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FXUS63 KDMX 181146
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
542 AM CST Fri Jan 18 2019

.SHORT TERM.../Today through Tonight/
Issued at 401 AM CST Fri Jan 18 2019

Confidence: High on snow timing and occurrence, medium to high on
amounts.

Models showing some divergence in the strength of this system as it
impacts the state today into tonight, with the northern trough
taking some time to fully phase with the stronger southern trough.
This creates a bit of a split in the heavier snow amounts across the
forecast area with two main areas of concern for heavier amounts
great than 6". The first heavier area likely across the northern
third of the forecast area with more intense snowfall rates this
morning as an area of snow spreads across the state this morning
into the afternoon hours associated with an upper level shortwave
currently over the western high plains into WY/SD/CO area. This wave
will continue to traverse eastward across SD/NE today and become
phased with the southern trough toward early this evening. Models
indicating an area of frontogenesis stretching across the north
central to northwest portions of Iowa this morning, with some
elevated slantwise instability present above the lift layer.
Therefore expecting a band of enhanced snowfall rates stretching
from areas of northwest Iowa into north central Iowa from 15Z to 18Z
and beginning to dissipate toward 21z. Therefore still expecting
snowfall amounts into the 6 to 8 inch range across the northern few
tiers of counties through late afternoon, with some additional very
light accumulations still possible through the evening hours and
snow tapering off late tonight.

Further south and west, have the area situated between the northern
heavier band and the surge of moisture northward associated with the
southern system into the south central/southeastern forecast area
this afternoon/evening. Therefore with some dry air to overcome and
lighter snowfall rates expected between the two stronger areas of
forcing, have trended snow amounts down especially across the
west/central portions of the forecast area with 3-4 inches in the
far west and around 4-6 inches in/around the DSM metro/Ames areas.
However majority of that accumulating snowfall in the DSM metro will
occur right around the afternoon commute timeframe with some
possibly higher snowfall rates during that time. Therefore could
still see travel disruptions with the short-lived heavier bursts of
snow across central IA by mid/late afternoon into early evening.

Across southern/southeastern/eastern portions of the forecast area
again see a possibility of heavier snowfall into the 6 to 8+ range
of snow with a strong surge of moisture northward into the state
this afternoon ahead of the digging upper trough moving across the
southern Plains today and into AR/Ohio Valley tonight into Saturday.
Trowal feature to approach the southern forecast area and be firmly
across northern MO. Again have another area of frontogenesis across
the south/southeastern counties this afternoon, with some
instability present aloft as well. Also have a very good cross-hair
signature indicating strong lift within the dendritic zone around
21z-00z which should allow for more intense snowfall rates in that
area again with the surging moisture. Again this is moving in around
the evening commute timeframe, so travel impacts are very
concerning.

Brisk north to northwest winds to pick up across the state this
afternoon and increase further tonight. Expect gusts to around 20-25
mph this afternoon with gusts to increase to 20-30 mph tonight.
Therefore blowing and drifting snow still an issue as well
especially in the north/central where more fluffier snow is
expected. It will be a bit harder to blow the southern snow which
will likely be more of a heavier/wet snow, but still could see
reduced visibilities to around 1/2 mile or less at times especially
in rural areas this evening into the overnight hours. As the snow
tapers off across the north late tonight, may see some easing of the
visibility restrictions even with gusty winds given the strongest
winds at that time will likely be across southern Iowa.

Made some adjustments to the WSW with regards to timing to try and
have start times adjusted to the onset of snow across the forecast
area, however did continue all of it until 12z Saturday with the
blowing snow potential tonight. However with the winds diminishing
slightly across the northwest to north and the snow tapering off,
could see the warning potentially ending a bit earlier if conditions
warrant.

.LONG TERM.../Saturday through Thursday/
Issued at 401 AM CST Fri Jan 18 2019

The long term remains very active. Beginning Saturday morning, the
deformation zone precip may still be occurring over far southeast
Iowa but the forcing is all east of the area by then so any precip
would be flurries or very light snow and only for a couple hours in
the morning. A tight pressure gradient will remain over central and
eastern Iowa so brisk wind with patchy blowing snow remains in the
forecast through at least early afternoon. During the afternoon the
wind diminishes some but as very cold air spills into the state, the
wind will be sufficient to drive down wind chill values by late in
the afternoon. Northwest Iowa will be in the cold air by early
Saturday morning so wind chill values Saturday morning northwest
could approach headline criteria.

Saturday night will see some of the coldest temps yet this winter
with areas at zero or below. The wind will be lighter and this will
result in very cold wind chills from 10 to 20 below but with the
lack of wind will likely not meet criteria.

On Sunday shortwave drops down the northwest flow over Iowa bringing
a shot at some light snow from roughly north central to southeast
Iowa. Models are not completely clear on the amount of moisture
with this system. It appears that the GFS...which is more bullish
on QPF is the outlier. Other models are much lighter on QPF but
with as cold as we will be, snow ratios will be quite high so it
won't take much qpf to get an inch of snow. After the system today,
it will pale in comparison, still something to watch.

As the next system digs into the west, it will push an upper ridge
over the region for Sunday night into Monday but by Monday night
through Tuesday a trough will sweep through the region with a
surface low ejecting from Kansas across south central to southeast
Iowa then into Illinois/Wisconsin. In addition to more snow, the
passage of the cold front associated with the surface low will bring
another shot of colder air to the state. Models are suggesting
decent moisture with this system but just where that will be is in
question. PTYPE will be in question with this system as well.
Soundings indicate a loss of ice introduction Monday afternoon so
while we may start as snow if the precip begins in the morning, with
the loss of ice, we will transition to freezing precip in those
locations. Ice introduction returns late Monday through Tuesday
when precip switches back to all snow. The ECMWF has the higher QPF
over much of eastern Iowa while the GFS is less and over far
northeast Iowa. This system is still 5 days away so still some time
to evaluate but this system should give moderate snowfall to some
area of central into eastern Iowa.

By Tuesday evening the system pulls out putting Iowa square in the
axis of an upper trough with a surface high moving in. More cold
air will drop into the state with overnight lows back in the single
digits above or below zero and highs Wednesday in the teens.

Late Wednesday night into Thursday an impulse looks to drop down the
northwest flow over us bringing yet another shot of light snow. QPF
with this system appears light and by this time there is little
agreement as to where the better chances for precip are. For now,
keeping some mention of snow across the northeast but confidence is
low on location. If the ECMWF is right, precip will fall more
west into south central.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 542 AM CST Fri Jan 18 2019

MVFR stratus will spread east this morning. Moderate snow will
begin NW by 15Z then gradually spread SE through the day IFR vsbys at
times in SN and BLSN. Conditions improve from NW to SE aft 06Z as
the snow pushes to the southeast. Winds will remain strong with
areas of BLSN likely through 12Z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from noon today to 6 AM CST Saturday for
IAZ026>028-036>039-045>050-057>061-070>073-081>083-092>094.

Winter Storm Warning from 3 PM this afternoon to 6 AM CST
Saturday for IAZ062-074-075-084>086-095>097.

Winter Storm Warning until 6 AM CST Saturday for IAZ004>006-015-
016.

Winter Storm Warning from 9 AM this morning to 6 AM CST Saturday
for IAZ007-017-023>025-033>035-044.

&&

$$

SHORT TERM...Beerends
LONG TERM...FAB
AVIATION...FAB
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