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000
FXUS63 KDMX 251143
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
643 AM CDT Thu Apr 25 2024

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- A few sprinkles or showers mainly southwest this morning, breezy

- The stretch of active weather begins tonight with mainly elevated
convection; hail and heavy rain the main concerns

- Ramping up severe weather potential into late Friday afternoon to
evening, especially over SW Iowa.

- Looking to remain active through the weekend, with continued
  severe weather and heavy rain on the table

&&

.DISCUSSION...
Issued at 405 AM CDT Thu Apr 25 2024

The surface pattern this morning features high pressure remaining
over the Upper Midwest region, while well to our southwest, the well
advertised system expected to bring more active weather later on
into tonight can be seen as the surface low continues to develop on
the lee side of the Rockies. Radar analysis over the past several
hours has shown weak returns across northern as well as southwestern
Iowa, though with low level dry air overhead despite increasing warm
air advection and some forcing has left dry conditions at the
surface. Not expecting to see much with this activity as this
initial wave passes through ahead of the main warm advection wing
expected to arrive tonight, with a few hundredths over
southwest Iowa by the afternoon at best. Otherwise,
temperatures currently in the upper 40s to mid 50s with broad
but light east/southeasterly winds keep rather comfortable
conditions. Despite the dryer air in the low levels, increased
moisture further aloft has allowed for cloud cover in the mid
levels to push across much of the state and will remain
throughout the day. The previously mentioned warm air advection
will continue to increase throughout the day, though cloud cover
may limit our high temperatures as highs are expected to reach
through the 60s. Resulting winds will be breezy later this
morning and continue throughout the day, with gusts upwards of
25-30mph.

As referred to above, active weather is expected to begin tonight as
the warm front in association with the deepening low in the western
portion of the Central Plains begins to lift northeast. Not much
change in thinking regarding potential severe weather, as model
soundings indicate low level warm air that would allow for more
elevated but scattered convection as the low level jet increases,
generally late evening into early Friday morning. Instability values
less than 1000j/kg look common over the state, along with effective
shear values between 30-35 knots and steep lapse rates, which would
mainly allow for a hail threat, though likely sub-severe as melting
hail may be a potential issue. Model guidance generally has rather
messy convection continuing through the morning, though the signal
continues to increase on a window for clearing to occur into the
afternoon across a good portion of the state. Into the later
afternoon to evening, CAMS as well as RAP for example show the
dryline entering the southwestern part of Iowa, with moisture ahead
containing dewpoints in the 60s, as well as moderate instability
values in the 1500-2000j/kg range, higher effective shear, though
similar lapse rates compared to the morning. This environment
generally looks more favorable for severe weather into Friday
evening, with large hail, gusty winds, heavy rain and even some
tornadoes on the table. SRH values in the 200-250m2s2 range are
present in the model soundings over the area, and even higher of 300-
400+ m2s2 values per the NAM, for a short time period. The SPC as a
result has increased the severe weather risk with an Enhanced over
the southwest portion of the state and a 10% hatched area for
tornado potential, while the Slight risk is still over much of
Central Iowa contains a 2-5% tornado risk. Definitely will be
focusing on all the details and updating as more data becomes
available, so stay tuned for updates. Regardless, the important
thing to keep in mind is that the potential for severe weather over
Central Iowa has been present several days out and has been fairly
consistent, so it is important to be prepared for the potential
hazards that are expected and make sure to have multiple ways to
receive warnings. To mention the hydro side of this system, PWATS
are not very impressive, with values around 1-1.25 inches, while
warm cloud depths are on the higher end in the 11-14kft range.
Looking more likely to see the higher amounts of rain focused more
on the northern and western portion of the state, especially Friday
morning with 1-2 inch values over the area. Not to forget the
synoptic winds, as warm air advection increases further throughout
Friday, gusty winds reaching over 30-35+ look common across the
state, especially into late Friday as the low level jet
redevelops directly over Iowa.

Beyond Friday, lingering precipitation is possible Saturday morning,
though much of the area could be dry. However, still looking at a
second system in the form of another developing surface low as the
broad trough over the southwest CONUS lifts northeast. Compared to
the previous system, the moisture availability looks a tad higher as
surface dewpoints reach into the mid 60s by Saturday evening. With
the low pressure looking to make a more direct passage across Iowa,
the developing cold front looks to move largely across eastern Iowa
and into IL/MO, which looking at the environment contains moderate
instability and shear to support severe weather Saturday evening
into Sunday. Heavy rainfall potential into Sunday also looks higher
given the previously mentioned higher moisture availability, with
the best potential for an inch or more of rain across southeastern
Iowa, which is highlighted by the Slight risk for excessive rainfall
in the Day 3 WPC outlook. Much uncertainty overall with the exact
evolution and resulting impacts are present, considering the active
weather before it that will play a role in these details, so will
have to wait to get further into the details until closer to the
weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 643 AM CDT Thu Apr 25 2024

VFR conditions are expected through much of the TAF period.
Mid-level clouds are expected to decrease a bit later this
morning before increasing through the afternoon, though low
clouds are expected to follow from southwest to northeast by
03z-06z as a warm front pushes through Iowa. Showers and storms
are expected late tonight and into Friday, with hail being the
main threat. Deteriorating conditions are expected with low
ceilings and heavy rain at times after 06z and beyond the
current TAF period. Winds out of the southeast are also expected
to increase throughout the day, especially into Friday as gusts
over 30 knots are possible across the state. Cannot rule out
isolated areas of LLWS as well over the area after 06z.

&&

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

&&

$$

DISCUSSION...Bury
AVIATION...Bury