Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 301138
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
538 AM CST Wed Nov 30 2016

.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
state.

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 12Z TAFS through 12Z Thursday morning/
Issued at 538 AM CST Wed Nov 30 2016

Widespread MVFR cigs across the sites with IFR over northeast Iowa
including vicinity of KMCW. The stratus will remain across the
state through the forecast period though cigs may raise to VFR
over southern Iowa overnight. Light rain and snow is expected at
KFOD/KMCW and possibly to KALO today. Breezy west to northwest
wind will continue through tonight.

&&

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

$$

SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Donavon


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