FXUS63 KDMX 272335

Area Forecast Discussion
National Weather Service Des Moines IA
635 PM CDT Wed Sep 27 2023

...Updated for the 00z Aviation Discussion...

.DISCUSSION.../Tonight through Wednesday/
Issued at 244 PM CDT Wed Sep 27 2023

Key Messages:
- Dense fog potential repeats tonight, most likely time 4a-10a
- Above normal temperatures Friday into next week; near record
  highs at times
- Limited widespread rainfall until middle of next week

Details: The upper level pattern shows a longwave trough over
northwestern North America and a ridge over eastern North American
with the closed low that had been spinning over Iowa moving into the
Ohio Valley beneath this ridge. The upper low has influenced our
sensible weather with light flow, which has resulted in repeat
nights of dense fog, and spotty showers. Both of these will be in
play in the near term forecast. First, the dense fog this morning
was stubborn, particularly the southside of Des Moines down toward
Osceola, not improving til about 10am. While visibility improved,
GOES-East Day Cloud Phase Distinction showed plenty of clouds over
much of the state by late morning into the early afternoon. Further,
some of the cumulus over northeastern Iowa are bubbly and may
produce a few spotty showers through the afternoon hours before
diminishing. Then, looking into tonight, clouds will amalgamate and
lower from north central Iowa into southeastern Iowa. This should
limit the fog potential in these areas, but to the southwest of this
cloud shield is where dense fog may develop after midnight.
Yesterday`s HREF performed really well handling the area of the fog.
For tonight, the latest/12z HREF shows the fog beginning to form
around midnight, but the period of the lowest visibility and densest
fog looks to be between 4am and 10am from around a line from Spencer
to Ames to Ottumwa and a few counties wide. Checking the HREF with
model forecast soundings, a favorable surface inversion for fog is
shown at Storm Lake, Carroll, Des Moines, Ames, and Ottumwa. Farther
to the northeast where clouds are expected, the inversion is
elevated indicating low stratus clouds instead of dense fog. The
clouds over the northeastern portion of the state may sag
southwestward resulting in fog that transitions to low stratus.
Soundings do show the surface inversion lifting, such as at Ames,
though the area will be somewhat uncertain until it sets up. While a
dense fog advisory is probable, will allow future shifts to assess
trends and location.

After the fog dissipates by mid-morning Thursday, will see the cloud
cover over the state be pulled to the northeast with areas from
Mason City to Waterloo and points north and east having some cloud
cover through the day. Southwesterly flow will become established
over the region by Friday into next week as the area will reside
between the amplifying longwave trough over the western CONUS and
the eastern CONUS ridge. Surface winds will also be breezy and
maximized from the latter morning hours through the afternoon with
forecast soundings from the NAM, GFS, and ECMWF ensemble show a good
deal of boundary layer mixing. As discussed yesterday, still think
the initial National Blend of Models` dewpoints and resultant RHs
will be too high. A quick check of 25th percentile dewpoints would
result in RHs 5 to 10% lower than currently forecast on Friday
through Monday, which would mean no worse than 30% at this point. As
for temperatures, they will be above normal with a fair amount of
confidence given the low spread in the ensemble data with the ECMWF
Extreme Forecast Index showing greater than 70 to 80% of its
ensemble members with anomalously high temperatures on Saturday over
western Iowa expanding across the state Sunday through Tuesday.
Thus, at times and in a few places, these highs will be near record
values. With the above normal temperatures, somewhat cured grass and
cropland fuels, and the breezy winds, slightly elevated fire weather
conditions will prevail. The winds are well below Red Flag Warning
criteria and the RHs are borderline so a headline event seems

Much of the period from Thursday into early next week will also
favor dry conditions. That said, with subtle shortwave troughs
passing through the region in the southwesterly flow, there are low
chances of rain or storms over varying sections of western Iowa.
However, the higher chances will be farther west with any severe
weather also west of our service area not due to a lack of
instability, but lack of shear. The probability of rainfall
exceeding an inch in this timeframe is just 15% from the total 100
members of the various global ensemble models. This will only add
onto the dry conditions, which will allow farmers to harvest. As we
head towards the middle of the week, the western CONUS longwave
trough will become cutoff with the northern branch sending a front
through the region as it moves eastward around Wednesday. This may
offer the next best chance of rainfall across the forecast area.


.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 635 PM CDT Wed Sep 27 2023

Once again MVFR to IFR ceilings and visibilities will move into
central Iowa late tonight although confined to about the northeast
half of the state. Farther southwest, only lighter fog is expected
at this time. Surface winds remain relatively light from the east
overnight increasing from the east southeast on Thursday.
VFR conditions are also expected to become widespread after 18Z on
Thursday as well.






NWS DMX Office Area Forecast Discussion