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000
FXUS63 KFSD 260408
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1108 PM CDT Thu Apr 25 2024

.KEY MESSAGES...

- Isolated to scattered showers and storms this afternoon. Additional
  showers and storms move in tonight through Friday. Strong to
  severe storms are possible tomorrow (greatest risk between 3
  and 9 PM), along with locally heavy rain.

- Shower and storm chances taper off Saturday, but increase once
  again on Sunday. Uncertainty remains, but those with outdoor
  plans should keep an eye on the forecast.

- Through Sunday night, widespread beneficial rainfall of an
  inch or more is expected. Some localized areas could see 2 or
  more inches of rain in this period.

&&

.DISCUSSION...
Issued at 354 PM CDT Thu Apr 25 2024

REST OF TODAY AND TONIGHT: Isolated showers and storms continue this
afternoon and early evening with increasing clouds as the first
vorticity lobe advects north a wave ejecting out of the Rockies.
Precip has been struggling a bit with dry air below the cloud bases.
This low will begin to deepen tonight. Breezy conditions this
afternoon continue through tonight, with tightening pressure
gradient and the potential to mix into stronger winds aloft. Expect
southeasterly winds 20-30 mph, with gusts around 40 mph. Isolated
gusts as high as 45 mph have been briefly observed (as well as
sustained winds over 30 mph); however, these have been sporadic and
isolated enough in nature to preclude any wind headlines. As we lose
peak mixing later this evening and tonight, should see a slight drop
in gusts. But, we could see some 40 mph gusts overnight if there is
any mixing into the strengthening LLJ. Temperatures this afternoon
range from the upper 50s (cloudy areas along the MO River) to the
lower 70s (clear skies in SD and MN). Continued southeasterly flow
helps keep lows in the 40s and lower 50s.

Once afternoon showers/isolated storms lift north out of the area,
we`ll turn our attention to the development just to our southeast in
NE through the evening hours. This activity, firing off the nose of
the LLJ and mid level vorticity lobe coincident with the WAA and
elevated warm front, will continue to lift north/northeast ahead of
the deepening low through the overnight hours. Instability is
elevated, and with values around 700 J/kg, severe weather looks
unlikely. Mid level lapse rates around 6 deg C/km could produce some
hail up to nickel sized in a stronger storm, and can`t rule out a
gust or two around 50 mph overnight if showers/storms are able to
pull down any of the stronger winds. Rain amounts will be quite
variable due to the convection but areas under stronger and/or
multiple thunderstorms could see close to an inch, with the highest
amounts across northwestern IA.

TOMORROW AND TOMORROW NIGHT: Low pressure continues to deepen as it
lifts northeast through the day on Friday, lifting into the southern
MO Valley by the late afternoon/early evening hours - becoming more
stacked as it treks into our area. Strong to severe storms are
possible, although there is some uncertainty as we wait and see the
answers to these three questions: if/how quickly do clouds and
morning convection clear up during mid day, if/how quickly can the
atmosphere recover, and where does the surface warm front end up.
Stratus through the day would help to temper some of the heating and
conditional instability, and thus limit the threat somewhat (less
instability to work with). Regardless, Friday is shaping up to be a
high shear/low CAPE possible severe weather event. MUCAPE varies
between model runs, but generally is at or below 1200 J/kg. However,
0-6 bulk shear values of 35+ knots (with some higher res models
showing 50+ knots) and low level (0-1 km) shear 35-40+ knots is
enough for supercells (or mini supercells) and a tornado threat. LCL
heights are hovering around 1500 ft, which is also supportive of a
tornado threat.  Mid level lapse rates also increase, especially if
we are south of the warm front and/or clear out, with values around
6.5-7.5 deg C/km supportive of the threat for large hail of half
dollar size - which is the main and most likely threat. Shear values
are more perpendicular to the front than parallel, so anticipate
that convection will be more discrete than linear. Damaging winds to
60-65 mph can`t be ruled out, although would be a greater threat if
more of the conditional instability is realized. So, greatest
concern tomorrow for severe weather falls between 3-9 PM, with the
risks of large hail, tornadoes, and damaging winds. Rainfall totals
again could be around an inch or more for some in the convection,
but most could see a tenth to one half of an inch in this time.

Otherwise, clouds and breezy conditions prevail with highs in the
upper 50s to upper 60s (again, depending on how far north the front
goes and if we can clear out) Low pressure slides into southwestern
MN by the early overnight, with lingering wrap around showers and
isolated storms. Lows fall into the upper 40s and 50s.

SATURDAY: Low pressure moves off to the northeast Saturday, but will
likely see some lingering wrap around precipitation through the day,
especially early and along the Hwy 14 corridor. Breezy with highs in
the mid 50s west of the James to the upper 60s southeast of IA/MN
Hwy 60, where the cold front might not have fully exited until later
in the day and skies might be more clear due to the position of the
low. Lows at night fall into 40s to near 50.

SUNDAY: Pattern Sunday is oddly similar to what we`re expecting
Friday and Friday night, with low pressure moving northeast out of
the Rockies into KS/NE. This low becomes stacked as it moves into
NE, with attendant warm front lifting north. More questions in the
track of this system as well as the timing, but could be another
high shear/low CAPE sort of day. This low is poised to move into the
MO River Valley a bit more quickly, during the late morning/early
afternoon Sunday. Similar questions regarding if/when we can clear
out, which would increase our conditional instability. Those with
outdoor plans should especially keep an eye on the forecast with the
chances for showers and storms. Will be keeping an eye on trends
here, with a potentially shifting severe weather risk.

When all is said and done through the weekend, beneficial rainfall
of 1-2" is possible to have fallen, as both the ECMWF and GFS
ensembles showing the entire area with an 80% chance or more of at
least an inch of rain. Mostly low (~30%) to moderate (40%) chances
of greater than 2" across the area, although there are higher
pockets in both the ensembles and the deterministic models.

EARLY/MID NEXT WEEK: Behind the low Sunday night, mid and upper flow
becomes more zonal with occasional shortwaves passing through.
Largely expect near to above normal temperatures with highs in the
upper 60s and 70s and lows in the 40s. Could see some periodic
showers or isolated storms with passing waves, but confidence is too
low to stray from the NBM at this time. Periodically breezy
conditions expected as well.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1108 PM CDT Thu Apr 25 2024

VFR ceilings have begun to deteriorate as stratus moves in, and will
continue to deteriorate to MVFR with scattered IFR for the remainder
of the period. Scattered showers and thunderstorms have begun across
the region. This first round is expected to remain sub-severe,
however dime to nickle size hail and stronger wind gusts up to 50
mph under thunderstorms are possible.

Winds are out of the southeast at 15-20 kts sustained with gusts 30-
35 kts. The highest gusts will shift from west to east during the
overnight period and will be predominate over northwestern Iowa by
Friday morning. Winds will gradually decrease through the afternoon
to less than 20 kts. There exists some concern for LLWS at KSUX near
the end of the period. Directional and speed shear may be present
between 1500 and 2000 feet AGL at 160 degrees and 45 kts.

A second, stronger round of showers and thunderstorms will develop
over northern Nebraska in the next few hours. Some of these storms
could be strong to severe with hail up to an inch and damaging wind
gusts. Several additional waves of showers and thunderstorms are
expected through Sunday as a low pressure system moves through the
area. Some of these storms could again be strong to severe.

&&

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

&&

$$

DISCUSSION...SG
AVIATION...AJP