Marshalltown, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

266
FXUS63 KDMX 091120
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
620 AM CDT Sat May 9 2026

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

.KEY MESSAGES...

- A cool front will move southward across Iowa today, bringing
  low (20%) rain chances at times; north in the morning and
  south in the afternoon to evening. A few thunderstorms are
  possible near the Missouri border, but severe weather is not
  anticipated.

- The front will bring gusty northwest to north winds across
  northern Iowa this afternoon, then notably cooler weather from
  tonight into Monday. Frost may occur in parts of northern Iowa
  Sunday morning and especially Monday morning.

- Warmer temperatures through next week, with the potential for
  more showers and thunderstorms late Monday into Tuesday. Any
  severe weather threat will be better determined in the coming
  days.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 148 AM CDT Sat May 9 2026

A large 500 MB low is currently centered near Hudson Bay,
extending cyclonic northwesterly flow down into the Midwestern
and eastern U.S. A subtle shortwave is rotating through this
flow over Minnesota early this morning, and will move over
Wisconsin and the Great Lakes today. Meanwhile, at the surface a
large high pressure area is spilling from south central Canada
down into the Dakotas. The leading edge of the influence of this
high pressure is manifesting as a wind shift, currently moving
into southern Minnesota and eastern South Dakota. Widespread
mid-level clouds and light showers accompany and follow the wind
shift at this hour and as the high pushes southeastward today,
this feature will track into and across our forecast area during
the day. The primary question is how much we will see, if
anything, regarding precipitation. While light rain is currently
being observed at a number of sites well to our north behind the
boundary, regional radar shows returns are scattered even there,
and with the upper forcing stemming from the shortwave moving
northeast of our area into Wisconsin, much of the impetus for
rain may fade out as the wind shift enters our northern counties
just after sunrise. Forecast soundings also show low-level dry
air in place, which will aid the gradual weakening or
dissipation of any light showers. Even so, the dry air has not
stopped isolated showers the previous two days, and have added
low (20%) PoPs in our north this morning accordingly. Any
lingering showers that do reach northern Iowa should fizzle out
toward midday as diurnal mixing deepens the mixed/dry layer and
the 500 MB shortwave moves even further away to our
northeast/east. However, this afternoon and evening meager
instability will develop in southern Iowa along and ahead of the
boundary, and low-level convergence along the wind shift should
help to generate isolated showers and thunderstorms in that area
so low (20%) PoPs are carried once again. Any impacts should be
low as only brief and spotty rain is anticipated with little
accumulation and only isolated lightning.

In addition to the low rain chances associated with the passage
of the boundary today, cooler air will also flow into Iowa as
the approaching high pressure area exerts its influence. In our
southern counties temperatures will warm nicely ahead of the
boundary, reaching highs in the mid to upper 70s. However, in
our northern counties temperatures will peak in the mid-60s just
afternoon, then remain steady or slowly fall later in the day.
Cold air advection will aid turbulent mixing and gusty winds
will develop, mainly across northern Iowa where gusts to around
30 MPH or higher are likely. Low dewpoints in the 30s will also
flow in, sending relative humidity levels down into the 20-30%
range at times this afternoon. However, green surface fuels will
mitigate or preclude any elevated fire weather concerns.

From tonight through Sunday night the high pressure area will
dominate our weather leading to cool, clear, and quiet
conditions. Early Sunday morning temperatures will fall into the
mid to upper 30s across northern Iowa and patchy light frost may
occur near the Minnesota border. On Sunday ample sunshine will
still allow temperatures to rise into the 60s across most of our
service area, but then Sunday night temperatures will plummet
once again after sunset. With the axis of the surface ridge
lying over northeastern Iowa early Monday morning lows will fall
even further, potentially to near freezing up around Mason City
where a Frost Advisory or Freeze Warning may become warranted.
This potential will be further assessed over the next couple of
days.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 233 PM CDT Fri May 8 2026

The Hudson Bay low breaks up and departs east into the overall
flow through the beginning of next week, releasing the western
CONUS ridging eastward into the midwest. This will boost
temperatures into the 70s and 80s through the first part of the
work week, while also opening the state up to a better gulf
moisture stream. Another wave will pass through the upper
mississippi valley region late Monday into Tuesday, meeting with
this moisture stream and bringing shower and thunderstorm
chances back to the area. The progression of the system has
slowed down a bit more into Tuesday, which could open up the
potential for better instability as the front passes Tuesday
afternoon. However, current deterministic guidance still keeps
most of the QPF tied to the main wave north and east of the
state, with little occurring over Iowa where the better
instability resides. A deeper dive into model soundings shows a
warm nose that is likely inhibiting parcels being lifted into
the unstable layer. Therefore, capping, magnitude of
instability, and frontal passage timing will all be something to
watch as more guidance comes in through the weekend, especially
with the healthy LLJ and wind fields associated with this
system. GEFS AI/ML guidance isn`t sold on severe weather
potential on Tuesday, likely due to the questionable instability
within guidance pulling probabilities down. Regardless, the
takeaway is that shower and thunderstorm potential returns to
begin next week, with a potential for at least a few stronger
storms, depending on the mentioned factors. The stronger winds
ahead and behind the system will also lead to breezy conditions
on Tuesday as the front passes. Temperatures moderate some into
Wednesday behind the front, but warm considerably through the
second half of the week as thermal ridging builds back eastward.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 620 AM CDT Sat May 9 2026

VFR conditions are forecast through the TAF period. Winds will
shift to northwest and increase today, especially at MCW/ALO
where gusts to 25+ KT will occur at times. Brief -SHRA also
possible, mainly at MCW this morning and perhaps DSM/OTM this
afternoon, but probability of any visibility reduction at any
terminal is so low (less than 10%) that it is not mentioned in
the TAFs.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...Lee
LONG TERM...Dodson
AVIATION...Lee