Marshalltown, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

128
FXUS63 KDMX 040546
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1146 PM CST Tue Dec 3 2024

 ...Updated for the 06z Aviation Discussion...

.KEY MESSAGES...

- Scattered snow showers possible (30-40% chance) across the
  north and northeast Wednesday. Low confidence (~20%) in
  visibility restrictions with snow.

- Wind Advisory in effect across the northern half of the area
  from noon to 9pm Wednesday. Gusts up to 50 mph (locally 55
  mph) likely (90% confidence) especially across the far north.

- Cold Thursday morning with wind chills in the negative teens
  across the north.

&&

.DISCUSSION...
Issued at 218 PM CST Tue Dec 3 2024

A scattered deck of upper level clouds moved across the state this
afternoon, primarily in the northern half. Southerly winds will work
to recover temperatures slightly today with areas across the south
just reaching above freezing. Tonight, a descending trough to the
northeast will drag with it a front. The arrival of the fropa has
trended slightly faster this forecast update which will limit highs
across especially the north tomorrow. Have lowered highs, keeping
northern Iowa right at freezing which it should reach in the morning
hours just before the fropa. Soundings show the potential for some
convective snow showers with steep lapse rates in the PBL. This
potential will be across the north in the morning, focusing in the
northeast through the daytime. Quick bursts of snow will be possible
as mixing maximizes, but snow will be blown around enough in the
wind to limit accumulation. One would be concerned that with
expected gusts tomorrow (more on the winds shortly), bursts of snow
could lead to visibility restrictions. For this area, the hi-res
guidance does highlight narrow bands of vsby restrictions associated
from simulated HCR snow showers, so reductions of 2sm or less cannot
be completely ruled out. However, a limiting factor will be the lack
of saturation in the DGZ, hindering the formation of more stellar
dendrites, leaving a snow pallet consisting of smaller, less
obstructive shapes. All this being said, confidence then becomes low
(~20%) for significant visibility reductions in the north.

Now, for the winds: have issued a Wind Advisory across the north for
Wednesday afternoon and evening. It is likely (90% confidence) to
see sustained winds of 25-30 mph with wind gusts up to 50 mph. It
cannot be completely ruled out to see 55+ mph gusts in the peak
mixing hours of the afternoon. This is supported by the magnitude of
winds at the top of the mixed layer, which eased slightly this
issuance, but still hit 50 kts for an hour or two in the far north.
Pair this with steep PBL lapse rates, a tight pressure gradient,
CAA, and subsidence from pronounced height rises, ingredients are
certainly in place to maximize mixdown of higher wind speeds. Trends
will be monitored to see what magnitude of winds will be realized.
For now, the Wind Advisory is representative. CAA preserves mixing
through the night, keeping gusty, but sub-advisory winds going until
Thursday. Clearing skies will also work to plummet temperatures into
the single digits for Thursday morning. The aforementioned winds
will produce wind chills in the negative teens north of Highway 20.
Highs will only warm to near 20 degrees.

In the extended, an upper level trough will stall over the southwest
CONUS and bring in a WAA regime for the weekend. A reinforcing open
wave enhances this regime for Sunday, pushing highs in the 50s that
day. The forecast for the following week remains uncertain as models
diverge on how they handle the eastward evolution of these
waves.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1147 PM CST Tue Dec 3 2024

Low level wind shear is coming to an end and concerns are
shifting to ceiling restrictions, increasingly strong and gusty
winds, and possible snow showers.

Winds from the southwest will become from the northwest through
the morning hours and increase in speed. Wind gusts have been
increased to at least 40 knots at the northern sites of MCW,
FOD, and ALO. It remains possible that a few wind gusts around
50 knots may occur in the afternoon hours over northern Iowa.
Accompanying the increasing winds will be lowering ceilings
into MVFR that will teeter around FL020 and may for a short time
drop below this FL. Have now included a short period of MVFR
stratus at DSM where confidence is above 50% in occurrence.
Narrow snow shower bands will likely develop and could bring IFR
restrictions to any terminal they pass over. For now, have
advertised this as a PROB30 at both MCW and ALO late morning and
early afternoon. Ceilings will improve late in the period into
VFR with the strong winds very slowly decreasing into the end
of the period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from noon to 9 PM CST Wednesday for IAZ004>007-
015>017-023>028-033>039-044>050-057>061.

&&

$$

DISCUSSION...Jimenez
AVIATION...Ansorge