561
FXUS63 KDMX 010818
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
318 AM CDT Sat Jun 1 2024

.KEY MESSAGES...

- Trending drier and brighter today

- Fog potential tonight, especially eastern half of the state

- Uncertainty on daytime Sunday rain/storm chances, but storms
  look to enter the state Sunday night with a few stronger
  storms and locally heavy rainfall possible

- Storms later Tuesday into Tuesday evening bring a higher chance of
  strong to severe storms and possible locally heavy rainfall

&&

.DISCUSSION...
Issued at 318 AM CDT Sat Jun 1 2024

GOES-East upper level water vapor imagery shows a closed 500mb
low meandering through Missouri early this morning. This is the
impetus for the low level moisture and lift that is causing the
scattered showers over portions of central Iowa at this hour.
This low will slowly move into Illinois and the upper Ohio
Valley later today and thus we should see conditions turning
drier with some clearing in at least western Iowa by later
today. Temperatures will be well into the 70s to around 80
degrees over the western half of the state while the slower
departing showers and clouds will keep temperatures lower in the
70s in eastern Iowa.

Tonight looks like a good setup for radiational fog development with
forecast soundings showing an inversion anywhere from 500 feet (RAP)
to around 1000 feet (NAM, GFS) deep. This combined with recent
rainfall, which keeps pooling of moisture beneath the nocturnal
inversion high, and light to calm winds as a transient high pressure
moves over the state should allow fog to form after midnight. High
resolution models as well as the 0z HREF mean visibility are
favoring the eastern half of the state (roughly east of I-35) for
fog development with the MAV/MET MOS guidance showing around a half
a mile visibility at Waterloo/ALO and under a half a mile at
Ottumwa/OTM. The latter is supported by 50-60% probability of a half
a mile or less visibility in the 0z HREF. As is typical with fog,
there are detractors and the NBM short is not as aggressive with its
visibility reduction.

After any fog dissipated Sunday morning, a subtle shortwave trough
will be approaching the state in the zonal flow with timing
differences remaining in the global suite of models. Low level
thermal lift ahead of this wave will allow for an increase in cloud
cover through the day. Confidence in details such as timing for
showers and thunderstorms is rather low looking at the spread in the
models. Some models such as the RAP, NAM, and ECMWF indicate shower
and thunderstorm chances increase over western Iowa persisting or
waning as they slowly move into central Iowa through the day. Other
models such as the NAMNest, HRRR, FV3, and NSSL WRF show the morning
activity dying leaving the bulk of the day dry across central Iowa.
Looking into Sunday night, there is better agreement in the
convective allowing models, though there is still inherent
uncertainty despite the agreement as the outcome of whatever happens
during the daytime Sunday may influence Sunday night`s storms.
Overall, it looks like severe convection kicks off over western and
central Nebraska that grows upscale/congeals into a line of storms
that advances towards Iowa reaching the state sometimes Sunday
evening or after midnight. This is shown in the latest FV3, NAMNest,
and the RRFS and as they reach our forecast area, their intensity
should be on a downward trend. The SPC`s day 2 outlook highlights
much of our forecast area with a marginal with a slight risk over
northwest Iowa and this seems to capture the risk fairly well.

On the hydrology front Sunday night, WPC highlights the western half
of Iowa with a slight risk of excessive rainfall. Precipitable water
values increase above 1.5 inches with favorable warm cloud depths
for efficient rainfall and forecast soundings Sunday night also are
juiced showing skinny elevated instability and near saturation over
much of the column. If this does end up being a progressive line of
storms as currently shown, this may lessen the flash flooding
concerns with those CAMs showing rainfall of 1 to 2 inches with
perhaps isolated pockets approaching 2.5 inches. Approaching those
higher end forecast rainfall values would near the 3 or 6 hour flash
flood guidance values. Of course, this doesn`t take into account the
uncertainty of rainfall during the day Sunday, which if it occurs
would prime conditions a bit more. Rivers show a little response,
but nothing beyond a return to action stage at some points based on
latest contingency forecasts.

The storms from Sunday night will be pushing out of the state Monday
morning, though NBM has broadbrushed PoPs through the day into
Monday night. Have lowered PoPs a bit in coordination with
neighboring office for part of Monday, which seems supported by the
idea that the shortwave energy will have moved east of the area and
some level of subsidence will be passing over the state. Highs on
Monday and into Tuesday will be a few degrees above normal in the
low and middle 80s. Storm chances do return later Tuesday as
shortwave vorticity lifts from Kansas into Illinois through the day
while a longer wave trough moving over the northern states deepens
and approaches the state. Moisture will pool ahead of this shortwave
and more so the cold front with precipitable water values nearing
1.75 inches and warm cloud depths at or above 3500m. 850-300mb flow
is weaker earlier in the day, but increases as the trough deepens
and the forcing approaches. While the mean flow is not as favorable
for flash flooding at this time horizon, convective elements may
still produce locally heavy rainfall and for now WPC has placed a
marginal risk of excessive rainfall over the state so will need to
monitor the flash flooding threat. As for severe weather risk,
instability really blossoms with MUCAPE nearing 3000 J/kg by late
Tuesday. However, the deep layer shear is displaced from this area
and closer to the front with hodographs generally showing linear
structure to messy/disorganized. So, at present would expect strong
to perhaps severe storms later in the afternoon into the evening as
the cold front pushes through the state.

As the storms depart Tuesday night and the cold front moves east of
the state, drier air with dewpoints back into the 50s will arrive
with high temperatures only stepping down a few degrees and more in
line with early June climatological values. The longer wave trough
mentioned above wraps up into a sprawling upper level closed low
over the Ontario and Manitoba provinces. Depending on its proximity
to Iowa, this could bring additional shortwaves and rain chances
into the region on northwesterly flow. Finally, while it will depend
on how much and where the rain falls this weekend into the first
part of next week, an examination of experimental HEFS (Hydrologic
Ensemble Forecasting Service) forced with GEFS rainfall does shows
several points in the Cedar and Des Moines basins returning to flows
above action stage with a 30% chance that ESVI4/West Fork Des Moines
River at Estherville and CEDI4/Cedar River at Cedar Falls exceed
minor flood stage late next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1136 PM CDT Fri May 31 2024

Scattered light showers will affect the terminals at times
overnight, but with little impact. Generally VFR conditions will
prevail, but with pockets and periods of MVFR ceilings that are
difficult to pin down in time. It appears at the northern
terminals MVFR ceilings will be most likely in the next six
hours, then a bit later at the southern terminals into Saturday
morning. Will indicate this trend in the 06Z TAFs, but
amendments are possible overnight.

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Lee