604
FXUS63 KDMX 210501
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1201 AM CDT Fri Jun 21 2024

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

.KEY MESSAGES...

- Isolated to scattered showers and storms possible through the
  night. Severe weather risk is low.

- Showers and thunderstorms possible over northern Iowa tomorrow
  afternoon. Heavy rainfall, wind and hail are the primary
  concerns, although a tornado can`t be ruled out.

- Additional severe weather is possible on Saturday afternoon,
  with all severe hazards possible.

&&

.UPDATE...
Issued at 922 PM CDT Thu Jun 20 2024

There was some concern this evening regarding the latitudinal
placement of the heavy rain axis overnight and into tomorrow,
and whether the Flood Watch needed to be started earlier. The
convection allowing models (CAMs) were not handling the O`Neill,
NE supercell well, which is currently rooted fairly low and in
the heart of the 925-850mb moisture transport and developing low
level jet. It was previously tracking slowly eastward per the
Bunkers supercell motion, and could potential continue that
movement along the Highway 20 corridor per the Corfidi upwind
propagation vector, but after weakening is now starting to lift
slowly northeastward following more of the mean wind. The
overall SW-NE moisture plume in the water vapor imagery suggests
a gradual northward focus into the night, with 0-2km moisture
convergence also targeting those areas right now. So, long story
short, confidence is current insufficient to move the headline
and higher PoPs farther south at the moment, although MCS trends
will continue to be monitored into the night as the airmass
remains ripe for heavy, efficient rainfall, especially
northwest.

&&

.DISCUSSION...
Issued at 307 PM CDT Thu Jun 20 2024

The front that brought a healthy amount of rainfall to portions of
southern Iowa overnight hasnt budged much through the day, keeping
plenty of moisture pooled over  the area. This boundary has begun to
develop convection again early this afternoon, but storms have been
relatively benign with little to no shear present through the layer.
Isolated to occasionally scattered storms may fester along and south
of this front through the afternoon and evening hours, but not
expecting any impacts beyond small hail and locally heavy rainfall.

As we head into tonight, we turn our attention to the northern CWA
as surface low pressure building to our west continues to pull large
amounts of moisture into the region. The evolution of this low will
bring a prolonged period of rainfall to the north starting tonight
and lasting intermittently into Saturday morning. Most of the
heavier rainfall tonight will stay north and west of the area,
mainly impacting SD and MN, however, still expecting some showers
and occasional thunderstorms in our northern counties overnight and
into early tomorrow morning. Although we arent expecting the
heaviest rainfall with this wave, PWATs will be increasing through
the night and into tomorrow, leading to moist profiles and efficient
rainfall for those that are impacted by precipitation.

Through Friday morning, the surface warm front will have caught up
to the better forcing and moisture aloft, increasing the magnitude
of showers and storms in northern Iowa by Friday afternoon. This
will result in both an increased chance for severe storms, as well
as an increased chance for heavy rainfall as the instability
increases along the boundary. First looking at the severe chances on
Friday, the parameter space isnt off the charts, but a few stronger
storms still look possible with warm, moist low levels and 2000
3000 J/kg of SBCAPE during the peak heating hours. The limiting
factor will be that most of the shear in the warm sector will be
fairly weak and unidirectional, resulting in messy hodographs.
Therefore, wind and hail would be the primary threats, while a
tornado would require a storm that is tied into the vorticity along
the boundary. This is reflected well in the SPC Day 2 outlook, which
places a slight risk for severe weather along the expected boundary
in northern Iowa.

In addition to the severe threat on Friday, heavy rainfall will also
be a concern. PWATs will have continued to increase through the day,
resulting in anomalously high values exceeding 2 over southern
Minnesota and into northern Iowa. This, combined with the better
instability present in the warm sector, will lead to very efficient
rainfall over throughout this region. To make matters worse, storm
motions will be out of the west, resulting in training convection
along the boundary. These heavy rain features have been well
captured by guidance, with ensembles and deterministic guidance all
in pretty good agreement for a swath of rainfall amounts of 3 to 5
inches along the SD, MN and IA borders and locally higher values in
the 6-8 range. Where the uncertainty resides is in the placement of
the surface front and the influence of mesoscale features on the
location of this heavy rainfall. As has been stressed in previous
discussions, setups of this nature during this time of year tend to
trend towards the location of the more favorable environments, as
cold pools/outflow boundaries spread out and produce new convection
south of the original boundary and in the more unstable air. This is
important because higher QPF amounts that had been over southern MN
the last few days are now trending south as CAMs begin to pick up on
the smaller scale features and leading to higher rainfall amounts in
northern Iowa. Unfortunately, these smaller scale features are often
not well forecasted by models, making the location of the heavy
rainfall uncertain.  WPC has shifted the moderate risk for excessive
rainfall further south into Iowa to account for this, and we have
locally issued a flood watch for Friday afternoon into Saturday
morning to account for this heavy rain potential.

Of course, its an active summer in Iowa, so this is far from the
end of our convective fun for the week. As the surface low pressure
tracks over the region, the cold front will drag behind it and be
the focal point for convection on Saturday as well.  Instability,
moisture and forcing will be easy to come by as this passes through,
with PWATs along the boundary well exceeding two inches and modeled
SBCAPE values again exceeding 2000 J/kg. The deep layer shear will
also be a bit higher (30-40 kts) on Saturday afternoon, which will
increase the chances for all modes of severe weather. As of right
now, SPC has a day 3 marginal risk over much of the area, and a
slight over northeast Iowa nearer to the low and where the better
shear will reside. This seems reasonable at this timeframe, but will
want to keep an eye on severe chances as this event starts to be
captured by higher resolution models.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1201 PM CDT Fri Jun 21 2024

IFR stratus was developing across northern IA, and that should
persist through at least 12z with some contributions from fog
development as well. VFR conditions should return area wide
later Friday morning however, with mainly dry conditions through
the afternoon. Additional convection is expected to develop
across northern IA Friday evening however, with sufficient
confidence to mention thunder even at this lead time.

&&

.HYDROLOGY...
Issued at 326 PM CDT Thu Jun 20 2024

With heavy rain still expected to fall from MN into northern IA over
the next few days, more rises on area rivers are expected with
renewed or increasing flooding possible. The river basin of most
concern is the Des Moines River basin above Saylorville Lake and
secondarily the upper Cedar/Winnebago River basins because those two
basins have their headwaters in southern Minnesota. Minor to
moderate flooding is expected per QPF ensemble hydrograph and HEFS
guidance. Our official river forecasts today included 48 hrs of QPF
whereas they typically include 24 hrs this time of year. The QPF
time horizon was extended because the longer term nature of this
rainfall event.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Friday afternoon through Saturday afternoon
for IAZ004>007-015>017.

&&

$$

UPDATE...Small
DISCUSSION...Dodson
AVIATION...Small
HYDROLOGY...Zogg