Altoona, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

718
FXUS63 KDMX 021139
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
539 AM CST Sun Feb 2 2025

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Mostly cloudy this morning with areas of dense fog, then
  yielding to a fairly mild afternoon with highs in the upper
  40s to mid 50s.

- A cool front will push through around Monday morning bringing
  lower temperatures for the rest of the week, with daily highs
  ranging in the 20s to 30s from Tuesday through Friday.

- Precipitation chances return around Wednesday, with the
  potential for freezing drizzle across much of the area from
  Wednesday morning into Wednesday night. There is a 40-50%
  chance of at least 0.01" of ice accumulation over much of
  central Iowa during this time.

&&

.UPDATE...
Issued at 416 AM CST Sun Feb 2 2025

As feared, the low stratus field over the northeast two thirds
of the area has begun to slowly move northeastward early this
morning, and the higher cirrus in the south is beginning to thin
out and move away as well. This has resulted in rapid decreases
in visibility observed in our western and southwestern counties
in the last hour, with dense fog now likely to expand across
much of the area in the next few hours before gradually
dissipating later this morning. Have issues a large Dense Fog
Advisory through 11 AM accordingly, and made associated updates
to the short-term forecast.

&&

.DISCUSSION...
Issued at 215 AM CST Sun Feb 2 2025

The forecast is relatively active compared to recent weeks, but
also generally non-hazardous with one glaring (glazing?) exception.

Currently a large low stratus cloud deck blankets most of the
northern two thirds of our forecast area, with thick higher
cirrus clouds streaming across the south. Light fog is observed
at times under the stratus, but with radiation significantly
hampered by the clouds no dense fog has developed. Short-range
model guidance is having trouble resolving not only the initial
range of the stratus deck, but also its evolution this morning
and through today. Some solutions clear the stratus out pretty
quickly overnight, with dense fog then rapidly developing in the
resulting clear area given the very light winds coming in from
the Missouri River valley. However, other solutions keep the
stratus more or less where it is, some of them through most of
the day today. However, satellite trends have shown very little
movement of the stratus deck for the past several hours, and
have thus favored the HRRR/RAP solutions which do not
significantly advance the stratus eastward until around midday
or so. With lingering cloud cover now anticipated, especially in
our northeastern counties where it could persist right through
the day, and with surface flow quite light and no appreciable
warm air advection, have tempered expectations for warming and
lowered high temperatures by several degrees this afternoon
closer to the HREF. Even so, they still range from the upper 40s
north to mid 50s southwest, and in any areas that do see skies
clear out by this afternoon it should still be a pleasant day
for early February.

Tonight a broad low pressure area will develop over the High
Plains, which an inverted trough stretching northeastward across
Iowa. South of this boundary, roughly south of Highway 20 or so,
light south southwest breezes should hold overnight and more
cirrus clouds are expected per forecast soundings, thus have
raised low temperatures in those areas into the mid to upper
30s. However, to the north of the boundary winds will come
around to north northeast and despite more potential for low
clouds near and north of the boundary, modest cold air advection
should allow temperatures to fall into the 20s in our northern
counties. There is also potential for fog development near the
boundary, however, have not included this in the outgoing
forecast yet as uncertainties in cloud cover would have a
significant impact, and also any such fog would likely develop
in a relatively narrow zone and at this range it is difficult to
tell precisely where the boundary and fog would lie by early
Monday morning. Also of note is that the potential for snow
across our far northern counties during this time has shifted
even further north into Minnesota, and the forecast for our
service area is therefore now dry.

The aforementioned surface low will move quickly eastward
through and away from Iowa on Monday morning, pulling a cool
front down across the area by midday with gusty north winds
ensuring through the afternoon. The guidance envelope on
temperatures has moved notably cooler on Monday as a result of
this earlier frontal passage, and have lowered highs by several
degrees as a result, though in our southeast the temperature
should still climb into the upper 40s or even lower 50s early in
the day before the front pushes through. Also boosted winds just
a bit in the afternoon, however they should remain well below
advisory levels.

By Tuesday night a large surface high pressure area will cover
much of the region, centered somewhere near northern Minnesota
and Lake Superior but with its influence stretching down all the
way into the High Plains and east northeasterly flow all across
Iowa. Concurrently, our mid-level flow aloft will be turning to
more west southwesterly ahead of an approaching trough, with
widespread forcing overspreading Iowa from late Tuesday and
Wednesday before the trough finally kicks through around
Wednesday night or so. The result of this scenario will be a
period of thick cloud cover and moisture advection aloft, but a
shallow layer of cool, somewhat drier east northeasterly flow
at the surface. This is a classic setup for freezing
rain/drizzle, as referenced in the opening of this discussion.
Forecast soundings are unanimous in depicting this outcome
across most of the area, excepting the far southeast where
temperatures may remain just above freezing, and the north where
deeper saturation may allow for ice crystal introduction
sufficient to result in light snow. However, for a large swath
across central and into eastern Iowa soundings depict a deep
saturated layer from around 700-750 mb down to near the surface,
and almost entirely below freezing. The lift within this layer
is somewhat modest so drizzle would be more likely than rain and
have advertised that in the forecast. The result could be a
prolonged period of freezing drizzle across much of the area,
however, there is one caveat. Although forecast soundings are
locked in on this freezing drizzle scenario, some solutions do
indicate a near-surface dry layer that may limit precipitation
potential, or at least delay the onset time for it reaching the
surface. Even so, the signal for freezing drizzle is strong
enough that 40-50% POPs are advertised around midday Wednesday
into the evening, with freezing drizzle forecast across much of
the area. The limited forcing within the saturated layer will
limit QPF potential and thus any ice accumulations, which is
supported by NBM probabilities showing a 40-50% chance of at
least 0.01" of ice across most of our area, but only a 10-20%
chance of at least 0.10" of ice. Regardless, any icing that does
accumulate on roads and other surfaces could result in
hazardous travel conditions, and have begun to more explicitly
message this potential threat.

The trough will kick through quickly sometime around Wednesday
night and push the low clouds and light precipitation eastward
out of our area, bringing quieter weather for Thursday and
Friday with no precipitation anticipated and daily highs in the
upper 20s to 30s. Another system will then approach quickly
around next Saturday or so and bring a return of
precipitation chances, but model spread is higher at that range
and any details of precipitation type or location remain
inscrutable.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 539 AM CST Sun Feb 2 2025

Low stratus clouds continue to blanket much of northeastern
Iowa, including MCW and ALO, while further southwest clearing
has allowed dense fog to develop which has reached FOD and
should reach DSM shortly and OTM in a couple hours. The combined
effect will be prevailing IFR or lower conditions this morning,
with gradual improvement after roughly 15Z and VFR expected by
around 18Z and thereafter through this evening. Toward the end
of the 12Z TAF period, later tonight, low MVFR ceilings should
work back into MCW and there is also a low potential (20-30%
chance) of LLWS which has not been advertised in the TAFs.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 11 AM CST this morning for IAZ004-005-
015-023>025-033>037-044>049-057>062-070>075-081>086-092>097.

&&

$$

UPDATE...Cogil/Lee
DISCUSSION...Lee
AVIATION...Cogil/Lee