874
FXUS63 KDMX 190820
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
320 AM CDT Thu Sep 19 2024

.KEY MESSAGES...

- Severe storms possible late this afternoon into early this
  evening with large hail and damaging wind gusts the main
  hazards. The coverage of severe storms remains uncertain and
  could be rather isolated.

- Warm, ample sunshine, and mainly dry during the day and evening
  Friday.

- Rain and storm chances return later Friday night into
  Saturday. Maximum coverage Saturday night into Sunday with
  chances highest over central and southern Iowa at 60 to 90%.

- Somewhere in Iowa, there will be rainfall totals of 1 to 2
  inches by Monday morning. Any severe weather is uncertain.

&&

.DISCUSSION...
Issued at 320 AM CDT Thu Sep 19 2024

Our weather pattern will become increasingly influenced by the
western US longwave trough and associated shortwave troughs as the
upper level low over the Middle Atlantic Coast departs allowing for
a more progressive pattern. The first shortwave trough that has a
closed low is spinning over the Montana/North Dakota/Saskatchewan
area per upper level GOES water vapor imagery. We`re just beginning
to get in on some of its forcing in the way of low level QG
convergence, which along with low level warm air advection and a low
level jet has been able to generate a few storms across western Iowa
this morning. The first storms trekked across northwest Iowa,
including Palo Alto, Emmet, and Kossuth Counties shortly after
midnight. Additional convection is now developing over west
central Iowa with more scattered coverage expected in the coming
hours as the QG convergence and warm air advection shift
eastward. So, will see these scattered showers and storms
through the morning and early afternoon hours, but expecting all
of this activity to be non-severe.

A chance for severe storms remains possible late this afternoon into
early this evening as a weak surface cold front, associated with the
aforementioned shortwave trough, swings into the state. While SPC
has expanded the slight risk across much of the forecast area, the
overall coverage of severe weather remains uncertain. This
uncertainty in coverage is driven in part by the lack of strong
surface convergence as the warm sector wind vectors will be more
parallel to the front while the cold sector vectors will be more
orthogonal to the front. Still, storms should form within the
favorable warm sector environment with limited inhibition around.
Thee storms should be isolated to perhaps a well spaced, broken
line of storms as mid-level cross boundary flow would limit
storm mergers in their early life cycle and allow for supercells
given shear values. Entrainment CAPE values are lower than this
time yesterday, but around 1500 J/kg with surface based CAPE
near 2000 J/kg per recent HRRR and RAP soundings over northern
Iowa. The lesser ECAPE values is due to the dry mid-level. This
dry air is aiding in favorable downdraft CAPE values anywhere
from 900 J/kg (RAP) to 1200 J/kg (HRRR), though it is
interesting to note that the convective allowing models (CAMs)
such as the 0z runs of the HRRR, NAMNest, RRFS, and WRF-ARW
hardly generate any wind gusts over 30 knots. Large hail will
remain a concern as well given the steep lapse rates and
effective deep layer shear over 40 knots and a linear hodograph
in this effective layer. The tornado risk seems rather limited
in our forecast area with a lack of backed surface and low level
flow to increase the low level curvature of the hodograph. In
addition, outside of perhaps northern Iowa, LCLs over 2000m will
not be favorable for tornadoes. The storms will push eastward
through the evening hours and largely out of our forecast area
by midnight.

Behind this front, which will drop into Missouri, large scale
subsidence will arrive and yield a dry and what looks to be like a
fair amount of sunshine on Friday. Another shortwave trough within
the base of the western US trough will begin to lift towards Iowa
Friday into this weekend. Deterministic models have a fairly similar
timing compared to one another, though the location varies a bit
still. With increasing southwesterly flow and warm air advection
ahead of this wave, will see rain and storm chances increase later
Friday night into the day Saturday. A lull in the rainfall is
possible during or around the afternoon, but more widespread rain
and storm chances return by Saturday night into Sunday as the
shortwave and associated forcing arrive. While the Colorado State
University`s machine learning random forest outlook shows low
severe probabilities over Iowa, severe weather chances remain
quite uncertain and agree with SPC`s day 3 discussion. Locally
heavy rainfall remains in play with ensemble mean probability of
an inch of rainfall having increased from this time yesterday
up to around 50%. 48 hour rainfall totals from a variety of
deterministic models (GFS, ECMWF/IFS, CMC, UKMET, ICON) and
experimental AI (Graphcast, AIFS) show 1 to 2 inch totals
possible somewhere in the state with a majority of these showing
central to southern Iowa as the favored location, though this
is not calling the shot given the spread that remains. Thinking
has not changed from last night that much of this rainfall will
infiltrate into the dry soil with any rises on streams staying
within bank.

This shortwave trough will exit early next week with rain and storm
chances diminishing Sunday night into Monday. However, another wave
will follow closely behind, which could bring passing rain or storm
chances through midweek. High temperatures Sunday into late next
week will be seasonally cool in the upper 60s or low 70s.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1145 PM CDT Wed Sep 18 2024

VFR to begin the period. Chances for showers and a few storms
will increase from west to east early in the morning. There
should be a break before more storms develop in the afternoon.
Gusty winds are possible with any storms in addition to MVFR
cigs and lower vsbys in rain. Breezy southerly winds much of
Thursday before switching with fropa by late in the period.

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Donavon