467
FXUS63 KDMX 090517
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1217 AM CDT Thu May 9 2024

 ...Updated for the 06z Aviation Discussion...

.KEY MESSAGES...

- Showers and storms likely by later this afternoon into the
  evening. Some stronger to severe storms possible with gusty
  winds the primary threat.

- Scattered to widespread rain showers linger overnight into
  Thursday. Total rainfall amounts of 1-2"+ possible over the
  northern half of the area, lesser amounts south.

- Quiet and dry to end the week. Shower and storm chances return
  early next week.

&&

.DISCUSSION...
Issued at 200 PM CDT Wed May 8 2024

Primary forecast challenge remains with convective and hydro related
trends over the next 12 to 24 hours. 19z GOES water vapor imagery
shows a large upper low centered over the western Dakotas along with
numerous embedded shortwave disturbances. At the surface a weak low
pressure center was noted moving into northwest Iowa, and a deeper
surface flow analyzed into western Missouri. Radar has shown returns
over portions of western and northern Iowa for much of the day, but
much of this has failed to reach the ground due to a substantial sub-
cloud dry layer and surface T-Td spreads of 20-30F.

High confidence in precip chances ramping up by mid to late
afternoon through this evening as forcing associated with a
mid-level shortwave rotates around the parent upper low. Hi-res
guidance has focused on an area of convection developing over
west central into northwest Iowa, and another corridor of
loosely organized convection over southwest into central Iowa by
22-00z. The storms then steadily track northeast through the
evening hours. Overall the severe threat will be limited by
marginal instability as the primary axis of deep instability
remains well to our south. Sounding interrogation suggests
downburst wind gusts will be the primary threat with any of this
acitivty given the aforementioned dry layer. Even with rather
limited surface based instability dcape is progged between
750-1000 J/kg. Cool mid levels and rapid flow aloft may support
a few cores with small to marginally severe hail. CAPE in the
lowest 3km may provide sufficient stretching for a few funnels,
but the dry air/high LCLs negates a meaningful tornado threat.

Instability wanes overnight as shower activity festers over northern
Iowa. Steering flow becomes sluggish as the upper low drifts
overhead. The slow progression will allow for some locations to
steadily pick up more rainfall on top of any amounts from this
evening. Thursday looks cloudy and cool with additional chances
for light rain and the upper low meanders. This will hold highs
close to 60, possibly not getting out of the 50s in some
locations. HREF PMM suggests rainfall totals over the next 24
hours of 0.5" or more generally along and north of I-80, with
higher amounts possibly exceeding 1-2" toward northern Iowa.

The decaying upper low is kicked out by Thursday night as
northwesterly flow is established aloft. Forecast is majority
dry to end the week with highs in the 70s. The only exception is
token lingering low PoPs in our far north/east associated with
a shortwave crossing the Great Lakes region. Most model/ensemble
guidance keeps precip off to the north and east, closer to the
deeper forcing tied to the upper disturbance. Temps warm a bit
early next week coincident with subtle ridging aloft.
Shower/storm chances return as a closed low slowly moves east
out of the Four Corners region. Flow aloft is very weak and
primary forcing mechanisms ill-defined, so it is difficult to
pinpoint a particular period for higher PoPs or glean potential
precip amounts. The broad PoPs provided by the NBM will have to
suffice at this juncture.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1217 AM CDT Thu May 9 2024

Clusters of TSRA will linger over northern Iowa in the next few
hours, with ALO the most likely terminal to be impacted.
Thereafter, SHRA will linger around MCW overnight then spread
southward again on Thursday morning and have VCSH/SHRA groups at
all terminals accordingly. It also appears prevailing MVFR
ceilings will spread across much of the area during the early
morning hours Thursday and persist for much of the day, and have
advertised this at all terminals as well. Occasional
thunderstorms or IFR ceilings may also be possible at times on
Thursday, however confidence in timing/location of such
conditions is too low to include in the TAFs at this range.

&&

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

&&

$$

DISCUSSION...Martin
AVIATION...Lee