Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 301905

National Weather Service Des Moines IA
105 PM CST Wed Nov 30 2016

Issued at 847 AM CST Wed Nov 30 2016

Have updated the forecast to reflect radar trends a bit better and
keep temps a bit cooler through the day. This should result in
snow falling through much of the day across the north and a
rain/snow mix further south as temps struggle to warm much today.
Already getting some snow grains down into the I-80 corridor, so
have extended the pops a bit further south and east as well at
least for the morning hours. Still some dry air to overcome in the
far southwest to south, so kept mainly slight chance or dry
conditions in those areas and categorical across the north through
the day. With the cooler temps could see some light accumulations
across the north, currently have around an inch to an inch and a
half far north near the MN border. Will be monitoring road
conditions and snowfall for possible advisory. Overall not
expecting advisory criteria snowfall of 3+inches, but it is the
first accumulating snowfall of the season in the north, so will
monitor for impacts due to that reason and if snow does seem to
end up more than expected at this time.


.SHORT TERM.../Today through Tonight/
Issued at 401 AM CST Wed Nov 30 2016

The upper level low pressure over southern Minnesota will finally
start moving east today as a vigorous short wave drops southeast
on the backside of the system this morning then wraps around and
helps eject the system east. This will bring cyclonic flow to the
state later this afternoon and evening. Deep saturation will
exist through the profile over the northern half of Iowa with less
moisture to the south. Weak vertical ascent will be in place
mainly along and north of Highway 20 through the afternoon and
will keep precipitation chances likely across that area.

A deep cold dome aloft will mean that the precipitation type will
be dependent on the near surface thermal profile. Temperatures
over northern Iowa are currently in the 32 to 35 range. These
temperatures should cool a few more degrees with the passage of a
weak boundary through the pre-dawn hours. Suspect that the model
soundings are being too optimistic in the diurnal range during the
day with too much mixing by the afternoon pushing temperatures
back to 36 to 38. Given the dense cloud cover and evaporative
cooling of any precipitation, these readings may be a few degrees
too warm. This scenario would keep the precipitation type mostly
light snow over the north with temperatures and wet bulb
temperatures not high enough to force melting. Accumulations up to
one inch may occur over the north with the focus over the
northern two tier of counties. The stratus will linger much of
tonight though profiles will dry aloft ending precipitation
chances all areas except for the far northeast.

.LONG TERM.../Thursday through Tuesday/
Issued at 401 AM CST Wed Nov 30 2016

The main forecast concern was focused on the persistent stratus
into the weekend and then an active weather pattern developing
late in the weekend and continuing into next week. Leaned more
towards the GFS as the ECMWF remains inconsistent with its between
each subsequent model runs.

Thursday through Saturday...large upper level low continues to
transition eastward during this period and finally allows for
surface high pressure to build into the state by Friday.
However, the mid-level stratus remains persistent with high RH
values b/t 925-850mb continue to advect in from the west-northwest
through Saturday morning. Potential for some breaks in the cloud
cover by Saturday as a bit drier air begins to entrain into the

Saturday night into Sunday...a 500mb shortwave trough looks to
outrun the deepening parent trough and likely becoming cutoff
over Mexico and west Texas during this time frame. The GFS is a
bit slower with the near cutoff low over Mexico and this solution
seems to be more reasonable as the northern shortwave trough
remains connected with the upper flow pattern. Regardless,
confident to increase pops 06-12z Sunday over the east-southeast
as both the GFS/ECMWF have some decent forcing and moisture
impacting this location. The precipitation looks to fall mostly as
all snow as there`s not a significant warm layer aloft to cause
any melting. Surface temperatures/wetbulb temperatures look to be
cold enough to warrant snow and leading to central Iowa`s first
widespread accumulating snowfall. Albeit, the QPF remains light
and thus snowfall amounts remain sub-advisory attm.

Next system looks to begin impacting the state by late Monday
night through Tuesday, in the form of a weak upper level
shortwave. Light snow chances exist, but there are still plenty
of discrepancies between the extended models. This is especially
true with timing and location of the aforementioned shortwave.
The GFS remains more consistent from run-to-run as the ECMWF
continues to be widely variable. The GFS is hinting on a deepening
trough towards the middle of the week with the surface low
tracking to the south over Missouri and thus Wednesday into
Thursday looks to be the window of any significant storm if the
GFS holds true.


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 105 PM CST Wed Nov 30 2016

MVFR CIGS to remain across the area through the period. Light snow
to continue, and could mix with some rain at the southern TAF
sites through the afternoon hours. Light snow to continue north
into the evening hours, before tapering off. Gusty westerly winds
expected through the critical taf period.





SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Beerends is the U.S. government's official web portal to all federal, state and local government web resources and services.