Grimes, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

737
FXUS63 KDMX 141835
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
135 PM CDT Wed May 14 2025

.KEY MESSAGES...

- Chance of thunderstorms (20-40%) overnight tonight, mainly
  north of I-80. A few stronger storms may produce large hail.

- Windy conditions at times Thursday afternoon and evening, and
  late Thursday night into Friday morning.

- Cooler temperatures and mostly dry from Thursday night
  through the weekend.

- Active weather pattern early next week, with some potential
  for severe thunderstorms especially between Monday night and
  Tuesday night.

&&

.DISCUSSION...
Issued at 235 PM CDT Wed May 14 2025

A deep trough extends over the Rockies, with a 500 mb low moving
across Montana this afternoon and a surface low developing over
western Kansas. As the trough deepens and digs into the High Plains
tonight, the 500 mb low will recenter roughly near the South
Dakota/Nebraska border and then pivot northeastward over Minnesota
by late Thursday night. In response to this evolution, the surface
low in western Kansas will intensify and move quickly northeastward
tonight and Thursday, wrapping into the developing 500 mb gyre.
Ahead of this system, theta-e and moisture advection will ramp up
over Iowa late today and overnight. Initially we will capped off
this afternoon, but increasing low-level moisture will help to erode
the cap and eventually lead to thunderstorm development tonight.
This is likely to occur in an arcing band within the zone of
strongest theta-e advection, oriented roughly northwest to southeast
across our forecast area. Depending on when convective initiation
occurs, the storms may be limited to our northeastern counties or
may develop earlier/closer to central Iowa. Most short-range model
guidance has been trending a bit later with initiation, depicting
scattered storms firing around 06-07Z perhaps near the I-80 or
Highway 30 corridors. With these elevated storms overnight, forecast
soundings indicate substantial instability and MUCAPE around 2000-
2500 J/kg, but very weak shear within the elevated layer. This
supports a threat of large hail with the stronger storms in the
arcing band, especially as it matures moving into northern Iowa
early Friday morning.

Farther west, stronger surface-based storms will fire across the
plains of western/central South Dakota and Nebraska late today and
this evening, then move eastward/northeastward tonight. As the
storms approach the northwest Iowa border overnight, likely after
midnight, they will weaken and send out a surging cold pool to the
east. Most high-resolution model guidance keeps any lingering
convection with this feature moving northeast into southwestern
Minnesota, just grazing or missing our service area. However, if new
storms are able to generate at the intersection of the advancing
cold pool and the arcing band of theta-e advection above the
surface, these storms could be near-surface based tapping into
stronger low-level shear and resulting in a threat of straight-line
winds in addition to hail. Right now the probability of this
occurring is low, but we will be closely monitoring trends today and
tonight. The Storm Prediction Center is appropriately advertising a
Marginal Risk of severe weather across much of western into north
central Iowa overnight.

Any overnight storms in our area will clear quickly to the northeast
on Thursday morning, later leading to a higher severe weather threat
downstream in eastern Minnesota and Wisconsin. As the parent 500 mb
low moves over eastern South Dakota on Thursday afternoon it may
spread a few showers or thunderstorms into our northwestern counties
in a grazing blow, however it is more likely these will remain north
of our area and POPs remain limited in the 20-30% range by the
afternoon. Of greater consequence is that as the large cyclone moves
by to our northwest strong and gusty westerly winds will surge into
our area, especially northwestern Iowa where some model output
indicates the potential for winds of around 30-35 KT with gusts
to 45+ KT during the afternoon/early evening. Later, as the low
becomes stacked and spins over Minnesota late Thursday night
into Friday morning, a tighter surface pressure gradient will
spread into our forecast area concurrent with moderate cold air
advection flowing in. This will support a second surge of winds
which will likely be stronger than the first. Several model
solutions are depicing wind gusts to around 50 KT in our
northwestern counties during this time, though there is some
uncertainty in how efficient mixing will be during the early
morning hours, and also the surface low will be slowly filling
in during that time rather than deepening. In any event, some
form of wind headline is likely for either Thursday
afternoon/evening, or Thursday night/Friday morning, or both.
However, with the more marginal nature of the first round and
the longer time remaining before the second round, and with
severe thunderstorm potential preceding both, will hold off on
any wind headlines at this time.

From later Friday through Sunday we will see relatively quieter
weather, with the big gyre moving eastward over the Great Lakes and
a surface high pressure ridge sliding across the Upper Midwest. This
will support cooler and dry conditions in Iowa, with highs Saturday
and Sunday ranging from mid/upper 60s north to mid/upper 70s south.
However, by late Sunday another large trough will have developed
over the western CONUS with a 500 mb low centered somewhere near the
Nevada/Utah/Arizona borders. This trough will subsequently progress
east northeastward into the central U.S., with long-range model
solutions bringing the 500 mb low to near the Nebraska/Iowa border
on Tuesday evening. Within this scenario we can expect several
rounds of thunderstorm chances early next week. The first round or
two will be associated with warm air/moisture advection on the
leading flank of the system, perhaps as early as Sunday night but
with higher chances by Monday night, and then more storms as the
main system moves through on Tuesday. At this extended range it is
diffuclt to pin down details of thunderstorm threats, but
synoptically this type of evolution in late May leads to severe
weather potential increasing with each round of storms, and Tuesday
afternoon/evening could be quite hazardous somewhere in the region.
We will be monitoring this potential in the coming days, especially
after getting through the severe thunderstorm and high wind risk
periods over the next couple of days.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1208 PM CDT Wed May 14 2025

VFR conditions will prevail through sunset. Low clouds and
widely scattered thunderstorms will develop overnight, resulting
in the inclusion of MVFR ceilings and PROB30 TSRA groups at
most terminals late tonight into early Thursday morning. Some
model guidance indicates IFR ceilings may be possible during
this time, but it is more likely they will remain between FL010
and FL020 and have not advertised IFR in the outgoing TAFs.
Toward the end of the period, later Thursday morning, winds
will turn to SSW at most terminals and become stronger and
gustier, with low clouds clearing out and VFR conditions
prevailing once again.

&&

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

&&

$$

UPDATE...Donavon
DISCUSSION...Lee
AVIATION...Lee