807
FXUS63 KDMX 301954
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
254 PM CDT Sun Jun 30 2024

.KEY MESSAGES...

- A few showers and storms possible Monday. The threat for
  severe weather is low.
- Storms expected Monday night with a strip of heavy rain
  possible over parts of northern Iowa.
- Strong to severe storms possible over central and southern
  Iowa Tuesday. Locally heavy rain may occur.

&&

.DISCUSSION...
Issued at 241 PM CDT Sun Jun 30 2024

A large area of surface high pressure is encompassing much of the
Upper Midwest today. Cool temperatures, low humidity and mostly
sunny skies are common under this system this afternoon. A broad
region with a rich moisture plume remains across much of the south
central and southeast CONUS and this clips into central Kansas and
southern Missouri. This moisture plume will begin to return
northward on the backside of the current high pressure system once
it begins to depart towards the central Great Lakes tonight and into
Monday.

For central Iowa, very dry air below 800 mb, that is being
reinforced by the surface high, will remain in place into through
much of the day Monday though it will be gradually moistening
through that layer from west to east. Theta-e advection will begin
to overtop this dry layer, again from west to east, beginning
tonight and will begin to bring some elevated instability into
central Iowa during the day. This will likely lead to some
convective bubbling above the dry layer over western and central
Iowa Monday, possibly not too dissimilar to what is ongoing over
parts of Nebraska and north central Kansas today. Mid-level
cloudiness will be on the increase as well and that will help keep
temperatures seasonably cooler once again with high temperatures in
the upper 60s to mid 70s.

A short wave trough will approach the area from the west Monday
night with lead short wave energy arriving into Iowa. This will help
tilt a strong low level jet into northwest/north central Iowa
allowing the deeper moisture axis with 2 inch plus PWAT values and
850 mb dew points of 16C or greater to arrive. Warm cloud depths
over 14 kft highlighted by saturating profiles in a warm
environment, will aid in warm rain processes. There will be the
potential for training of precipitation/storms as the mean storm
motions will be near parallel to a slow moving boundary thus a strip
of heavy rainfall could occur, most likely south of where the
previous very heavy rain fell last week. The 850-300 mb mean wind
flow if 40 to 50 kts so that could be a limiting factor to heavy
rainfall.

Surface based convective development is expected along the
descending boundary on Tuesday. While most guidance would suggest
this boundary will be over central Iowa Tuesday afternoon though the
exact location of the boundary may be influenced by the previous
convection to the north and any potential outflow. The deep layer
shear and in particular speed shear, is better Tuesday and would be
supportive of some organized convection including a few supercells
along the potential for splitting cells. Supercell motions
would move storms off of the boundary meaning any organized
activity could limit the training potential. Severe storms will
be possible with the primary threats being large hail and
damaging winds.

A reprieve in the weather arrives for Wednesday followed for
more active weather potential for the 4th of July holiday. The
system arriving for Thursday is fairly pronounced with a closed
low moving into the Northern Plains and may present the
potential for more severe weather. Another strong upper level
system arrives over the weekend followed by the upper flow
devolving into a long wave trough next Sunday and into the
following week.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1224 PM CDT Sun Jun 30 2024

Shallow flat cumulus will continue to develop across Iowa this
afternoon with bases near 4-5 kft. It is possible to become
locally bkn this afternoon but conditions will remain VFR. The
cumulus will dissipate later this afternoon and evening. Mid-
level cloudiness will increase later this afternoon and into
Monday morning. Cigs will remain VFR through the TAF period but
a few showers may approach KFOD/KDSM late Tuesday morning. The
wind will remain mostly light today into tonight before becoming
breezy/gusty from the southeast Tuesday morning.

&&

.HYDROLOGY...
Issued at 250 PM CDT Sun Jun 30 2024

Reference the main forecast discussion above for finer details
about two potential rounds of heavy rain that may occur Monday
night and Tuesday. Heavy rain over northern Iowa is possible
Monday night with the other over central and southern Iowa
Tuesday afternoon and evening. Most streams over northern Iowa
have crested and are beginning to descend from last weeks heavy
rainfall. In addition, many storage areas are beginning to lower
and dry since these past events and are available for some water
storage again. Areas along the parts of the Des Moines River
and in particular the recent record crests along the West Fork
of the Des Moines would take a significant amount of rain to
return to those levels and at this point, the heavy rainfall
should be south of the previous location. Despite the expected
heavy rain potential upcoming, nothing showing up in the HEFS
and other ensemble forecast that would suggest significant
additional flooding even in the 10th percentile forecasts. This
rainfall may prolong flooding at some locations. One area to
watch is the Saylorville Lake area storage as additional heavy
rainfall upstream could cause more rises there than currently
forecast, which does not use future rainfall.

&&

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

&&

$$

DISCUSSION...Donavon
AVIATION...Donavon
HYDROLOGY...Donavon