated with surrounding offices to
also issue a wind advisory. Bufkit winds in the mixed layer
showing a period of 26 to 35 kts winds across the north, with an
area 30 to 40kts in the eastern CWA by this afternoon. Therefore
will continue the wind advisory through 6pm. Winds will be handled
in the winter weather advisory for the northern portions of the
CWA.

&&

.SHORT TERM.../Today through Tonight/
Issued at 333 AM CST Tue Jan 10 2017

Big changes in the weather are expected today in terms of
precipitation and temperatures with a wintry mix likely impacting
the northeast third of the forecast area sufficiently to degrade
travel somewhat through early afternoon once the precip begins.
Although confidence in precip is fairly high, how that will
materialize in precip type details is only low to medium as models
are struggling to handle the changing thermal profile. A noted short
wave/PV anomaly continues to push through the TX/OK/KS vicinity and
is expected to quickly expand precip into Iowa, certainly more so
than what is currently upstream. Although a dry layer is precluding
precip at the moment, 1-3km warm/theta-e advection will continue to
erode that through the morning and eventually saturate the column.
This lift will wane but give way to higher based DPVA/kinematic
forcing which is expected to blossom precip 12z-15z.

The big question will be precip type and unfortunately the models
are struggling to handle the warm advection well. The NAM is way too
cool with 09z obs already several degrees past expected highs
in spots. Thus have tried to trend to at or above the warmer end
of guidance in most areas, although the GFS MOS may be too high in
spots. To compound the problem with surface temperatures, a warm
layer aloft around 1.5km could produce any combination of sleet or
freezing rain before temps cool sufficiently to flip the precip
to all snow as the system passes. Thus the current forecast has a
wintry mix this morning along and north of a Sac City to Hampton
line with winter impacts eventually a tier or so counties farther
south as we switch to snow toward midday. Although it won`t last
long the snow in the deformation zone may blow around for a bit
with sharply increasing wind on the back side of this system.
Forecast soundings suggest gusts may push 40 mph by early
afternoon in the cold air advection mixing, but luckily these
winds should not be coincident with falling snow for too long and
snow amounts are not expected to be much, typically an inch or
less, so do not expect much of the blowing snow/low visibility
problem once the snow stops. With all these items have opted to
issue a winter weather advisory for portions of west central and
northern Iowa 6am-3pm. Confidence in when the impacts begin is not
high as the precip conditions will change quickly, so there is
some uncertainty how or if the morning commute will be impacted in
the advisory area.

Temps should peak no later than midday and then fall fairly quickly
north into night night. The next system will quickly develop another
warm front into Iowa tonight however and create quite the thermal
gradient south to north toward morning. This will lead to more
uncertainty, especially along the frontal zone somewhere near the
Highway 20 corridor. Overnight lows may vary from near 10 above
toward the MN border all the way up to the upper 20s south.

.LONG TERM.../Wednesday through Monday/
Issued at 333 AM CST Tue Jan 10 2017

Not much change from the going forecast...primarily because model
discrepancies haven`t changed.  On Wednesday an upper level trough
will move across the region while at the surface a Canadian high
will push across the Northern Plains into Iowa and drive a cold
front through the state between 15Z and 21Z.  Forcing looks pretty
impressive during the mid morning through late afternoon and with
the colder air moving we will likely see some precip tied to the
frontal passage. The quality of moisture will again be an issue
leading to a period of drizzle/freezing drizzle and eventually
changing to snow, first across the North then East before ending.

Wednesday night through Thursday morning another short wave will
push across the state bringing yet another chance for precip.
Northern Iowa will likely be snow while further Southeast a wintery
mix will be more plausible.

For Thursday night and Friday a second high surges South out of
Canada bringing a reinforcing shot of cold air to the state.  Temps
look pretty cold during this period with lows -10 to +10.  During
the day Friday the high quickly shifts East switching the winds to
an Easterly direction and bringing in temps that are not quite as
cold for Friday.

The weekend still looks very messy with models still not close to a
consensus. Friday afternoon and night some energy gets ejected out
of the base of a deep low and trough over the Southwest U.S. as a
low lifts out of the Southern Plains spreading some precip up
into Iowa during this time. Soundings indicate the precip would
be all snow at this point but just how much moisture will be
available is up for debate. That moves off for Friday overnight
into Saturday afternoon. For Saturday night through Monday night
the Euro and Canadian models both have a closed low tracking
across the mid section of the country but the Euro is further
North and West thus putting us more in the warm sector. The
Canadian is faster, further South and colder while the GFS leaves
an open wave coming across us. While precip is expected from this
system timing, ptype and how much moisture will we be dealing with
is all still unclear. There will be a wintery mix but how long it
will last and where any ice might be is far from being determined
with any certainty.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1231 PM CST Tue Jan 10 2017

Precipitation expected to end this afternoon from west to east and
has mostly changed over to all snow. Gusty northwest winds also
bringing BLSN at ALO and MCW leading to vsbys below one mile.
VSBYS to improve this afternoon as snow ends and just have some
BLSN mainly around MCW. Winds should diminish toward evening and
back around to the south late tonight. CIGS to be tricky with
mainly MVFR conditions through much of the period. May have a
period of VFR tonight before low clouds move back in for
Wednesday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 3 PM CST this afternoon for
IAZ004>007-015>017-023>028-033>039-044>049-057>060.

Wind Advisory until 6 PM CST this evening for IAZ050-061-062-
070>075-081>086-092>097.

&&

$$

UPDATE...Beerends
SHORT TERM...Small
LONG TERM...FAB
AVIATION...Beerends



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