Area Forecast Discussion Issued by NWS Des Moines, IA
000
FXUS63 KDMX 121754
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
1254 PM CDT Tue Mar 12 2024
...Updated for the 18z Aviation Discussion...
.KEY MESSAGES...
- Warm again today with highs near 70. Chances for storms south
central into southeast Iowa this evening.
- Widespread 0.25-0.50" rainfall Wednesday night into Thursday.
- Severe weather possible over the southern half of the state
Thursday afternoon and evening. Large hail is the primary
threat at this time.
&&
.DISCUSSION...
Issued at 240 AM CDT Tue Mar 12 2024
It`s been a rather mild overnight so far as southwesterly winds have
prevented full boundary layer decoupling and temps holding nearly
steady in the 50s. The warm airmass will lead to another day of
well above normal highs, although upper clouds ahead of an
approaching shortwave may peel max warming potential back a few
degrees. Still running with highs around 70 for all areas. All
indications suggest winds today will not be as gusty as Monday.
The sfc pressure field relaxes as a weak boundary sags south
toward the central tier of the state. The airmass north of the
boundary is quite dry and RH values may fall into the teens
during the peak heating hours. This is certainly a concern on
the fire weather front, however headlines are not necessary at
this time with winds progged to be below critical thresholds.
LLJ amplifies this evening ahead of a compact shortwave pushing off
the central Plains. Most global/hi-res guidance is in agreement with
convection developing aoa 00z within a corridor of 500-1000 J/kg
MUCAPE from eastern KS through MO and into south central/southeast
IA. Soundings suggest any convection in our area will be elevated
with parcels rooted between 850-800mb. Steep lapse rates above
this level and effective shear of 20-30 kts could promote some
loosely organized updrafts capable of producing near-severe
hail.
A deep upper trough is established over the western conus Wednesday.
Several shortwave disturbances are embedded within the mean
trough and will deliver a chance for widespread rainfall from
later Wednesday and into Thursday. Sfc low deepens over KS with
a sharpening warm front extending north and eastward. Precip
chances increase by late Wednesday into Wednesday night as
theta-e advection ramps up and showers and storms blossom within
a zone of phased kinematic and thermal forcing mechanisms.
Precip chances continue into Thursday as another shortwave breaks
off from the parent upper low, now attempting to become cutoff over
the far southwest US. An eroding deformation zone slides
through and provides support for additional shower activity
across the northern half of the forecast area. Surface low
migrates into southwestern Iowa by Thursday morning with a warm
front extending eastward through the state. A warm and
increasingly humid warm sector in place over the southern
portion of the state suggests the potential development of
surface based convection. Stratus clouds within the warm sector
may inhibit destabilization and keep capping in place, however
potential clearing during the afternoon along with deeper
synoptic forcing with the arriving upper shortwave. Any surface
based convection would be working beneath an EML with steep
lapse rates and within a highly sheared environment, although
much of that contribution is in the mid to upper levels as low
level kinematics are not overly impressive at this time. Thus
mainly looking at a favorable environment for supercells and a
threat for large hail. The full scope of the threat, including
the potential for tornadoes, will need to be closely monitored.
Conditions dry out Friday and into Saturday, although
temperatures will be cooler with highs mainly in the 50s. A deep
upper low crashes southward into the Great Lakes region and
pushes a cold front through the state by Sunday. Blustery
northwest winds + highs in the 30s and 40s will make it feel
noticeably cooler. Model soundings suggest a few snow showers
are possible as cold air advection and diurnal heating steepen
low level lapse rates, but dry air will be a limiting factor.
Thus only mentioning slight chance PoPs over a handful of our
northern counties at this time.
&&
.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1239 PM CDT Tue Mar 12 2024
VFR conditions continue for the start of this TAF period across
all terminals. Winds will gradually decrease tonight at all
sites and switch direction across northerly terminals from
north northwesterly to easterly by Wednesday morning. May see
some showers and thunderstorms near the KOTM vicinity and
southern Iowa after 02z. Confidence on any direct impacts for
this terminal is low at this time, thus have opted for VCTS
mentions. Reductions in visibility are possible with any
showers and storms. Ceilings will lower to MVFR across KOTM and
areas southeast by Wednesday morning.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...Martin
AVIATION...Castillo/Bury