637
FXUS63 KDMX 202010
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
310 PM CDT Thu Jun 20 2024

.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.

&&

.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 /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1241 PM CDT Thu Jun 20 2024

A moist airmass remains over the area, leading to a difficult
aviation forecast through the morning. MVFR conditions are
expected to stick around through the next few hours, but
uncertainty remains on if/when improvement to VFR will be seen.
This time of year, ceilings generally improve quickly through
the day as the surface warms and mixes. However, moist air and
instability aloft has hindered this improvement, which has led
to prolonged periods of low ceilings lasting into the
afternoon. As of right now, not expecting MVFR ceilings to go
anywhere real soon, but still anticipating some improvement
later this afternoon as mixing occurs and cloud cover lessens.

Beyond ceilings, expect light and variable winds through the
night, with the potential for more MVFR to IFR ceilings
overnight and into the morning hours at northern sites (KMCW,
KALO and KFOD). Further south, conditions will be warmer and
dry out some, keeping ceilings in VFR. A few isolated showers
and thunderstorms are possible through the afternoon and
overnight, but confidence is too low to include in TAFs so will
amend or add in future issuances once impacts appear more
likely.

&&

.HYDROLOGY...
Issued at 213 PM CDT Wed Jun 19 2024

Little change in thinking from previous discussion. Heavy rain still
poised to fall over basins in the north and west over the next few
days. Main concern remains the West Fork Des Moines River Basin with
heavy rain possible in its reaches in northwest Iowa as well as into
Minnesota. Ensemble output brings sites nearest the state line into
minor flood stage from a mix of local rain and upstream runoff.

&&

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

&&

$$

DISCUSSION...Dodson
AVIATION...Dodson
HYDROLOGY...Jimenez