928
FXUS63 KDMX 141138
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
638 AM CDT Wed May 14 2025

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

.KEY MESSAGES...

- Thunderstorms possible tonight and Thursday. A few strong
  storms may occur, mainly over northern Iowa.

- Strong to very strong winds may occur at times Thursday into
  Friday.

- Warm again today then a bit cooler by the end of the week and
  into the weekend with highs in the 60s and 70s.

&&

.UPDATE...
Issued at 358 AM CDT Wed May 14 2025

Update for thunderstorm chances tonight into Thursday along with the
potential for strong winds at times Thursday through Friday. Surface
cyclogenesis is ongoing over northeast Colorado this morning ahead
of a closed upper low that is moving across the Utah and Idaho
region. The surface low pressure will settle into northwest Kansas
for most of today and deepen before lifting northeast into Nebraska
late this afternoon and evening. Strong theta-e advection will lift
into central Iowa late this evening and this could trigger elevated
thunderstorm development. While there will be pronounced elevated
mixed layer (EML) capping for today, the theta-e advection will
moisten while also eroding the EML overnight and how saturated the
layer around 850 mb becomes will determine areal coverage of the
elevated storms. The vertical ascent will be focused near 850 mb and
if the EML is eroded enough, free ascent for convective development
is expected. Lifting air parcels from this level results in MUCAPEs
of 2000-3000 J/kg, which is plenty for the potential for large hail.
Storm relative deep layer sheer profiles are generally weak with
little to no speed or directional shear so any updrafts likely have
limited lifetimes though elevated convection processes and inflow is
a bit different than surface based convection.

A more sustained area of convection will be ongoing over Nebraska
this evening and is expected to move into northwest Iowa around
midnight or after. These storms will be surface based along the
first boundary moving into the area. These storms should be
weakening as they arrive as they start to loose their surface based
instability and at some point, the cold pool should start to surge
ahead of the line. The one potential caveat is if these storms are
able to link into the corridor of elevated instability that is
lifting into northern Iowa at that times. it is possible damaging
winds may occur with these storms. More storms may refire over parts
of north central and northeast Iowa Thursday morning and if that
occurs, an attendant severe weather threat with large hail and
damaging winds would be possible. The latest trends are this
development may be to the northeast of the DMX county warning area.

There are several opportunities for strong synoptically driven winds
Thursday into Friday. This is mainly over northern/northwest Iowa.
There remain mixed signals in the strength of the wind as the first
lobe of cold advection arrives immediately behind the low Thursday
morning along with pressure height rises. A greater signal for
headline criteria wind is Thursday night into Friday as mixed layer
winds in excess of 50kts. There is > 50% confidence that wind
headlines will be needed at some point during this period and there
a non zero chance that a few areas reach High Wind Criteria.

Finally, we continue to pay attention to early next week as a strong
storm system moves into the Midwest which may being an increasing
chance for thunderstorms along with severe weather chances.

&&

.DISCUSSION...
Issued at 235 PM CDT Tue May 13 2025

Surface dew points in the 60s were able to reach the eastern part of
the state today, however saturation aloft looks less impressive than
it did yesterday, restricting the cloud layer to only a couple
thousand feet. Similarly, the instability has decreased, leaving
less available to list parcels. Have continued the trend from the
overnight forecast in keeping PoPs below 20%. Dew points are on the
increase today and cloud cover will keep the southeast closer to the
80 degree mark.

Southeast winds remain again on Wednesday as a trough approaches
from the Rockies. This trough will deepen and negatively tilt just
to the north and west of the state, developing a closed low. The
initial theta-e advection wing arrives after sunset Wednesday. While
the NSE will have weaker shear, MUCAPE values of around 1500 J/kg
will be plenty enough to support marginal hail in thunderstorms that
form on this moisture axis. The question mark is if the EML can be
overcome overnight. Models illustrate this well in their simulated
reflectivity, struggling to produce updrafts until during the day
Thursday in the northeast. The dry slot will be on the approach,
but so will the better wind profiles, so the severe threat for
these initial storms is not zero as they approach places like
Mason City in the morning Thursday (<10% confidence).

Now for the main part of the system associated with the severe
weather outlook: A line of storms will form near the surface low
along the cold front. The better wind profile will lag behind it as
it enters Iowa, but cloud layer shear of 40kts will line up in
proximity overnight. Marginal hail and damaging winds will be
possible, but confidence at this point is under 10%. CAMs and
synoptic guidance are split on how this line will evolve across the
northern half of the state late in the night and early Thursday.
This is due to a couple of synoptic factors: 1) Will the EML be
overcome and 2) will the invading dry slot undercut existing
updrafts? Synoptic guidance leans pessimistic in regards to storm
coverage, whereas CAMs preserve the line, lifting it into Minnesota
through the morning. At this time, there`s a 50% confidence in
precipitation in northern Iowa Thursday, but confidence is not high
in these maintaining severity at this time (<10%). Confidence is
lower for this precipitation existing farther south than Highway 20
because of the EML.

The upper low will slow/stall in Minnesota into Friday with another
front making its way in from the northwest. Precipitation chances
linger nearest the low center in far northern Iowa. Another trend to
watch will be the behavior of the next synoptic front that arrives
in the state Friday afternoon. On top of the low to the north,
another wave to the southwest with lee cyclogenesis will generate
more lift and some moisture advection back into the state. Over
50kts of deep layer shear will be at play. Gradient winds will
approach advisory criteria nearest the northern low in northern
Iowa.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 636 AM CDT Wed May 14 2025

VFR conditions into this evening. Southeast wind will be breezy
at times this afternoon. Cumulus development also this afternoon
though any bases will be AOA 4kft. Thunderstorm chances increase
overnight, mainly after 06z. Cigs will increase also with MVFR
or potentially lower cigs developing. Some guidance suggesting
IFR cigs at a few locations late in the period but the
confidence in these cigs remain low enough to exclude from the
forecast at this time.

&&

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

&&

$$

UPDATE...Donavon
DISCUSSION...Jimenez
AVIATION...Donavon