Area Forecast Discussion
Issued by NWS Des Moines, IA

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092
FXUS63 KDMX 212151
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
351 PM CST Wed Feb 21 2018

.SHORT TERM.../Tonight through Thursday/
Issued at 351 PM CST Wed Feb 21 2018

Very messy forecast beginning late tonight through Thursday night.
We will remain in broad southwest flow aloft as a subtle shortwave
pushes into the state.  Theta-e advection will increase across the
south especially late tonight and Thursday.  Soundings show deep
saturation and sufficient forcing to produce snow initially beginning
in the south and southwest spreading to the north and northeast
during the morning.  Based on QPF and snow ratios we should see
around an inch of snow in the southwest.  As the morning wears on,
soundings show that we keep a substantial warm aloft but lose our
ice introduction so we will transition across the south from light
snow to a period of freezing rain then freezing drizzle for the
afternoon.

During the afternoon we are expecting some accumulation of light
snow to occur across the north...on the order of a half inch or
so...with light ice amounts across the south...on the order of five
hundredths of an inch.

Then during Thursday evening a second shortwave and a better shot of
forcing comes across central Iowa which will enhance the snow across
the north and produce some freezing rain across central Iowa.  We
are looking for an inch of snow north in the evening with up to a
tenth of an inch of ice across portions of central Iowa on top of
the .02 to .05 ice accrual amounts from earlier in the afternoon.
After 06Z the system will lift fairly quickly off to the northeast
and while there may be lingering freezing drizzle in this
period...additional ice accrual will be minimal.  A winter weather
advisory will be issued for the event beginning at 09Z across the
southwest primarily for the snow.  Elsewhere will start the advisory
at 15Z.  Primary thinking here is that we will not see much snow in
the morning but we will begin to transition from snow to freezing
rain which is expected to make late morning travel increasingly
hazardous.  Adjustments will need to be made as the event nears but
the messaging for the morning will be out.

.LONG TERM.../Thursday night through Wednesday/
Issued at 351 PM CST Wed Feb 21 2018

The upcoming system should be exiting the area at onset with the
attention turning to the next system to end the week. Fair weather
and seasonal temperatures are anticipated Friday through Friday
evening but the current WA/BC short wave/PV anomaly is expected to
drop through the base of the western trough Friday Night with its
leading effects reaching southern Iowa by early Saturday. This
system has deepened from run to run over the past several days and
is now a well mature extra-tropical cyclone passing near or
through Iowa. Yet another round of a light freezing rain/freezing
drizzle mix should be underway Saturday morning as strong
warm/theta-e advection strengthens in relatively shallow moisture.
By midday a noted deformation zone should begin developing
somewhere near NW Iowa with impressive theta-e advection and
frontogenetic forcing phasing in this area with concurrent
cyclogenesis somewhere from central IA to the immediate MS River
Valley depending on model solution. The lift in this
frontogenetic/deformation zone is quite impressive with deep
phased forcing and an associated jump in QPF. The depth of the
Dendritic Growth Zone is not extreme and a relatively warm column
may limit snow ratios somewhat, but some models also depict
extreme lift in the DGZ as well which could offset snow density in
the other direction. Thus have kept snow ratios in the low teens
for now on average, but this will have to be watched. Present snow
amounts are in 3-6 inch range NW through 00z Sunday, with more
possible into the evening. Thus confidence in a band of moderate
to heavy snow is increasing, but unfortunately placement
confidence has not due to model variability as it could vary from
primarily affecting FSD/MPX areas rather than ours. Blowing snow
could be a significant issue as well with wind gusts 30+ mph
possible.

Farther south and east impacts should be less with any
freezing precip melting by afternoon and warm advection increases
west of the inverted trough and eventual surface low development.
It is not mentioned in the forecast as of yet, but there is some
token MUCAPE which could result in convective in the warm sector,
and even possibly thunder snow in the deformation zone.

The system should be fairly fast moving however with dry,
seasonal late February weather into Sunday and Monday. Precip
chances will increase somewhat once again into late Tuesday and
especially Wednesday however but confidence is low. The models are
quite variable with the evolution of the western trough, the
degree of southern and northern stream phasing, strength of
individual short waves and the associated surface low track.
Precipitation could be either rain or snow.

&&

.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1125 AM CST Wed Feb 21 2018

Warmer air aloft will move in and produce cigs which will lower
between 09Z and 14Z to the MVFR category.  Initially TAF locations
will be cold enough for some light snow and northern locations will
remain cold enough for the precip to remain as snow through the
valid time of the TAF forecast.  Occasional IFR conditions are
possible aft 15Z for all TAF locations but this would be very
localized. Southern TAF locations will see precip change over to
freezing drizzle as some drier air near surface noses in.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 AM Thursday to midnight CST
Thursday night for IAZ004>007-015>017-023>028-033>039-047>050-
059>062-073>075.

Winter Weather Advisory from 3 AM Thursday to midnight CST
Thursday night for IAZ044>046-057-058-070>072-081>086-092>097.

&&

$$

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



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