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321
FXUS63 KDMX 140429
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1129 PM CDT Thu Jun 13 2024

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

.KEY MESSAGES...

- Severe storms with damaging wind gusts, large hail over
  southern Iowa developing later this afternoon and moving out
  of the state this evening

- Periodic shower and storm chances return later Friday night
  through Sunday morning; locally heavy rainfall possible with a
  low severe risk at this time

- Elevated heat and humidity Sunday into at least early next
  week

&&

.DISCUSSION...
Issued at 148 PM CDT Thu Jun 13 2024

GOES-East Day Cloud Phase Distinction RGB has shown cumulus and
convective clouds along I-90 with most of the showers and the
handful of thunderstorms largely staying over far northern Iowa or
southern Minnesota. This activity was being forced by a weak
shortwave that is now moving through Minnesota. Our attention will
be on two surface boundaries that are over the state and which we
are monitoring this afternoon for convective initiation (CI). The
first is the surface front, which has pushed well into southern
Iowa. The second is a moisture discontinuity lagging behind a county
to two with dewpoints in the 50s behind it and well into the 60s to
low 70s ahead of it. This boundary may be the source for much of our
CI in the next few hours. Beyond the storms this afternoon, heat
south of this boundary has built with temperatures nearing 90
degrees and heat index a few degrees higher in the low 90s.

Latest SPC mesoanalysis shows mixed layer CAPE up to around 2000
to 2500 J/kg south of the moisture discontinuity boundary,
which is in line with latest HRRR model soundings. Forecast
storm inflow also points to wider updrafts and this along with
the thick CAPE profile and steep low and mid-level lapse rates
point to large (>1")to significantly (>2") large hail as storms
rapidly develop between 20z/3p and 21z/4p. While the hail risk
will remain as storms drift southeastward late this afternoon
and early this evening, damaging wind gusts potential will be on
the rise during this same time window. Convective allowing
models continue to point to severe (>58 mph) and possibly
significant (>75 mph) wind gusts over the southern two tiers or
so of Iowa counties along and east of Highway 169. The risk for
the significant wind gusts has been trending southward over the
last 18 hours or so and may end up more so over northern
Missouri, though we may still see some severe gusts over far
southern Iowa. While initial storms may have good stretching
for tornado potential, high LCLs, poor storm relative helicity,
and the lack of streamwiseness in soundings point to tornadoes
being unlikely.

The cold front will push south of the state early this evening with
the most sensible weather change being lower humidity and dewpoint
values back into the 50s on Friday. Highs on Friday will be similar
over northern Iowa, but at least several degrees lower over southern
Iowa. Moving into the forecast this weekend, GOES-East upper level
water vapor imagery shows a closed low nearing the California coast
today. This low will open up and move northeastward toward Iowa
bringing a lead wave Friday night into Saturday with the main
shortwave arriving Saturday night into Sunday morning. Both of these
will bring chances for thunderstorms. While instability will build,
shear is looking more marginal for anything more than a low severe
risk. Heavy rainfall parameters may be the more impressive part with
precipitable water values topping 2 inches from western Iowa arcing
up into parts of northern Iowa Saturday afternoon into Saturday
night with deep warm cloud depths. Latest NBM paints probabilities
of greater than 2 inches of rainfall in a 24 hour period over
northwest Iowa between 15 to 30%. This is also where the latest
ECMWF extreme forecast index has 50-70% of its members with an
anomalous event with a shift of tail of zero over the Siouxland area.

Much of the rainfall will be moving east of the state by Sunday
afternoon as the main shortwave moves into the Great Lakes. At the
same time, a ridge will build over the eastern US with the high
centered over the central Appalachians while a longwave trough sets
up over the western US. With Iowa between these two, the question
becomes whether the ridge`s influence will be sufficient to keep
shortwaves moving through this flow far enough away to limit storm
chances with more elevated heat conditions. Or, does the trough
impinge more on the state with higher storm chances and heat
mitigated either in magnitude, time, and/or area? Ensemble data
points to the ridge being the favored outcome early next week with
the GEFS mean being the detractor and bringing slightly higher PoPs
into the state. With the ridge favored, this would point to heat
being the larger story vs storms. At this time, the experimental
HeatRisk is pinging both Monday and Tuesday with a level 4 of
4/extreme over portions of eastern into central Iowa. While daytime
heat index values near 100 degrees these days, the HeatRisk is
likely keying on the anomalous overnight lows, which are in the
middle 70s over this area. All this to say that for central Iowa
this will be the first multi-day heat episode and at least those who
are more sensitive or susceptible to heat should take
precautions.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
-- Changed Discussion --
Issued at 1127 PM CDT Thu Jun 13 2024

VFR conditions with light winds and mostly clear skies should prevail
through the period. A few locations have observed some brief
visibility reductions this evening, likely in low lying areas
where cooler air is pooling. Expecting this to be fairly
transient so have not included in TAFs at this time. If
visibility reductions become more persistent or widespread,
will amend accordingly.
-- End Changed Discussion --

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Dodson