845
FXUS63 KDMX 171132
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
632 AM CDT Sat May 17 2025

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

.KEY MESSAGES...

- Notably cooler weather through the weekend, with winds
  gradually decreasing through today.

- 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.

&&

.UPDATE...
Issued at 424 AM CDT Sat May 17 2025

Winds this morning will continue to gradually decrease, with gusts
generally up to 30-35mph falling into the 15-25mph range especially
into the evening. Given the very slow nature of the departing low
pressure over the Upper Midwest, have slowed the decrease of clouds
across the state. The main concern of note otherwise at least
through Sunday afternoon is the fairly isolated potential for frost
in the far northwest Sunday morning, as conditions are generally
favorable but low confidence as temperatures are just a bit warmer,
and winds light so any impact on any frost formation would be
minimal. Decided to hold off on any frost headlines but should be
re-evaluated in the next forecast package given these latest trends.

Active weather is still expected to ramp up Sunday, which per
various deterministic as well as CAM guidance generally depicts a
boundary arriving into southwest Iowa just after 6-7pm Sunday and
tracking northeast across the state overnight into Monday morning.
Ahead of this boundary, temperatures increasing into the 60s with
dewpoints largely in the upper 40s to low 50s will allow for
instability to increase around 1000J/kg in MUCAPE, along with
effective shear values around 45-55 knots and steep lapse rates.
These storms continue to look fairly elevated in nature, with hail
and gusty winds the main concerns. These atmospheric variables,
along with an increasing LLJ across the region will help to generate
storms into southwest Iowa Sunday evening into Monday. In terms on
convective mode, most solutions generally show scattered shower and
storm activity lifting southwest to northeast across Iowa, though
NAM guidance depicts more of a defined MCS developing out of eastern
Kansas and passing across Iowa. At this time, scattered convection
looks more likely given the nature of the tracking boundary, but
cannot rule out MCS activity through the morning hours into Monday.
The Marginal risk area for severe weather from the SPC across the
southwest portion of the state, and even a Slight risk in the far
portion of southwest Iowa into Nebraska and Missouri seems
reasonable. On the hydro side, PWATS around 1-1.25 inches with warm
cloud depths around 12-15 kft, paired with the slow moving nature of
storms looks to bring rainfall amounts above 0.5-1 inch or more,
though antecedent dry conditions should result in limit impacts
in terms of possible flooding.

The active pattern looks to continue into Monday, especially by the
afternoon to evening as the surface low begins to track just west of
Iowa. A Slight Risk for severe weather is currently outlined
mainly along and south of I-80 per SPC guidance, as warm air
spreads over most of the state boosting instability around 2000
J/kg and shear around 55-60 knots leading to fast moving storms.
Further details will be better known in the coming days, which
will largely be affected based on how the convection slated for
late Sunday into Monday plays out. Shower and storm chances
remain through midweek, with the highest potential for severe
weather south by Tuesday, before decreasing into the evening.
Will continue to keep a very close eye on these details over the
next few forecast packages.

&&

.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 /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 632 AM CDT Sat May 17 2025

Widespread MVFR conditions remain as low stratus continues to
pass across the state. These areas of low stratus are expected
to pass overhead until about 19-20z at KDSM and KOTM, and
closer to 22-00z at the northern terminals before ceilings
gradually improve. Winds out of the northwest will remain breezy
through the afternoon with gusts generally up to 25-20 knots,
then will decrease into this evening.

&&

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

&&

$$

UPDATE...Bury
DISCUSSION...Lee
AVIATION...Bury