500
FXUS63 KDMX 171946
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
246 PM CDT Fri May 17 2024

.KEY MESSAGES...

- Isolated storms possible Saturday afternoon east.

- Several periods with storm chances from Sunday through Tuesday
  night and again by late in the week. A few periods may contain
  severe weather and heavy rain chances.

&&

.DISCUSSION...
Issued at 245 PM CDT Fri May 17 2024

An overachieving upper short wave moving into northwest Iowa and
eastern Nebraska today, resulted in some very light shower across
far northern Iowa earlier today. The main impact was passing mid-
level cloudiness that brought brief periods of mostly cloudy
conditions north.

The thermal ridge will move over the area tonight before being
pushed further southeast as a boundary moves into the state. This
boundary is a weakening cold front that will be stretched between
low pressure over southern Canada and another low pressure across
Texas. This surface boundary will start to undercut the thermal
ridge, which will effectively cap surface based parcels, at least
initially. Trends have been towards a stronger cap and greatly
limiting any thunderstorm potential. The convective allowing models
have been a bit more robust on convective initiation but these seems
aggressive considering the lack of mid-level cooling source and the
lack of strong forcing. The Gulf moisture will also be blocked to
the south thus moisture will be lacking. That said, enough
uncertainty remains that have kept isolated storm mention over the
southeast forecast area. Should a storm develop, it could produce
gusty winds or small hail as there is enough room above the cap for
updraft acceleration. The boundary will quickly be followed by low
pressure with even drier air moving into the state.

The forecast still looks quite active beginning Sunday and
persisting through Tuesday night. The upper flow will begin to
transition to southwest flow Saturday night and into Sunday. The
Gulf flow north will become re-established Saturday night and will
fuel storm development across Nebraska and Kansas Saturday night
ahead of an approaching short wave. The main uncertainty on this
activity is when it will reach central Iowa. It will be elevated and
will be able to move over the drier near surface layer so it could
arrive by early afternoon and earlier to the west. Additional storms
are possible Monday and Monday night. The timeframe of most interest
appears to be Tuesday/Tuesday night as there is a near merger in the
northern and southern streams leading to a strong jet max but also
the southern stream short wave arriving first and followed up by the
northern stream short wave overnight or into Wednesday. Per usual
this far into an active period, previous convection can play a
significant role in subsequent boundary location. In this case, the
earlier Tuesday convection could result in the second round being
further south. Either way, severe storms will be possible and could
have an attendant heavy rain threat as well with PWAT values over
1.50" still appearing likely. Thursday should be a transition period
followed by another potential active period Friday into next
weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1225 PM CDT Fri May 17 2024

Breezy southerly winds will persist into this afternoon and
increase some through 20z. The mixed layer will decouple this
evening, likely after 01z though the wind will remain in the 5
to 8 kt range. The wind will increase mid to late Saturday
morning and become more southwesterly. Sct cumulus near 5 kft is
expected over southern and eastern IA this afternoon along with
passing mid level cloudiness. VFR conditions will persist
through the forecast period.

&&

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

&&

$$

DISCUSSION...Donavon
AVIATION...Donavon