746
FXUS63 KDMX 220805
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
305 AM CDT Sun Sep 22 2024

.KEY MESSAGES...

- Lingering categorical showers/iso storms southeast half today
  and showers returning south Monday (25 to 40% chance)
- Much Cooler as Fall Begins today
- Tuesday rain chance?
- Lower confidence and possible complex late week pattern

&&

.DISCUSSION...
Issued at 305 AM CDT Sun Sep 22 2024

.Short Term /Today through Monday/...

Confidence: Medium to High

Forecast pretty much on track with relatively stable model solutions
and decent consensus the past few runs. Tonights subjective sfc
analysis at 04z shows low pressure over east central Iowa with a
warm front extending east into northern Illinois; a trailing cool
front from the low into southwest Iowa and northeast Kansas along
with a secondary, stronger cold front extending nearly due west from
the low near Marshalltown back to Carroll and then northwest into
southeast South Dakota. Aloft at H850, Iowa and much of the Central
Plains remains east of the main H850 trough; resulting in continued
moisture transport into our area overnight through most of today
(Sunday). A large plume of +10C dew points is still being forced
northeast into our area at this time. High pressure over the
Southeast combined with the elongated trough from southern Canada
southwest to New Mexico is assisting to focus the moisture along the
boundary. Over the past few hours, the trailing cold front has
become more active with a line of showers and thunderstorms from Des
Moines to Omaha back west to Kearney. Though we are already drying
out in the north at this time, the southeast half will continue to
see showers and some isolated storms today as the upper level trough
becomes more parallel to the mid level flow and begins to slow its
southeast progression through the day today. The secondary cold
front now edging southeast is bringing with it a rather cool airmass
which will be present for the day today. H850 temperatures are
expected to drop to 8C in the north to the lower teens in the
southeast by afternoon. Clouds are expected to hold on in the
southeast half and will also fill in farther north today as an H500
system associated with the H850 southwest trough begins to move east
this afternoon/evening. There remains some uncertainty as to how
quickly the systems will track far enough south to no longer impact
the region with clouds/precipitation. The HIRES models show a
consensus of keeping more clouds and even some precip in the south
through Monday as the H500 trough pulls east northeast. Ensemble
guidance supports this scenario as well; however the deterministic
GFS and EC are shading the main moisture channel far enough south to
keep most, if not all of the precipitation later this evening and
Monday in Missouri. Given the NAMs north bias, it also generally
keeps most, if not all of the additional rain showers south in
Missouri. With the signal being consistent the past few days in some
of the data, will lean toward the NBM which does bring back some
showers later Monday morning and afternoon in the south and
southeast (chances at 25 to 40%) with light amounts of up to a tenth
of an inch of rainfall possible. Clouds will linger over the
southeast as well on Monday as the southern stream wave brushes Iowa
with some precipitation.  Highs today will be 20 degrees cooler than
yesterday as afternoon readings reach the upper 60s northeast to the
lower 70s in the north central areas. With lingering showers south,
mid 60s will be the general trend there.  Tonight will be rather
cool as lows dip to the lower to mid 40s with lower 50s in the
south. Monday will see a similar spread of slightly warmer
temperatures north where more sun is anticipated; highs ranging from
the lower 70s north to the upper 60s in the south/southeast.

.Long Term /Monday Night through Saturday/...

Confidence: Low to Medium

Confidence in the extended has dropped considerably over the past 24
hours due to a number of factors. First, though it appeared
yesterday that Tuesday would be dry, the deterministic synoptic
models are now picking up on a wave currently located near Lake
Winnipeg, dropping it south southeast into Iowa by Tuesday with a
few showers and perhaps an isolated storm. This originally was
expected to drop south and remain slightly west of Iowa, but the
exiting upper level system may slow down enough to now allow the two
systems a better chance to phase. To varying degrees, all the 00z
NAM, EC and GFS solutions show an increase in clouds and a slight
chance of showers Tuesday afternoon. The current blended approach in
our forecast shows no increase in rain chances and we may need to
revisit that again later today and tonight as more data arrives.
Beyond that, confidence erodes further as the medium range models
are struggling with an H500 low and the apparent tropical
storm/potential hurricane reaching the eastern Gulf states by
Thursday. For the first few days of the extended, temperatures will
recover to the 70s and last through Thursday.

Secondly, as we move through the middle of the week, several
possible scenarios are now being introduced by the medium range
models with significant differences in the late week/early weekend
forecast. The deterministic GFS is the most progressive of the
model suites with a large ridge at H500 building across the west and
extending to the Northern Plains with H850 temperatures warming back
up to 18 to 22C by Friday over Iowa. The EC, on the other hand ends
up phasing the two H500 waves (tonights with the Tuesday Canadian
ripple) into a cut off H500 low over southern Missouri by Thursday.
With the approaching apparent TS/hurricane in the Gulf by late week;
the deterministic GFS pushes the system up the eastern seaboard
while the EC merges the H500 closed low and the soon to be
extratropical cyclone over the mid Mississippi River Valley,
bringing an abundance of moisture back into the Central Plains and
Iowa by Friday and Saturday. The difference between solutions GFS/EC
is about 4 inches of rainfall over MO, with the EC depositing nearly
7 to 8 inches of rain across southern MO and 1 to 2 inches into the
southeast half of Iowa. For grins, the deterministic GEM is similar
to the EC, but phases the H500 and tropical system in the deep
south, rather than closer to Iowa. Backing out of these model
suites, the ensemble EC and GEFS are both suggesting some component
of moisture being drawn back into Iowa by Friday and lasting into
the weekend with highs tempered back to the lower to mid 70s for the
period. From a climatological standpoint, it seems out of favor to
get a cut off H500 low this time of year in the mid Mississippi
River Valley. But with that being said, we do have quite the
blocking high over the eastern CONUS right now and the mean H500
ridge may maintain its position into midweek stretching from New
England to the southeast states. The difference between the GFS and
EC might mainly be due to the generally faster, more progressive
nature (bias) of the GFS compared to the EC.  For now will need to
maintain the PoPs in the extended period Thursday through Saturday
due to the odd possibility that the cutoff EC solution, supported by
the ensemble guidance, comes to fruition. If it does, next weekend
looks rather wet for much of the area. Details will begin to be
ironed out already by Tuesday or Wednesday. By then the trend toward
a cutoff H500 or progressive pattern will be readily apparent in the
data.


&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1142 PM CDT Sat Sep 21 2024

Rain with a few thunderstorms expected to continue across the
south overnight and into the morning hours. Over the next 6
hours, some scattered thunderstorms will be possible over KDSM
with locally MVFR cigs possible. Gusty winds expected after 12z,
especially in the north where 25kt gusts are possible. Winds
will ease and skies will gradually clear after 18z.

&&

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

&&

$$

DISCUSSION...REV
AVIATION...Jimenez