-- Discussion containing changed information from previous version are highlighted. --
290
FXUS63 KDMX 111752
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1252 PM CDT Tue Jun 11 2024

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Low thunderstorm chances north today, but moreso late
  afternoon and evening north & east Wednesday with strong to
  severe storms possible.

- Additional storms, possibly severe, expected south Thursday as
  well.

- Not to advisory level as of yet, but elevated heat and
  humidity south Thu, and central and south Sun and Mon with
  highs often in the 90s. The first noted heat episodes of the
  season.

&&

.DISCUSSION...
Issued at 348 AM CDT Tue Jun 11 2024

Although unseasonally low dewpoints are in place early this
morning, the pattern will turn much more summer-like this week
and into early next week. Our H5 heights will be around 576dam
today, but increasing central and southern Plains upper ridging
will commence into midweek keeping those at bay to the north for
an extended period with IA on the southern fringe of the
westerlies allowing for increasing heat and humidity and
intermittent thunderstorm chances. The immediate concern will be
the evolution of upstream showers from NE through Siouxland
into MN. These should continue to weakly drift across mainly the
northern half of the state today driven by fairly deep and
phased thermodynamic and kinematic forcing ahead of the current
Dakotas upper trough, but within minimal instability. A few
convection allow models (CAMs) are also hinting at additional
surface based development along the trough`s associated surface
front far northeast by late afternoon or early evening, although
coverage (if any) should be limited. If they do occur, a few
could be strong with wind and/or hail. 0-6km shear could exceed
40kts, but MLCAPEs <1000 J/kg should temper severe potential.

Fair conditions with seasonal conditions can be expected
tonight, but that will change by tomorrow when low level
moisture and thermal ridging is expected to push through the MO
Valley, especially the latter. This should boost highs into the
upper 80s or lower 90s with dewpoints back to 60F+ in many
areas. Theta-e advection aloft will introduce some chances for
convection north and into MN for the morning and by midday with
more surface based potential by afternoon. The surface
reflection ahead of the upper trough skirting the US/Canadian
border will be weak, but noted instability is expected to build
ahead of it with 03Z RAP projecting 2500-4000 J/kg MLCAPEs over
far northern IA into MN. Experimental Entrainment CAPEs note
some decrease from traditional MLCAPEs due to dry soundings, but
stronger storm relative inflow ~30kts should offset that
keeping healthy updrafts in place. Although timing and the
speed of southern advancement is uncertain, most CAMs suggest
scattered but robust convection developing in these areas and
drifting into northern and eastern IA by the late afternoon
evening. Examination of forecast soundings note the degree of
warming more evident that moisture return resulting in high
based convection in a relatively dry environment. This suggests
damaging wind would certainly be a threat, DCAPEs are 1000-1400
J/kg, and with deep shear 40+ kts and elongated hodographs,
large hail is in play as well with a Slight Risk (2/5) across
northern IA. There is a non-zero tornado potential as effective
SRH does jump quite a bit by early evening with the start of the
low level jet, but that would be short lived with high
LCLs/LFCs suggesting any organization would quickly become
outflow dominate, and would have to coincide with some of the
slower solutions that don`t push the convection and associated
outflow farther south through IA. Hi res guidance is limited
beyond 00Z Thu, but there appears to be some heavy rain
potential as well with the NamNest and HRW FV3 both noting small
clusters of 2-5" possible, especially during the evening fed by
what would be a 40kt low level jet. The moisture parameter
space, isn`t climatologically high for mid June, but even
seasonal values are capable of these heavy rain rates,
especially if slower moving discrete cells occur and realize
those higher ESRH values.

The convection should weaken into the night with the loss of
insolation as it sags southward, but the front may reignite
again south during peak heating Thursday. Dewpoints should surge
through the 60s to 70F just ahead of the boundary south,
resulting in high instability and adequate 35+kt deep shear
allowing for another round of brief severe weather potential
south and into MO. With more moisture and lower cloud bases,
there will be some tornado potential, but weaker low level
kinematics should limit that somewhat with wind and large hail
still the primary concerns. These dewpoints and highs in the
lower 90s south will be one of the days ushering in what will
eventually be several days of elevated heat and humidity
concerns by early next week with heat indices well into the 90s
south and the corresponding Wet Bulb Globe Temp Risk into the
High Category as well.

After a brief break from precip Friday with the presence of
upper level ridging, a summer-like pattern will be more
persistent with intermittent chances for thunderstorms, as well
as increasing heat and humidity early next week. Convective
details are of course uncertain as of this lead time, but the
ridge should progress sufficiently to introduce some southwest
flow aloft upstream and start the progression of at least some
weak short waves through the MO Valley, and what may be the
passage of a stronger wave over the weekend keeping storms in
the forecast off and on into early next week. The severe
weather instability and shear parameter space via the 00z
deterministic solutions doesn`t appear sufficiently phased to
elevate the severe potential too much, however considering the
season there would be at least a low end severe threat, which
the latest available 10/00Z CSU machine learning guidance
suggests as well with relatively low severe weather
probabilities Sat-Mon.

There appears to be more confidence in additional elevated heat
and humidity however with the NBM noting highs well into the
90s central & south Sunday and Monday, with projected heat
indices a few degrees higher, possibly touching 100F in a few
spots south. The corresponding Wet Bulb Globe Temperature Risk
category increases to Moderate-High central & south Sunday with
more widespread High coverage Monday. This will be the first
noted heat episode of the year, so those planning or
participating in outdoor activities during these times, and even
south Thursday, should note heat safety rules and
accommodations.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
-- Changed Discussion --
Issued at 1238 PM CDT Tue Jun 11 2024

VFR conditions remain in place across the area with scattered
morning showers having festered out. Although a few additional
isolated showers or storms are possible in northern into
northeastern Iowa this afternoon into early evening, have
removed VCSH mentions from KMCW due to low confidence in a
shower/storm actually impacting the TAF site directly given the
very isolated coverage at best and overall chances only around
20%, if that. A few breezy winds this afternoon will become
light again tonight before increasing again Wednesday morning.
-- End Changed Discussion --

&&

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

&&

$$

DISCUSSION...Small
AVIATION...KCM