Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 141132

Area Forecast Discussion
National Weather Service Des Moines IA
632 AM CDT Fri Jun 14 2024

 ...Updated for the 12z Aviation Discussion...


- Fair weather today with seasonally warm temperatures

- Prolonged period of elevated heat and humidity Sunday through

- Intermittent chances for showers & thunderstorms return west
  overnight tonight, and continue area wide well into next week
  with severe weather and locally heavy rains possible at times.


Issued at 344 AM CDT Fri Jun 14 2024

A typical summer-like pattern will remain in place across Iowa
for the foreseeable future with Plains ridging today, a short
wave passage this weekend, and then IA between a persistent
northwest CONUS trough and broad eastern CONUS ridging, which
appears to be locked in place for an extended period, at least
through next week and the following weekend. This will keep off
and on chances for convection in the forecast along with varied
degrees of heat and humidity, and diminished confidence on the
extent of that warmth by the end of next week. Before things
turn more active again, there will be plenty of sunshine through
much of the day today with little in the way of moisture and
forcing. Surface high pressure will keep highs at seasonal
levels with atypically low dewpoints and humidities for mid
June. It won`t last long however.

Our attention will then turn to the slow moving PV anomaly
currently moving into AZ, whose associated broad vertical motion
field will return chances for showers and storms to western
sections overnight, and much of the area Saturday and Saturday
night. Initially this will be generally weak convection in low
instability with increasing warm/theta-e advection through the
MO Valley. However by later in the day and into the night, low
level moisture and increasing humidity will stream back into IA
increasing our chances for stronger and potentially more surface
based convection.

Confidence is not great, but the severe weather potential and
Slight Risk (2/5) highlighted in the SPC Day 2 outlook for peak
heating Saturday is interesting. MLCAPEs build to at least 2-3K
J/kg west by late afternoon and early evening. NAM, GFS, and
NamNest soundings all show marginal effective shear, no better
than the 30s(kts). However examination of low level kinematics
highlights at least some tornadic potential with LCLs <1000m,
and 0-1km shear and SRH ~20kts and ~200+ m2/s2 respectively
through 00z. While they are not extreme or persistent, low level
streamwise vorticity values reach 0.02/s at times with the
percentage of streamwise ingest 90+ percent at times. Although
the loss of inhibition and increasing CINH may temper the
potential, these values increase even further into the evening
with the increasing low level jet, so there will be at least
some potential into the early evening as well. The lack of
higher effective shear should limit organization and longevity
somewhat, but nevertheless all modes of severe weather would be
possible, and this is somewhat corroborated by 00z GEFS based
CSU tornado, wind, and hail severe probabilities 5 and 15
percent respectively. The lack of effective shear may increase
the heavy rain potential as well with the 00Z GFS noting a
35-40kt low level jet impinging on a warm front either along the
IA/MN border or just north, potentially fueling whatever
convection is ongoing into the night. With anomalously high
moisture parameter space, including precipitable water values
approaching 2" and 3.5-4.0km warm cloud depths, efficient, locally
heavy rainfall is possible with both the HRW FV3 and NamNest
noting isolated clusters with several inches of rainfall, and
even the lower resolution parameterized deterministic ECMWF
solution notes similar amounts.

There should be a brief respite Sunday with mainly dry
conditions, however this will also start what may be an extended
period of elevated heat and humidity concerns. What looks to be
a extended period of northwest CONUS troughing and eastern
ridging starts to be locked into varied degrees for at least
several days. There will be intermittent low chances for
thunderstorms early next week, but confidence on the extent of
that is low due to varied NW-SE solutions of the
Rockies/northern Plains jet and eastern upper level ridging.
There appears to be higher confidence with respect to at least
some elevated heat and humidity however with persistent
southerly flow for several days. Highs and heat indices should
often be in the 90s, with heat indices touching 100F+ at times.
There will be little relief at night either with muggy lows in
the 70s early next week, which will be near record or record
high minimum temperatures at times. Breezy south winds may make
it feel a bit better, and offset any Wet Bulb Globe Temperature
Risk categories more than High (3/4). However, the experimental
NWS HeatRisk output notes Extreme values (4/4) by Monday and
Tuesday, mainly driven by duration and the lack of any nighttime
relief more than any individual daytime extremes toward
advisory criteria, but the duration of the event may eventually
lead to headlines nonetheless.

By the end of next week, confidence in the large scale pattern
(NW trough/sprawling eastern CONUS ridge) remains high. However
the degree of the upper ridge retrograding, how far west it
expands, will have a big impact on our local Iowa weather with
solutions ranging from drier conditions and more persistent
elevated heat, to a bit more seasonal values with more chances
for showers and storms. The current consensus blend and NBM
suggests the former more tempered heat and a bit wetter GFS-like
solutions, however there are solutions such as the EC 00z
deterministic run which leans toward more expansive upper
ridging from the central Plains through the east coast, and
resultant highs back into the 90s with less precip. Regardless,
this looks to be the first prolonged heat episode of the year,
whether it extends late into next week or not, so those planning
or participating in outdoor activities next week should note
heat safety rules and accommodations.


Issued at 632 AM CDT Fri Jun 14 2024

There is high confidence in VFR conditions through the period
with clear skies through at least midday, and then increasing
high and then mid level cloudiness this evening and into overnight.
There is some potential for light showers early Saturday
morning west, but confidence is certainly insufficient to
mention at this lead time.