821
FXUS63 KDMX 172322
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
622 PM CDT Tue Sep 17 2024

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

.KEY MESSAGES...

- Chances for showers and storms in the west late Wednesday
  through Wednesday night, and across more of Iowa Thursday
  afternoon into Thursday night. There is a potential for severe
  weather late Thursday, mainly in northern Iowa.

- Another period of more active weather, with higher rain and
  storm chances, later this weekend into early next week.
  However, details of timing and location are unclear at this
  range.

&&

.DISCUSSION...
Issued at 202 PM CDT Tue Sep 17 2024

A weak mid-level shortwave moving northeastward across the
region sparked some light showers and a few thunderstorms over
northwestern Iowa this morning, but as the shortwave has moved
away these have degenerated and the area is now dry, though a
few additional light showers are lingering to our west near the
Missouri River valley. In wake of this system a lull is
forecast tonight, with some mid-level clouds but no additional
precipitation anticipated.

By Wednesday morning a large gyre will be centered around
southeastern Montana, but any eastward advance will be stymied
by a blocking pattern in place partially due to the remnants of
the tropical cyclone sitting over the Carolinas. This blocking
pattern will shunt the Montana low slowly north northeastward
into central Canada from Wednesday onward. A weak surface
trough, effectively a cool front, trailing from the system will
generate showers and thunderstorms over eastern Nebraska on
Wednesday afternoon that will bleed eastward into area area
later Wednesday into early Thursday. Instability and shear will
be limited during this time, and while there may be a marginal
risk of severe weather to our west, nearer to the front, in our
service area the risk will be minimal.

On Thursday the aforementioned trough/cool front will be moving
across our area late in the day, and should again provide a
focus for convective initiation. However, by then instability
will be higher, potentially around 1500-2500 J/kg2, and forecast
soundings show good speed shear aloft but more or less
unidirectional, and with weak near-surface flow/convergence.
This supports initially strong updrafts capable of hail for a
brief period, perhaps transitioning into an isolated strong wind
threat, with any such threats relatively higher further north
toward the parent low. SPC has outlined this potential well with
a Slight Risk of severe weather across northern Iowa and a
Marginal Risk elsewhere.

Any convection late Thursday/Thursday night will clear out of
our area by Friday, with the surface boundary pushing just south
and southeast into Missouri and Illinois before stalling out.
This will provide a short break of dry and quiet weather on
Friday, but by Friday evening another late mid-level low will
have sunk into the southwestern U.S. somewhere around the Grand
Canyon, with steering flow over Iowa beginning to shift toward
southwesterly thereafter. This should push the stalled surface
boundary over northern Missouri back toward Iowa, and as the
southwestern U.S. low subsequently ejects northeastward bring a
period of higher rain chances sometime this weekend. However,
the long-range/global models are having problems resolving the
evolution of this scenario with regards to the track and timing
of the low, extent to which the preceding surface boundary
retreats, and resultant temperature/precipitation effects in
Iowa and there are wide disparities from run to run. For now
will maintain generally 40-60 POPs during that time. It is too
early to diagnose any severe weather threats for the weekend,
but there is certainly some potential for heavy rain given
seasonably high PWATs and the possibility of a nearly stalled
boundary nearby roughly parallel to the steering flow.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 621 PM CDT Tue Sep 17 2024

VFR conditions prevail through the current TAF period.

&&

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

&&

$$

DISCUSSION...Lee
AVIATION...Martin