Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

214
FXUS63 KDMX 202344
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
644 PM CDT Fri Sep 20 2024

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

.KEY MESSAGES...

- Spotty showers and thunderstorms in southern Iowa early
  Saturday morning, then more widespread thunderstorms in the
  afternoon and evening, continuing through Sunday.

- A few stronger storms are possible on Saturday with pockets
  of heavy rain possible on Saturday and Sunday (especially in
  southern Iowa).

- Cooler Sunday through next week with highs in the 60s and 70.

&&

.DISCUSSION...
Issued at 237 PM CDT Fri Sep 20 2024

High pressure has settled across the area today resulting in plenty
of sunshine, mostly sunny skies, and low humidity. Temperatures this
afternoon have warmed into the mid to upper 80s this afternoon with
dewpoints in the 50s. A quick zoom out on satellite tells us this
quiet day will be short lived. Off to the west two phased waves are
visible, one across the desert southwest and the other across
the northern Rockies. These will move across the area this
weekend, bringing chances for stronger thunderstorms and heavy
rain through the weekend.

The first wave of precipitation will arrive around midnight or
shortly after as spotty showers and thunderstorms lift into the area
out of Missouri on the nose of a slug of warm air advection out
of the southern plains. There is still some variability amongst
the CAMs as to placement with most keeping the bulk of the
activity into Missouri through Saturday morning. The better
forcing arrives later Saturday afternoon when more widespread
shower and thunderstorms are expected across the area. MLCAPE is
generally near or under 1000 J/kg with modest shear which would
prevent better updraft organization. As such, a few stronger
storms are possible with hail or a stronger wind gust, but at
this time the overall severe threat is on the low side. The
southern closed low lifts into the area late Saturday into
Sunday, with lingering forcing from a trailing boundary behind
the northern wave. This will prolong precipitation across
central and southern Iowa on Sunday and into early Monday.
PWATs increase in southern Iowa on Sunday, approaching 2 inches.
There remains uncertainty as to the placement and precise
magnitude of the rain totals for the weekend, but most ensemble
guidance indicates southern Iowa will be the winners with
lesser amounts north. The EC is the most robust with 2-3" of QPF
south of I-80 while the GEFS keeps totals closer to 1.5 inches.
HREF guidance is in for the first half of the event and shows
pockets of 3+ inches possible in isolated areas.

Much cooler air filters in on Sunday with highs in the 60s expected.
Temperatures remain cool next week, near to a few degrees below
average for this time of year. Precipitation trends into next week
are less clear with significant differences between synoptic models
middle of next week. The GFS features a large closed low across the
area while the Euro keeps this feature well off to the southwest.
Details will be refined in the days to come.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 641 PM CDT Fri Sep 20 2024

VFR conditions expected over the next 6-8 hours. some showers
and storms may form in southern IA between 6 and 8z this
evening. Confidence was too low to include in KDSM and KOTM at
this time, but will evaluate again at 06z. Widespread showers
and storms approach from the south and west after 18z with the
first round affecting northern terminals before 00z. These
showers will be along a front that will bring in a wind shift
in the afternoon. High confidence rain affects southern
terminals shortly after the conclusion of the TAF period.

&&

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

&&

$$

DISCUSSION...Hagenhoff
AVIATION...Jimenez