291
FXUS63 KDMX 110852
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
352 AM CDT Thu Jul 11 2024

.KEY MESSAGES...

- Pleasant summer day today with light winds.

- Heat arrives for this weekend. Heat index values near to above
  100 possible Saturday and Sunday afternoons. The heat may
  persist over central and southern Iowa on Monday.

&&

.DISCUSSION...
Issued at 346 AM CDT Thu Jul 11 2024

Upper level low pressure is over east central Iowa early this
morning. A few showers and storms are still ongoing across parts of
east central and into southeast Iowa in response to the system. The
precipitation is co-located with the system`s upper level cold core
evident around 600 mb and higher. This activity will be diminishing
over the next several hours as the upper level system begins to
depart to the east. A few areas of fog have also developed early
this morning  with the fog is located along and south of Highway 34
over far southern Iowa and also over parts of far northwest Iowa.
Additional expansion of the fog is possible this morning in areas
with the clear skies.

Otherwise for today, surface high pressure will control the weather
with light winds today. Some shallow and flat cumulus development is
possible by this afternoon though the cumulus over northeast Iowa
could have more vertical extent than central Iowa though no
precipitation is expected. Warm advection will begin to lift into
southwest Iowa tonight but the moisture advection will be lagging
and a very dry air mass will remain over much of Iowa and any
precipitation should remain south into Missouri. The moisture
advection will begin to pivot into Iowa on Friday but precipitation
is not expected as it will take time for profiles to saturate and
the Elevated Mixed Layer (EML) will become established which will
cap off any surface based convection.

The low level jet will tilt into southern Iowa Friday night and will
bring some elevated instability to that part of the state. A few
elevated storms are possible if enough saturation can occur aloft
and near the forcing to release some of that instability. The EML is
expected to become more pronounced on Saturday and Sunday as more
warm advection aloft occurs and a portion of the thermal bubble to
the west arrives. This brings the million dollar question that we
have been discussing the past several days, how warm will it get
this weekend? All points discussed during these past several days
remain valid. One, the boundary layer should mix well below the EML
but it is capped due to the EML somewhere below 850 mb. Two, surface
dew points will be on the increase due to moisture advection and a
very big second this time of year, we are entering peak
evapotranspiration of Iowa crops. What impact will the mixing have
on the dew points. The most like scenario is dew points will be in
the low to mid 70s by mid afternoon. Three, the NBM temperatures
remain near the 90th percentile or higher and are likely overdone.
That said, the NBM spreads are fairly low and the 925 mb
temperatures at 12z each morning are in the mid 20s C which would
support a launch into the 90s by the afternoon. Finally, Four, does
it matter meaning higher temperatures with lower dew points or lower
temperatures with higher dew points will result in similar heat
index values. Expecting heat index values of 95 to low 100s on
Saturday and roughly 100 to 105 on Sunday. Heat will be the key
messaging this forecast period.

A buckle in the western upper high is still on track sometime Monday
or Monday night which will drop a boundary into the state. The exact
timing will play a role on if the heat remains Monday. At this time,
some relief into northern Iowa is expected but the heat may linger
central and south. A more active period in the weather looks
possible by mid next week as a stronger upper level system passes
through the region.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1048 PM CDT Wed Jul 10 2024

A few showers linger in eastern Iowa, otherwise activity has
ended across the area. Overnight patchy fog remain possible,
however data is split on where this will be impactful at TAF
sites. Have included MVFR mention at KMCW/KFOD where fog is more
likely. Amendments may be necessary as conditions change
through the early morning. Otherwise winds shift to out of the
east/southeast with VFR conditions to finish out the period.

&&

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

&&

$$

DISCUSSION...Donavon
AVIATION...Hagenhoff