Altoona, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

645
FXUS63 KDMX 151919
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
219 PM CDT Thu May 15 2025

.KEY MESSAGES...

- Strong and gusty winds this afternoon and evening, then again
  during the day on Friday. Wind Advisories are in effect for
  both periods across much of northern and western Iowa.

- A few light showers northwest this afternoon, and more
  widespread across northern Iowa on Friday, but with no impact.
  Otherwise, the forecast is dry until at least late Sunday.

- More active weather is forecast early next week, with higher
  (60-80%) rain chances at times between Sunday night and
  Tuesday. Severe thunderstorms and heavy rain may be possible
  during this period, but confidence is low.

&&

.DISCUSSION...
Issued at 218 PM CDT Thu May 15 2025

Today`s weather has unfolded largely as forecast, with a large
cyclone stacking and deepening over the Dakotas and moving
quickly north northeastward toward the North Dakota/Minnesota
border. A dryline trailing from the system is sparking strong
thunderstorms over Minnesota, but the boundary has already moved
northeast of our forecast area. An area of scattered light
showers is moving across northwestern Iowa and may just clip
areas around Estherville this afternoon, but with no impact and
other than this the forecast is dry through tonight and into
Friday morning. However, with a tightening pressure gradient and
deep mixing we are seeing strong and gusty winds spreading
across our area and these will continue until sunset when they
should diminish somewhat as mixing decreases. The current
advisory remains in effect, and while a couple of reporting
sites outside the advisory have occasionally reached criteria,
it has not been consistent or widespread enough to warrant
expansion.

Overnight tonight, while surface winds diminish somewhat, some
short-range models are tracking a very subtle shortwave impulse
moving over central Iowa and producing spotty light QPF.
However, forecast soundings are quite dry and most solutions
remain dry, so have maintained POPs below 10% accordingly. On
Friday the stacked and slowly filling low will drift eastward
across northern Minnesota, with an expanding shield of stratus
and showers advancing around its southern flank and into our
area from the northwest. Have maintained POPs in that area,
diminishing from likelies (60-70% chance of rain) in our
northwestern corner to lower chances (20-30%) near the Highway
30 corridor and just sprinkles south of that. In any event, no
significant rainfall or thunder is anticipated with this
precipitation. Of greater consequence is another surge of strong
and gusty winds during the day, this time from the west
northwest. While forecast soundings indicate notably shallower
and less efficient mixing beneath the low cloud deck, stronger
winds just off the deck and moderate cold air advection will aid
in momentum transport and we should see advisory level winds
once again, perhaps even over a slightly larger area than today.
In light of this, and with winds expected to reach criteria in
our northwestern counties just after sunrise, have elected to go
ahead and issue another Wind Advisory now for Friday,
encompassing the same area as today`s advisory plus several more
of our southwestern counties. This may need to be expanded or
time-adjusted a bit tonight as more data comes in.

On Friday night, as the large storm system moves away over the
Great Lakes, winds should gradually diminish and clouds and any
lingering sprinkles will move out. This will give way to a brief
spell of quiet and relatively cool weather as a surface high
pressure ridge slides across the Upper Midwest around Saturday
and into Sunday. High temperatures Saturday afternoon will only
range in the 60s to lower 70s south, and lows Sunday morning are
forecast in the upper 30s northeast to upper 40s south; a
notable change from our long recent spell of unseasonable
warmth.

By Sunday morning the next storm system will be taking shape as
a 500 mb trough digs down over the western U.S. and Rockies. A
reinforcing vorticity maximum will move from around Seattle down
into the larger trough during the day on Sunday, and the entire
system will then move eastward toward the Midwest and Iowa in
the early part of next week. However, the interaction between
the two vorticity maxima wobbling around the larger cyclonic
flow is not being well-resolved by the long range models,
resulting in differences between solutions that could have
significant implications for hazardous weather potential in our
service area. It is likely that an initial surge of theta-e and
moisture advection, along the inflection zone where 500 mb flow
turns to southwesterly in advance of the approaching system,
will produce a swath of rain moving across Iowa from southwest
to northeast on Sunday night or Monday. However, potential for
heavy rain or severe weather is unclear at this time, as is the
timing of this progression. After that confidence steadily
decreases as the large, discombobulated trough approaches. Long
range models have been trending slowly but steadily southward
with heavy rain and thunderstorm chances since yesterday, but
still keep these over or just south of southern Iowa and have
maintained high (70-80%) POPs Monday night and Tuesday for now,
tapering off thereafter as the large system moves away to the
east. Despite the maintenance of high POPs in a desire for
forecast consistency, overall confidence in the evolution of
this system has faded, and will need to wait for better
resolution of this scenario before additional details can be
sussed out. Nevertheless, certainly the potential for severe
thunderstorms and/or heavy rain remains for our area early next
week, and should be monitored over the next several days.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1219 PM CDT Thu May 15 2025

VFR conditions are forecast through the TAF period. Strong and
gusty southwest winds are expected for much of the period,
especially this afternoon and early evening, then again by late
Friday morning. This will result in crosswinds of 20-30 KT at
times on some NW/SE runways. Low clouds and -SHRA will spread in
from the northwest on Friday, but not reach the terminals until
after 18Z and thus will be advertised in subsequent TAF
issuances.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...

Wind Advisory until 8 PM CDT this evening for IAZ004>007-
015>017-023>026-033>037-044>048.

Wind Advisory from 7 AM to 9 PM CDT Friday for IAZ004>007-
015>017-023>026-033>037-044>048-057>059-070-071.

&&

$$

DISCUSSION...Lee
AVIATION...Lee