805
FXUS63 KDMX 251120
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
620 AM CDT Tue Jun 25 2024

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

.KEY MESSAGES...

- Heat advisory southwestern Iowa today

- Severe storms most likely 4p-10p today with large hail and
  damaging wind gusts the most likely hazards

- After a dry period Wednesday into part of Thursday, storm
  chances and locally heavy rainfall return late Thursday
  through Friday night

- Hydrologic discussion below main discussion details ongoing
  river flooding and future locally heavy rainfall potential

- Drying out this weekend with more seasonal temperature and
  humidity this weekend

&&

.DISCUSSION...
Issued at 359 AM CDT Tue Jun 25 2024

While tonight/early this morning may seem like a lull in the active
weather pattern and it is from the standpoint of a largely blank
radar across central Iowa as of 4am, record river flooding is
ongoing in parts of the state along with stifling heat if you`ve
steeped outside. The active pattern looks to continue as Iowa
will reside on the edge of the 500mb ridge that is centered over
the Desert Southwest into the southern Plains.

GOES-East upper level water vapor imagery shows two convective
clusters atop the ridge with one tracking over Wisconsin and
another, weaker cluster back over South Dakota. The only
concern for convection before sunrise would be if these could
build back to the west along a slowly sagging southward cold
front into the capped atmosphere over central and western Iowa.
This sagging cold front, though, will be the focus for
additional storm chances, which could happen as early as mid to
late this morning. This is most reflected in the NAM and ARW
cores and this activity would likely stay well below severe
limits. Have this in the forecast as low chance PoPs over the
southern forecast area.

The front will help to bring some very minor relief to northern
Iowa, but southern Iowa will still be feeling the oppressive heat.
Have not made any changes to the heat advisory over our southwestern
forecast area as this lines up with current heat index values that
are near or a little above 105 degrees. It is possible that this
could be expanded across all of southern Iowa, but given uncertainty
in whether there could be possible storm development and residual
cloud debris from this into the afternoon, have opted to not make
any change for now. And as a reminder again, all users should be
wary of dewpoints from AWOS sites, which can report erroneously high
dewpoint values and thus inaccurate heat index values.

Late this afternoon or early this evening, convective allowing
models (CAMs) are showing development of severe storms in an
environment of strong instability with MLCAPEs at or above 3000
J/kg, 30 knots plus of deep layer shear favorable for storm
organization, and mid-level lapse rates of 7 to 8C/km. Supercells
with large hail greater than 2 inches in diameter and damaging wind
gusts of 60 to around 70 mph will be the main hazards with
these storms. As for any tornadoes with these supercells, low
level storm relative helicity (SRH) values are quite low and
forecast soundings do not suggest streamwise ingest. Winds would
need to back locally to increase SRH to realize tornado
potential. While this looks unlikely, with storms forming along
this boundary I won`t count it out altogether in late June.
Eventually, these storms will congeal into a linear, forward
propagating system with damaging wind gusts becoming the main
concern as they move east-southeastward and then southward into
Missouri. With these storms being more progressive and south of
Highway 20 largely, this will lessen the flash flooding
potential with deterministic models and 0z HREF localized
probability matched mean being 1 to 2 inches with isolated 3
inch amounts. See hydrologic discussion below for more details.
One final note is that a few of the extended HRRR runs have
shown a second round of severe storms moving into and through
central Iowa late in the evening into the early morning hours of
Wednesday, which would also have damaging wind gust potential.
The rest of the CAMs do not show this as the late
afternoon/early evening clear out the area of the instability.
This potential will need to be monitored as each round of
convection can change the environment by lowering the
instability or leaving boundaries so confidence decreases in
time with each successive round of storm potential.

As the cold front pushes south, aloft the trough will become a bit
more amplified and help to push the ridge a bit southwestward as
well. This will bring heat relief to all of Iowa on Wednesday with
highs ranging from the upper 70s over northern Iowa to the middle
80s over southern Iowa and dewpoints around 60 degrees instead of
low and middle 70s. In addition, this will squelch the instability
away from the state for a good 24 to 36 hours and give a dry period.
The next chance for storms will come later Thursday and Thursday
night as a subtle shortwave trough tops the then deamplifying
ridge. This combined with strong low level thermal lift and a
strengthening 850mb low level jet will bring storms advancing
from west to east across Iowa. While there may be some attendant
severe risk that will become better defined in the coming days,
of higher concern will be the heavy rainfall risk with
precipitable water values surging to around 2 inches and deep
warm cloud depths of 4000 to 4500m - quite favorable for late
June. Broad brush rainfall amounts are 1 to 2 inches with the
probability of an 1 inch in 6 hours from the 100 member ensemble
at 20% Friday morning. Additional storms and rainfall are
possible as a cold front moves into and through the state on
Friday. Once the cold front clears the state Saturday, a
sprawling zone of high pressure will pass through the region.
More seasonal temperatures (middle 70s to middle 80s Saturday,
70s Sunday) along with lower humidity are forecast. As this high
shifts east early next week, return flow of Gulf moisture will
begin and thus a return of storm chances.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 620 AM CDT Tue Jun 25 2024

Aviation concerns this period will be the low confidence in
thunderstorm timing and location. First up are the storms
developing just before the start of this period over southern
Iowa. Since this could impact OTM in a few hours, went with a
short prevailing line. Additional storms may develop later this
morning or earlier this afternoon, but the more likely time
will be late this afternoon into this evening over central and
southern Iowa. This is where there was enough confidence to
include SHRA lines, but uncertainty in placement have held
thunder back or more restrictive restrictions. Storms should
clear by the end of the period with VFR conditions.

&&

.HYDROLOGY...
Issued at 359 AM CDT Tue Jun 25 2024

Major to record flooding is ongoing or continues to be forecast on
the West Fork of the Des Moines River early this morning. While
sites in this basin have shown a small decrease or leveling off of
their river level, a secondary crest is still working its way down
from southern Minnesota for Estherville and Emmetsburg. At Humboldt
and Fort Dodge, their first crest is still coming and may be a bit
prolonged or slower to fall because of this secondary crest.

For the rainfall forecast today as mentioned in the discussion
above, rainfall amounts are generally 1 to 2 inches with localized
amounts of 3 inches. NASA SPoRT relative soil moisture (RSM) shows
more limited capacity over northern Iowa, say roughly north of
Highway 20. Fortunately, much of the rainfall for later today into
tonight is forecast to fall south of Highway 20 where RSM is
lower/more capacity is available. This is also where USGS daily and
7 day streamflows shows river basins in the normal percentile versus
northern Iowa at much above of high streamflows. Thus, again
suggesting some capacity for rainfall that falls today.

The next chance for rainfall will be later Thursday through Friday
night. It remains to be seen what kind of flash flood risk may
result with the latest WPC excessive rainfall outlook at slight (2
out of 4) for a good portion of Iowa. While the experimental HEFS
(Hydrologic Ensemble Forecasting Service) based on GEFS QPF points
to mainly within bank rises at the 10% chance exceedance level, it
will depend on where and how much rain falls in the basins. This
will ultimately determine whether a given river response is a
quicker rise, higher or prolonged crest, or a slower fall.

Make sure to stay up to date on the latest forecasts and river
information, avoid flooded areas and rivers, and never drive around
barricades or through flooded areas.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon today to 7 PM CDT this evening for
IAZ070-081-082-092>094.

&&

$$

DISCUSSION...Ansorge
AVIATION...Ansorge
HYDROLOGY...Zogg