349
FXUS63 KDMX 292325
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
625 PM CDT Mon Apr 29 2024

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

.KEY MESSAGES...

-Dry tonight, with clearing skies and winds turning light

- Showers and thunderstorms return Tuesday afternoon to evening,
with severe weather remaining on the table

- Continued chances for showers and storms Wednesday and Thursday,
with remaining severe potential and heavy rainfall possible

&&

.DISCUSSION...
Issued at 348 PM CDT Mon Apr 29 2024

Conditions have shaped out to be relatively quiet as expected, as
surface analysis shows the surface low pressure currently over
Minnesota/Wisconsin, though low clouds remain across the northern
two-thirds of the state after looking further into satellite
imagery. This low pressure however has been tracking northeast
through the day, which has allowed for this cloud cover to begin
breaking apart and gradually pushing out of the state. This clearing
trend will continue, with skies turning clear across the state into
this evening. As a result of today`s cloud cover, temperatures vary
quite a bit over the state, with far southern Iowa seeing early
afternoon temperatures in the 60s, while over northern Iowa where
the thickest clouds are seeing values in the upper 40s to low 50s.
Winds as expected so far have been breezy out of the west/southwest
as the system departs out of the region, with reported gusts
generally around 25-30 mph. These winds will gradually decrease as
upper level ridging makes its way across the region tonight.
Clearing skies with increasing subsidence will allow for rather
efficient radiational cooling overnight which will allow for
temperatures to cool into the 40s, while far northern Iowa will
likely see values dip into the upper 30s. Cannot rule out isolated
patchy fog across the northwest given low level moisture and calmer
conditions as indicated by HREF guidance, though confidence on
occurrence remains on the lower end given the small window of
opportunity.

Expecting a quiet start to the day as upper level ridging gradually
departs to the east. Increasing southerly flow will result in warmer
air advecting into the region, with highs into the afternoon
reaching into the mid to upper 70s across the state. Winds will turn
rather breezy with the tightening pressure gradient ahead of
the next system, with gusts upwards of 25-30 mph expected. Into
Tuesday afternoon, a shortwave ejecting eastward into Iowa, with
increasing theta-e advection into the state and moderate
forcing for lift is expected, with showers and thunderstorms
expected to develop along the resultant cold front. Though
variance among CAM guidance currently exists, the general trend
has this development beginning over northwestern Iowa, before
extending further south across western Iowa and gradually
pushing eastward across the state after 3- 4pm. A deeper dive
into model soundings shows moderate MLCAPE values in the
1500-2000 J/kg range, effective shear values up to 40-50 kts,
and very steep lapse rates around 7.5-8 C/km into the late
afternoon to early evening hours. Drying in the low levels
continues to be evident as well with an inverted-v sounding over
portions of southwest Iowa and moderate DCAPE values. Given
this parameter space, would expected the primary threats to
remain large hail and damaging winds, with a tornado threat
remaining lower end but not zero given the aforementioned drier
air and higher LCL heights, despite higher end SRH values and
favorable hodograph structure. SPC guidance therefore keeps the
Slight Risk along and west of I-35, though Marginal further east
as storms are generally expected to become weaker and more
elevated with generally less favorable conditions available for
storms to maintain themselves. Though heavy rain could occur at
times with any stronger storms, not expecting any higher-end
rain totals with this round, generally under an inch.

A brief reprieve is expected Wednesday morning, though a developing
low pressure system on the lee-side of the Rockies per model
guidance is generally expected to make a gradual lift into the
Central Plains later in the day, with a surface warm front pushing
into southern Iowa and gradually lifting north/northeast. Timing
differences exist between deterministic models, with the NAM and GFS
a bit slower as most activity looks to remain south of I-80, while
the Euro has widespread activity across the state as showers and
storms move into the region by the afternoon/evening. Of interest
particularly would be through the evening Wednesday into Thursday
morning as a strengthening 50+ knot LLJ strengthens directly
overhead, along with a stronger push of theta-e advection. The
overall environment looks favorable for potential severe weather,
especially over southwest Iowa where the triple point is expected to
generally be located as current guidance suggests. Moderate MUCAPE
around 2000+J/kg and steep lapse rates per soundings are indicated
over southern Iowa, along with effective shear values around 30kts
would allow for a potential of hail and damaging winds, though
tornado potential is not zero as favorable turning in the low levels
is indicated, though LCLs again look to be too high. Active weather
continues through Thursday as Iowa looks to remain in the warm
sector at least into the afternoon, with ongoing chances for on and
off showers and storms. As the low pressure lifts northeast through
the day, a trailing cold front looks to follow into the afternoon to
evening, though uncertainty increases as location of this front
varies. Generally speaking, eastern Iowa looks to see the highest
potential for the cold front to impact, with additional chances for
showers and storms. Temperatures in the 70s paired with higher
moisture content as dewpoints increase further in the mid to upper
60s will allow for plenty of instability overhead, along with
favorable shear and very steep lapse rates over the region.
Considering hydro details with the system evolution Wednesday
through Thursday, PWATS through the event are on the moderate side
nearing 1.5 inches with higher warm cloud depths, which will allow
for periods of heavier rainfall and rising river levels, with
widespread totals of 1-2 inches expected and some locally higher
amounts possible. Overall, the potential for flooding still looks to
be on the lower end, but will continue to evaluate further. This
doesn`t go without saying that there are details that will need
further refining over the next few forecast packages as more details
become known.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 623 PM CDT Mon Apr 29 2024

The MVFR cloud deck over northern Iowa should continue to depart
the area through the early evening. Winds will also begin to
diminish as the sun sets over the next hour or two. Otherwise,
clear skies with light and variable winds are expected tonight.
TAFs reflect the expected prevailing direction, but may not be
representative of the whole period due to light flow. Tomorrow
becomes gusty as another storm system approaches. Fog chances in
the morning seem to be low, but still possible mainly near KFOD.

&&

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

&&

$$

DISCUSSION...Bury
AVIATION...Dodson