878
FXUS63 KDMX 141745
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1245 PM CDT Wed Aug 14 2024

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

.KEY MESSAGES...

- Back-to-back rounds of thunderstorms this morning and again
  this evening into Thursday morning. Heavy rain is possible
  both rounds with a few strong to severe storms possible
  tonight.

- Additional storm chances Thursday afternoon into evening.

- Warmer and drier forecast this weekend.

&&

.DISCUSSION...
Issued at 340 AM CDT Wed Aug 14 2024

Multiple rounds of showers and storms are expected today through
Thursday with heavy rain and strong to severe storms all
possible.

Attention early this morning is on the first round as a lead
shortwave ahead of the surface low noted over the northern High
Plains at 06Z moves eastward through Nebraska. Storms continue to
traverse southern into central Nebraska early this morning and are
expected to continue into Iowa fueled by favorable moisture
transport into eastern Nebraska, increasing theta-e advection, and a
35-45 knot low level jet. As this activity arrives in western to
southwestern Iowa, SPC mesoanalysis and hi-res guidance continues to
depict marginal instability in Iowa which will limit storm strength
and as such, generally expect a continuing weakening trend with this
convection, as has already started to occur in eastern Nebraska. The
warm air advection wing will be the first to arrive which was just
arriving at the IA/NE border in far southwest Iowa around 715Z with
this activity gradually lifting northeastward. Then, the main line
arrives shortly after generally moving eastward across southern
Iowa, though precipitation extends well north and west behind the
main line. Lots of lightning has been noted with both the warm air
advection wing and the main line which will remain the primary
hazard along with locally heavy rainfall given the more minimal
instability values previously mentioned. With pwats around 2",
precipitation amounts may near or exceed 1" over especially
southwest Iowa, with localized higher values possible, into midday.
Coverage should diminish as storms push eastward through the morning
hours as the overnight low level jet fades with time with most
guidance in pretty good agreement of only lingering precipitation in
northern Iowa by early to mid afternoon. With this, clearing should
begin to occur over at least far southwest into southern Iowa
setting the stage for round two later this evening.

The extent of clearing and as such, atmosphere recovery and
resultant instability values heading into round 2 will have a
significant impact on storm mode and strength. Forcing continues to
look stronger with this second round as the aforementioned low nears
the area dragging a front with it, in addition to the potential for
any remaining outflow boundaries from the morning activity. Latest
guidance has varying amounts of MUCAPE remaining heading into the
evening hours with amounts varying from 1000 J/kg to robust values
of a few thousand J/kg. While these highest values may be offset
from the increasing shear, there remains the ingredients for some
strong to severe storms with damaging wind gusts and possibly large
hail. However, soundings do continue to suggest the potential for
long, looping hodographs, low LCLs and enough helicity to keep a
conditional tornado threat if any storms can become surface based
despite otherwise elevated storms being the most likely. As noted in
the previous discussion, HREF helicity tracks fade with time through
the late evening hours with overall hi-res guidance continuing to
depict messy storm modes which would all lead to a low tornado
threat overall, but it is conditionally higher than zero. With pwats
once again near to exceeding 2", favorable warm cloud depths, and a
stronger low level jet in most guidance, heavy rain will also remain
a threat this evening into early Thursday, especially given the rain
this morning. HREF Localized Probability Matched Mean 24-hour QPF
values ending 12Z Thursday reach towards or even exceed 2" in much
of southern Iowa with pockets of higher values, and additional
little areas of similar amounts in isolated locations further
north and east. Thus, we will continue to have to monitor for
any localized flooding issues especially with the repeated
rounds of rain, and given the overall amounts into Thursday,
some ponding in especially low-lying areas seems likely.

Round 2 storms exit the area by mid-morning Thursday (note, round 2
begins near/after 00z tonight) but additional showers and storms are
possible on Thursday afternoon into evening. Guidance is not in good
agreement at this time with the amount of instability present
on Thursday afternoon with the better forcing also generally
east of the area. Timing of the frontal passage, any lingering
outflow boundaries, and amount of recovery after early day
storms will all influence the strength of any storms that occur,
which could also be east of the area. Will continue to monitor
the mesoscale details that will impact our storm threat
Thursday, but for now, see the SPC Day 2 Outlook for some
further details.

Although some wrap around showers could occur especially east on
Friday as the low to the northeast consolidates further, conditions
generally look to dry out for much of the weekend though some
isolated precipitation chances remain as some waves move through the
otherwise continuing to build northwest flow as the upper ridge
continues to nudge further east into the region. Temperatures will
be closer to normal levels with higher humidity as we continue
through the week.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1245 PM CDT Wed Aug 14 2024

MVFR to IFR conditions are expected for much of the forecast
period as a system passes through the Midwest. Showers persist
across the north this afternoon with widespread convection
overnight helping the poor aviation conditions persist. Surface
wind gusts in and around the storms may approach 35-40kts this
evening in western Iowa. Low ceilings persist overnight into
Thursday morning with some minor improvement by late in the
period. Surface winds are expected from the southeast this
afternoon becoming more southwest toward the end of the
forecast.

&&

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

&&

$$

DISCUSSION...KCM
AVIATION...Cogil