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000
FXUS63 KDMX 232321
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
621 PM CDT Tue Apr 23 2024

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

.KEY MESSAGES...

- Light winds tomorrow, then breezy again on Thursday.

- Widespread showers and thunderstorms starting late Thursday
  and lasting through Friday. Severe storms expected.

- Additional showers and thunderstorms expected through the
  weekend.

&&

.DISCUSSION...
Issued at 402 PM CDT Tue Apr 23 2024

>> Winds Diminishing Tonight, Quiet Wednesday Then Breezy Again
Thursday

Surface low pressure passing to our north has pulled it’s associated
surface boundary through Iowa today, leading to a few
sprinkles/light rain showers south through the morning hours, and
widespread gusty winds this afternoon. Observed winds early this
afternoon have ranged anywhere from 20 to 30 mph with gusts over 40
mph at times, suggestive of some efficient mixing today. With this
efficient mixing, conditions have dried out quite a bit already with
dewpoints in the mid 20s to low 30s as of early afternoon. While
fire weather concerns aren’t overly high due to recent rainfalls and
greening vegetation, strong gusts and dry conditions may still lead
to erratic fire behavior this afternoon.

High pressure filters in tomorrow, leading to light winds and mostly
clear skies through the day. This cooler air mass will keep
temperatures more seasonal in the mid 60s as flow steadily flips to
more easterly then southeasterly through the day. As we get into
Thursday, the well advertised late week system will begin to make
it’s way towards the state. However, before it arrives, tightening
pressure gradients ahead of the surface low pressure will again
result in breezy conditions on Thursday and Thursday night.

>> Showers and Thunderstorms Late Thursday through Friday

By late Thursday, the warm air advection wing ahead of the
approaching trough will begin to arrive in our area, bringing
increasing shower and thunderstorm chances through the overnight
hours and becoming widespread by Friday morning. Strong wind fields
and increasing instability in the warm sector of the low will lead
to an increasing chance for severe weather through Friday.
Thunderstorms overnight Thursday into early Friday morning will
initially be elevated and mainly supported by a strong LLJ. However,
the instability will be relatively low (~500-1000 J/kg), but a few
stronger storms would still be capable of producing hail into Friday
morning. After sunrise, surface heating and the advection of warm,
moist low level air should help to eliminate capping in place,
leading to the possibility for surface based storms by mid-day.
Similarly, instability will be increasing through the day with
deterministic guidance putting a nose of ~2000 J/kg of SB CAPE over
the southern half of the area Friday. That being said, the better
deep layer shear looks to stay a bit further south in the area along
the Iowa/Missouri border. The CSU machine learning probabilities are
keying on this area as well, showing the highest likelihood for
strong storms along and south of the border. That being said,
probabilities stretch well into our area, and with the favorable
instability, severe storms will still be possible further north as
well. This is depicted best in the Day 4 SPC outlook, which has a
15% chance for severe storms well up into north central Iowa.

In addition to the severe chances on Friday, widespread heavy
rainfall looks possible. Current ensemble and deterministic guidance
looks to be favoring southern and eastern Iowa, which matches
conceptually with where the best moisture transport and highest
PWATs are located. LREF grand ensemble show roughly a 50% chance of
an inch or greater of rainfall in 24 hrs for Friday over much of
central, eastern and southern Iowa. This is reflected in WPC QPF
guidance, showing widespread areas of an inch or higher for Friday.
Finally, not to be completely overshadowed by the showers and
storms, strong synoptic winds are expected Friday as well with
strong low level flow aloft and tight surface pressure gradients.
That being said, the magnitude of winds mixed down to the surface
may be somewhat limited with rainfall ongoing much of the day and
broad-scale lift present.


>> Additional Convection Saturday into Sunday

Rainfall from the first wave will begin to diminish as the low
tracks northeast late Friday night into Saturday morning. A very
brief lull in storms then sets up Saturday morning, with a good
amount of uncertainty in what comes next. The over-arching
progression will be a secondary wave ejecting north behind the first
wave, bringing additional shower and thunderstorm chances on
Saturday night and into Sunday. That being said, guidance is still a
bit sporadic on the track of this secondary wave, which will dictate
the severe impacts in our area. There also looks to be a boundary
that sets up between the two waves, that could fire some convection
late Saturday afternoon/evening before the second wave arrives, but
this will be dependent on the location of the boundary as well as
the ability to overcome the dry air slotted in the mid levels.
Therefore, not going to dig too far into the details of the second
wave quite yet given the variability. However, the important
takeaways are that more rainfall is likely again Saturday into
Sunday, with the potential for additional severe weather chances on
both days, currently favoring southern Iowa.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 620 PM CDT Tue Apr 23 2024

VFR conditions prevail through the current TAF period. Gusty
northwest winds should quick taper down over the next few hours
and remain at or below 10 kts.

&&

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

&&

$$

DISCUSSION...Dodson
AVIATION...Martin