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000
FXUS63 KFSD 251133
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
633 AM CDT Thu Apr 25 2024

.KEY MESSAGES...

- Rain and storms return this evening and night and persist
  through Friday. Strong to severe storms are possible on
  Friday.

- Beneficial rainfall is expected tonight with a broad quarter
  to three quarters of an inch of rain expected, highest over
  northwest Iowa.

- Chances for rain continue through the weekend with
  probabilities increasing to 30-50%+ for exceeding a half an
  inch of rain on Sunday.

&&

.DISCUSSION...
Issued at 355 AM CDT Thu Apr 25 2024

Sprinkles to very light showers begin the day today. These sprinkles
and light showers are expected to dissipate through the rest of the
overnight hours. Today will be a mostly quiet day as cloud cover
builds into the region. This will hold high temperatures similar to
what they were yesterday with highs into the mid to upper 60s but
could be a touch cooler. An upper level wave will begin to eject
into the Plains which will set the stage for rain this afternoon,
evening, and night. Lee cyclogenesis will take place in the front
range of the Rocky Mountains as the wave ejects. Low level pressure
gradients will tighten ahead of the developing surface low which
will result in a strengthening low level jet (LLJ) and southeast
gusts up to 30-40 mph. Warm air advection (WAA) will strengthen in
response to the LLJ and bring the chance for isolated showers this
afternoon. Sounding profiles are mostly dry aside from a moist layer
between 800-700 mb so any rain that does fall during the afternoon
will be light as only a few hundredths to possibly up to a tenth.
The LLJ will further strengthen into the evening and overnight
hours, continuing gusty southeast winds and rain chances during this
period of time. A surface warm front will push into Kansas but the
elevated front will move into central Nebraska. While still south of
our area, a mid level wave along with a 500 mb speed max look to
encounter the elevated warm front and produce convection on it.
Elevated instability is expected ahead of this elevated warm front
but the instability will be weak on the order of about 500 J/kg of
CAPE. Given the weak instability, severe weather is not expected but
isolated small hail is possible. The other aspect to this overnight
precipitation is beneficial rainfall. Moisture transport will
strengthen between 925-850 mb which will allow for a widespread
quarter to three quarters of an inch of rain to fall, highest over
northwest Iowa. Isolated higher amounts are possible.

The upper wave pushes into the forecast are on Friday which will
bring the chance for strong to severe storms with it. However, this
chance for severe storms remains conditional as it remains uncertain
how overnight rain and storms will affect the environment. If the
low levels remain stable via low level stratus blanketing the area
as the occluding system pushes into the Missouri River Valley, then
severe weather chances will be low and would be confined to elevated
convection with a large hail threat. However, if the low levels are
able to clear out and destabilize as the wave approaches, then
severe weather will be possible. In this scenario, the surface warm
front will rapidly advect northwards and push into the Missouri
River Valley and allow convection to develop as the main wave and
left exit of an upper level jet streak couple with the
baroclinically generated vorticity along the warm front. With
surface dew points moistening up to around 60 degrees F,
sufficient instability will be realized up to 1,000-1,500 J/kg
of CAPE. Wind shear will be sufficient at 30-40 knots of deep
layer shear. Hodographs reveal that there is clockwise curvature
in the low levels with 0-1km storm relative helicity (SRH)
values up to 150-200 m^2/s^2. This along with 0-1 km shear
values between 20-30 knots and low LCLs would support an
environment capable of tornadoes along with large hail and
damaging winds. In fact, storm relative inflow looks to be
aligned with 0-500 m vorticity vector, signaling the potential
for nearly streamwise vorticity to be ingested into the storms
that develop. Taking everything into consideration, think that
the the most likely scenario is for the overnight rain and
storms to prevent the low levels from destabilizing during the
afternoon hours and thus keep low level stratus across the area.
This is supported by hi-res ensembles as they all show barely a
30% chance for 2-5 km updraft helicity exceeding 75 m^2/s^2 and
widespread cloud cover throughout the day. The main timeframe
for storms is between 4 pm to 10 pm. Outside of storm chances,
Friday will be a cooler and more humid day as high temperatures
only warm to the upper 50s and 60s while dew points moisten to
the 40s and 50s. Winds will remain gusty out of the southeast
with gusts up to 30-40 mph expected, strongest along and east of
I-29. Low temperatures will remain on the mild side, only
falling to the upper 40s and 50s overnight.

An active pattern remains aloft for Saturday as another upper level
wave ejects into the Plains. A cold front will lie southeast of the
forecast area thanks to Friday`s wave. Given the front`s placement,
severe weather is not expected but light showers may be possible.
High pressure will slowly be pushing into the area from the
northwest resulting in marginally breezy northerly winds. High
temperatures will warm to the upper 50s to upper 60s from northwest
to southeast across the area while low temperatures will fall 40s
and 50s.

The same upper wave from Saturday will push into the Northern Plains
on Sunday. Medium range guidance varies on how the surface low will
track along with where the surface front will lie. If the front was
to back further to the northwest, then higher theta-e air could be
introduced to the area and may introduce chances for storms. If not,
then deformation precipitation on the back side of the system would
be more likely. Ensembles only show about a 30% chance for CAPE
values to exceed 500 J/kg. They also show a widespread 30-50%+
chance for exceeding a half an inch of rain. This is further
supported by cluster analysis which has all clusters showing a broad
40-60% chance for exceeding a half an inch of rain. Aside from rain
chances, high temperatures look to remain in the 50s and 60s with
lows falling to the 30s and 40s.

Nearly zonal flow sets up for most of the week next week. With
westerly flow aloft, next work week look to be mainly dry with highs
in the 50s to 70s, coldest on Monday, and lows in the 40s and
50s.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 633 AM CDT Thu Apr 25 2024

VFR conditions begin the TAF period. VFR stratus is pushing into the
area early this morning from northeastern Nebraska. Sprinkles to
very light showers are possible beneath the stratus, mainly by KSUX.
Southeast winds will gust up to 30-35 knots this afternoon across
the area and will continue to gust that high for the remainder of
the TAF period.

Chances for rain will increase this evening and night across the
area as ceilings lower to MVFR levels. IFR ceilings are possible
late in the period but confidence is not high enough to include in a
TAF at this time. Thunder is also possible during the overnight
hours but confidence is also too low in any specific location seeing
thunder so have omitted from all TAFs at this time. Will continue to
monitor trends over the coming hours. Showers along with MVFR
ceilings will finish the TAF period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.

&&

$$

DISCUSSION...Meyers
AVIATION...Meyers