797
FXUS63 KDMX 030729
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
229 AM CDT Mon Jun 3 2024

.KEY MESSAGES...

- Showers and storms passing through this morning, highest chances
  over northern Iowa

- Severe storms, locally heavy rainfall possible later Tuesday into
  Tuesday evening

- Favoring dry conditions midweek into next weekend with lower
  humidity and seasonal temperatures

&&

.DISCUSSION...
Issued at 229 AM CDT Mon Jun 3 2024

Scattered storms were developing over northwest Iowa with the
convection over Nebraska greatly diminished from its earlier
intensity in the night. There was a short-lived hail core up near
Hawarden around 1230am, which produced 1.25 inch hail. Convective
allowing models (CAMs) such as the HRRR, RRFS, RAP initialized well
with placement, though the HRRR is a bit hot on wind gusts in
Nebraska given the lack of any reports. The northwest Iowa
storms should continue off to the east-northeast into southern
Minnesota and north central Iowa and will have to watch for any
healthier cores for possible hail. As of the Nebraska
convection, the leftovers will continue a downward trend, but
still may get a few gusts of 30 to perhaps 40 mph. Will have to
watch as some guidance shows a weak MCV that may try to kick off
new development toward sunrise. Even if this does occur, it
looks to stay below severe limits.

As these storms depart later this morning, the expectation is for a
minimum in activity across much of central Iowa into tonight with
just some weak vorticity passing overhead for forcing. Attention
turns to the Pacific Northwest region late Monday as a longwave
trough moves eastward and digs and negatively tilts into the upper
Midwest into the day Tuesday. MLCAPEs at or topping 2000 J/kg are
likely ahead of a surface cold front Tuesday afternoon with
marginal deep layer shear for storm organization. Lapse rates
have looked modest staying largely at or below 7C/km the last
few nights, but both the NAM and GFS show an area over southern
Iowa with low level rates above 7C/km. Forecast soundings show
quasi-unidirectional flow with speed shear with this flow
favoring large hail as storms develop. Soundings also show dry
air in the 800-600mb layer and sub-cloud layer and while
downdraft CAPE values are low, this sounding profile may support
gusty downburst winds. As for flash flooding, the heavy
rainfall parameters continue to look favorable with high
precipitable water values, deep warm cloud depths, and 850-300mb
flow somewhat parallel to the cold front. That said, rainfall
amounts do not look too alarming and largely between 1 to 2
inches in small areas. One would have to push to the NBM 90th
percentile to see amounts above 2 inches. While our area missed
some of the potential priming rains yesterday/yesterday evening,
forecast rainfall would near the lowest values of 1 and 3 hour
flash flood guidance in our area. As for rivers, modest rises
are possible through the week. However, outside of the river
flood warning and watch already out on the West Fork Des Moines
River forecast points, ensemble QPF 72 hour best forecast
rainfall would not have any other forecast points exceeding
flood stage. This is supported by the experimental HEFS
(Hydrologic Ensemble Forecasting Service) and its 50% chance of
exceedance showing similar.

The severe weather threat and storms will end by late evening
Tuesday or the very early morning hours of Wednesday and the cold
front will sweep east of the state with lower dewpoint air and more
seasonal temperatures following. The longwave trough will wrap up
with some differences on the exact evolution, but the idea though
will be that Iowa will be beneath northwesterly flow with surface
winds averaging from the northwest through the end of the week.
Thursday looks to be the breeziest day with winds averaging 15 to 25
mph. Forecast is favoring dry conditions, but depending on placement
of shortwaves rotating through the northwesterly flow could see a
passing shower.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1219 AM CDT Mon Jun 3 2024

A broken line of thunderstorms approaching from eastern NE will
weaken as it moves across the area overnight, resulting in a
low confidence forecast but still with a good chance, 40-60
percent, of thunderstorms at any given terminal overnight. Have
thus maintained VCTS groups at all five TAF sites, between about
07Z and 14Z, and introduced short TEMPO groups at FOD and DSM
where confidence is a little higher in at least rain and briefly
reduced visibility/ceilings. There will also likely be a period
of MVFR ceilings at MCW/ALO around or just after sunrise, but
from late Monday morning onward VFR conditions are forecast with
perhaps a few scattered showers/thunderstorms by the afternoon.

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Lee