818
FXUS63 KDMX 011720
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1220 PM CDT Tue Oct 1 2024

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Much cooler today with highs in the 60s to perhaps a few low
  70s.

- Dry with above normal temperatures returning by Wednesday.

- Low relative humidity and gusty winds combined with very dry
  cropland vegetation will lead to fire weather potential on
  Wednesday.

&&

.DISCUSSION...
Issued at 250 AM CDT Tue Oct 1 2024

A cold front passed through overnight in response to a strong short
wave moving across the Northern Plains and Manitoba/Ontario. The
cold front is now exiting the far southeast corner of the state. The
boundary was very moisture starved and was barely able to muster
even a few clouds as it moved through. In addition to the cooler air
arriving, dew points that were in the 50s to low 60s Monday
afternoon, have fallen into the 30s behind the boundary passage.
High pressure is quickly moving into Iowa in the wake of the
boundary and dew points under the high are even drier and in the
20s. It is very likely that dew points by this afternoon mix out in
the 20s as well with relative humidity values dropping into the
teens and 20s. Temperatures will be much cooler today than the past
several days but given the very dry air and only partial sun
filtering from passing high level thin clouds, readings should be
able to exceed guidance values by 1-3 degrees. Therefore, expect a
temperatures mainly in the upper 60s with a few low 70s.

The area of high pressure will begin to shift to the east/southeast
tonight with return flow developing and warmer temperatures expected
on Wednesday. High temperatures on Wednesday will be in the mid 70s
to low 80s. Breezy to gusty southwest winds are also expected. Mixed
layer winds will be strongest over northwest Iowa and in the mid to
upper 20 kt range and closer to 20 kts far southeast. Warm advection
rising motion will dampen this mixing a bit. The wind will be aided
though by an increased pressure gradient of 20-25 ubars/km, which is
not strong by any means but sufficient to offset some of the warm
air dampening of the mixed layer winds. The net and translating the
wind to mph is gusts of 20 to 30 mph. Minimum relative humidity
values will again be in the upper teens and 20s. The dry air and
good mixing should again led to warmer than guidance/NBM
temperatures by 1-3 degrees.

The elephant in the room for Wednesday is fire weather risk and
potential. Observations from across the forecast area is most tall
grasses are at most 50 percent cured and likely does not pose a high
fire weather risk. The biggest risk will be field fires during peak
harvest season as crop foliage is currently very dry and cured and
the potential for fire growth will be much higher over the cropland.
Will highlight this in the fire weather forecast. Our office will
continue to gather information today to decide how to handle this
type of situation with likely wind and grass curing below Red Flag
criteria but cropland is more.

The rest of the forecast over the next 7 days is for a lack of a
better term, boring. The aforementioned short wave that passed to
the north of Iowa overnight will evolve into a large upper low over
northern Canada and will help lead to westerly flow through Iowa
much of this week with a little less certainty by next weekend.
Overall remaining dry with above normal temperatures. It appears to
be a good setup for good mixing during the daytime and strong
shallow decoupling at night, meaning warmer days and cooler night
with larger diurnal ranges than forecast guidance would suggest.
Therefore, continuing the trend of siding closer to NBM 75th during
the day and NBM 25th at night is a more likely scenario outcome.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1220 PM CDT Tue Oct 1 2024

VFR conditions are forecast through the current TAF period.

&&

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

&&

$$

DISCUSSION...Donavon
AVIATION...Martin