394
FXUS63 KDMX 181117
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
617 AM CDT Wed Sep 18 2024

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Still warm with increasing clouds today and increasing rain
  and storm chances tonight through Thursday evening. Severe
  storms possible late Thursday afternoon into early Thursday
  evening.

- Lull in rain chances on Friday

- Rain and storm chances return this weekend into early next week
  with timing and location details remaining uncertain.

&&

.DISCUSSION...
Issued at 311 AM CDT Wed Sep 18 2024

Early morning upper level water vapor imagery shows three
prominent areas of low pressure/shortwave troughs. The first is
an upper low over the northern Rockies with a plume of moisture
associated with convection to the east from the northern into
the central Plains. The second is descending down the West
Coast. Both of these are within the mean longwave trough over
the western US. To the east, a low continues to spin over the
Carolinas region, which will begin to drift to the northeast and
move along the Eastern US Seaboard through week`s end. It`s
slow departure will result in the a slow eastward progression of
the western US trough as we move through the rest of the week
into this weekend resulting in a transition to a more active
weather pattern with rain and storm chances.

Filaments of high clouds from the convection associated with the
first low are starting to move into western and central Iowa early
this morning. This low will occlude as it moves over Montana and the
stronger QG convergence will remain west of the state. This leads to
the biggest forecast change in the near term - that is slowing down
the arrival of rain chances while also lowering their chances today
into early this evening. Despite clouds increasing today, highs will
still reach the middle 80s, which is a touch lower than yesterday.
Breezy winds from the south and later southeast will prevail as well
with seasonably high humidity. As this occluded low slowly drifts
northeastward into southern Canada, its surface front along with the
stronger low level QG convergence will move into Iowa later tonight
into Thursday. This forcing and more so a strong low level jet
upwards of 40 knots pointed into Iowa should help for scattered
thunderstorms to develop, particularly toward sunrise Thursday. SPC
maintains a marginal risk over northwest Iowa with instability and
shear marginal in our service area keeping chances for severe
weather minimal. These storms may fester through the morning as they
move off to the east. The higher chance for more widespread storms
and severe weather will be late Thursday afternoon into early
Thursday evening along the cold front. The convergence along the
front may be somewhat weak with a more parallel warm sector wind
vector with a more perpendicular cold sector wind vector. Still,
convergence will be relatively maximized along it and this should
help storms to form. Entrainment CAPE values upwards of 2000 J/kg,
deep layer shear around 30 knots, and favorable (e.g. 7-8C/km) low
and mid-level lapse rates for parcel acceleration will result in a
severe risk - namely damaging wind gusts and hail. Downdraft CAPE
values are north of 1200 J/kg with forecast sounding at MCW showing
plenty of dry air in the mid-levels and to a lesser degree sub-
cloud. The lapse rates raise concerns for large hail, though the
freezing level around 13kft is a bit high and given warm subcloud
air, this may aid in some melting as the hail falls. While upper
level winds may allow storms to remain somewhat separate, low level
SRH values and streamwiseness are not impressive for tornadoes
unless winds can locally back to increase the curvature. SPC
continues and slightly expanded the slight risk over northern Iowa,
which lines up with where the better parameter space for severe
weather resides.

These storms will exit to the east toward or a little after midnight
Thursday night/early Friday morning while the front will settle near
the Iowa-Missouri border or over northern Missouri. Friday continues
to look like a minimum in rain chances as large scale subsidence
slides across the region. Meanwhile, the mean longwave trough with
the new upper low at its base will begin to slowly progress eastward
across the Southwestern US toward the central US. As our flow
becomes southwesterly, this will funnel in higher than normal
precipitable water values to the state later Saturday through Sunday
night with values of 1.5 to 1.75 inches. This flow should also help
to lift the front back into the state. Details regarding timing and
location of rainfall do still remain uncertain and this results in
widespread, prolonged rainfall chances in our forecast this weekend
into early next week. Both the deterministic ECMWF and GFS bring a
surface low somewhere over Iowa late Sunday or Monday and preceding
this low`s arrival, there will be shower and storm chances. It is
interesting to note that the experimental AI models (e.g. AIFS,
Graphcast) are focusing their rainfall either south of the state or
over the southern half of Iowa.

While hard to pinpoint anything down with regards to any severe
weather chances this weekend into early next week, locally heavy
rainfall is possible given the favorable warm rainfall environment.
The 12z ensemble probabilities show up to around a 30% chance of 1
inch of rainfall in 24 hours on Sunday into Monday. Given our dry
antecedent conditions with no more than a half inch of rainfall over
the last 14 days per radar estimates and CoCoRaHS gauge data and
NASA SPoRT upper level relative soil moisture no higher than 30%,
much of this rainfall will be able to infiltrate outside of any
hydrophobic areas or rates. Latest experimental HEFS (Hydrologic
Ensemble Forecasting Service) data supports this with reasonable
worst case output showing just a few gages in the Cedar Basin
reaching action stage in the next 10 days.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 617 AM CDT Wed Sep 18 2024

High clouds are moving over the state, but expect conditions to
remain VFR through the period. Shower and storm chances increase
late today, particularly tonight after 6z. However, confidence
of any SHRA/TSRA occurring at a given site is too low to
include a mention. Winds from the south will be breezy again
today before turning to the southeast and weakening to some
degree tonight.

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Ansorge