195
FXUS63 KDMX 162352
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
652 PM CDT Fri May 16 2025

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Strong and gusty winds through this evening, with A Wind
  Advisory in effect across much of northern, western, and into
  central Iowa.

- Patchy light rain through early tonight, mainly across the
  northern half of Iowa, and notably cooler weather through the
  weekend.

- More active weather is forecast from late Sunday into early
  next week. Severe thunderstorms and heavy rain may be possible
  at times between Sunday night and Tuesday, but confidence in
  timing and details is low at this range.

&&

.DISCUSSION...
Issued at 203 PM CDT Fri May 16 2025

Things have unfolded largely as forecast today. A large gyre is
spinning over northern/central Minnesota, and this morning a
weak shortwave impulse rounding the base of the cyclone
generated some sprinkles/light showers across central and
southern Iowa but generally with only a trace of rain. Meanwhile
a surface trough is moving across our forecast area like a
spoke on the wheel, currently picking up on radar and
approaching the Des Moines and Waterloo areas. Ahead of this
trough winds are from the southwest and skies are mostly sunny
with temperature climbing all the way to 82 at Oskaloosa, but
behind the trough winds shift to westerly, skies become mostly
cloudy with a few sprinkles/light showers, and temperatures have
fallen to 49 at Estherville. Winds are strong and gusty
everywhere, but particularly behind the trough and across our
favored northern and western areas where a couple of sites have
sporadically reached High Wind Warning criteria, but the
majority of the area/time have remained solidly within Wind
Advisory criteria. As such, no changes have been made to the
ongoing advisory which remains in effect through 9 PM.

As the large cyclone moves eastward into the Great Lakes region
and continues to slowly fill in tonight, winds will gradually
diminish after sunset and any lingering light rain will move
out. A high pressure ridge will then build in from the north
Saturday and Saturday night, leading to mostly clear skies by
Saturday afternoon along with cooler temperatures through the
weekend. On Saturday temperatures will remain in the 60s despite
good insolation late in the day, and in the north closer to the
ridge axis and where clouds may linger longer through the
morning, temperatures may not even reach 60 degrees. After
sunset temperatures should then fall quickly Saturday night,
especially in areas where winds go very light or calm, and
Sunday morning lows are forecast in the mid to upper 30s in our
northern counties and mid-40s in the south. At this time the
lowest forecast temps are 37-38 and winds are expected to stay
light (rather than calm) with the core of the ridge over
Minnesota, so have not included any frost in the forecast.
However, it is possible isolated, sheltered locations in our
northwest could see a light frost by sunrise Sunday.

The general scenario for early next week has not changed
appreciably since yesterday, but with some increase in
confidence levels regarding the details of the evolution. By
Sunday morning a trough will have dug into the western U.S.,
with a two-phase appearance of vorticity maxima, one near the
Utah/Arizona border and the other over Washington state. During
the day Sunday the northern vortmax will sink south
southeastward while the southern one lifts slowly northeastward,
and thereafter the two will do-si-do and eventually congeal into
one large mid/upper low over the High Plains by Monday evening.
The entire system will then move slowly eastward over Iowa
before passing to the east around Wednesday night. At the
surface, the aforementioned high pressure ridge will lie across
the Upper Midwest and Iowa Sunday morning ahead of the storm
system, with cool and dry conditions in place. By Sunday evening
however, things will begin to change as the western storm
system approaches, turning steering flow over Iowa to the
southwest with warm air/moisture advection aloft surging in.
Forecast soundings show this rapid warming/saturating aloft, but
also maintain a dry near-surface layer consistent with easterly
surface flow out of the retreating ridge. While some
deterministic model runs paint copious QPF across our forecast
area Sunday night, others (most notably the NAM) are more
bearish as a result of this low-level dry air. Regardless of
whether precipitation is widespread or more scattered, and how
early Sunday night it is able to commence, model soundings are
nearly unanimous in depicting increasing instability aloft,
especially in our southwestern counties where MUCAPEs will
increase to around 2000-2500 J/Kg by late Sunday night. While
shear in the elevated layer is not overly strong, it is enough
combined with the strong instability to support a hail threat
during this time, again mainly in our southwest, and this is
well illustrated by the D3 Marginal Risk from SPC.

During the day on Monday, surface cyclogenesis will result in a
deepening low over the OK/TX panhandle and the development of a
warm front arcing across northeastern KS and northern MO. North
of this boundary, over Iowa, convective evolution during the day
is uncertain as warm air advection continues aloft but is
undercut by cooler surface flow from the east northeast, and
broad but nebulous forcing overhead is unfocused by a lack of
surface boundaries/heating. This should result in a relative
lull in precipitation for most of the day, but with a
maintenance of POPs in the forecast to account for scattered
rain/storms especially with the degree of saturation just above
the surface. By late in the day Monday thunderstorms should
develop rapidly near the surface warm front, perhaps just to our
southwest, and move northeastward into our service area in the
deeper steering flow. As the mid/upper low moved in Monday night
and mean winds back around into it, storm motion will slow and
high POPs will be maintained right across Iowa. This will lead
to a potential for slowing/erratic motion of individual
convective elements and prolonged potential for heavy rain,
leading to an increasing concern of hydrologic impacts. However,
these are mitigated by the very dry antecedent conditions, and
limited by a relative lack of confidence in such details 3-4
days out. In addition, there is certainly some potential for
severe thunderstorms late Monday/Monday night, however, if the
surface warm front does stay south of our area then any storms
would likely be elevated for us, and with profiles saturated the
threat of any hail/wind would appear fairly low and mostly
confined to our southern counties. That being said, we are still
several days out from this system and any northward shift in the
location of the boundary would have a significant effect on our
severe weather probabilities, so the situation definitely bears
watching over the next few days.

By Tuesday the large storm system will be filling in and
gradually weakening, then moving off to the east, with any
precipitation in our area diminishing in intensity on Tuesday
and ending sometime around Tuesday night or so. This will be
followed a brief period of deep-layer ridging that should
provide a respite of quieter weather roughly around
Wednesday through Thursday or so, followed by an establishment
of relatively brisk west northwesterly 500 mb flow later next
week which may lead to a return of intermittent precipitation
chances and a prevention of any significant warming.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 644 PM CDT Fri May 16 2025

Trends this issuance favor a longer duration of MVFR for
northern sites, reaching as far south as KDSM by the early
morning hours. Light rain to taper off by 06z. Gusty winds will
slowly ease through tomorrow morning, becoming less gusty after
18z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 9 PM CDT this evening for IAZ004>007-
015>017-023>026-033>037-044>048-057>060-070>072-081-082-092-
093.

&&

$$

DISCUSSION...Lee
AVIATION...Jimenez