540
FXUS63 KFSD 191150
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
650 AM CDT Sun May 19 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms spread in this morning
  with gusty south winds.

- Additional storms developing and/or moving through this
  evening and overnight bring potential for isolated strong to
  severe thunderstorms (SPC Level 1 of 5 risk). Large hail is
  the primary threat with locally heavy downpours and damaging
  wind gusts being secondary.

- Confidence is increasing in a widespread heavy rain with
  storms Monday night through Tuesday morning. During this
  24-hour period, high confidence (>70%) in a widespread 1+" and
  a moderate chance (40-60%) of a widespread 2+" of rain with
  some locations possibly exceeding 3".

- Isolated severe thunderstorms are possible Monday night with a SPC
  level 1 of 5 risk near and south of a line from Jackson MN to
  Sioux Falls SD to Lake Andes SD.

- Scattered severe thunderstorms are possible Tuesday afternoon
  with a SPC level 2 of 5 risk across much of northwest Iowa and
  far eastern Nebraska.

&&

.DISCUSSION...
Issued at 431 AM CDT Sun May 19 2024

TODAY: Early this morning, dry surface high pressure builds eastward
and GOES-16 satellite images shows an area of scattered showers and
storms tracking northeast through NE. 00z high resolution model
guidance suggests this subtle mid level wave will take more
northeasterly track than previously expected, resulting in high
chances for scattered showers and storms across northeast NE and
northwest IA this morning. Further northwest, another area of
elevated showers has developed along the nocturnal low level jet
nosing into central SD, ahead of a sfc low in eastern WY. Expect
this activity to translate east into eastern SD with the warm sector
through the morning and early afternoon, though there is lower
confidence in exact timing and how far south rain would extend. The
00z NAMNest seems to have captured ongoing activity the best, so
have slightly favored this guidance in the rain chances. This would
suggest higher confidence in some breaks in the rain for much of the
region this afternoon, especially west of I-29.

Cooler today with a southeast breeze. May see gusts in the 30s mix
down with showers, but severe weather is not expected until at least
mid afternoon due to a largely stable airmass. Persistent cloud
cover will make it tough to heat much today. Expect highs in the
70s, aside from perhaps closer to 80 degrees near and west of the
James River Valley where mixing into warming air aloft and breaks in
the clouds are more likely between waves.

TONIGHT: High-res models suggest potential for late afternoon or
early evening storm development as 1000-2000 J/kg MUCAPE develops
near and west of the Missouri River. Low confidence on whether this
would be in southwest SD or central SD depending on the progression
of the dryline/sfc low and whether any more isolated storms could
develop out ahead near the sfc warm front. With the stronger upper
jet forcing well to our northwest and south, any strong to severe
storms would favor multicellular mode and northeastward storm
motion, aiding training storms. Modest mid level lapse rates of 6-7
C/km and skinny CAPE favor large hail as the main threat with
damaging wind gusts up to 60 mph and locally heavy downpours being
secondary. HREF probability matched mean QPF suggests tonight`s
storms would produce a swath of at least 0.25" of rain east of the
James River Valley and isolated pockets of 0.50-1.00" across
portions of southwest MN. High confidence in storms exiting east of
the area by sunrise Monday morning.

MONDAY: The sfc low stalls somewhere from southwest MN to north
central IA Monday with a stationary front/inverted trough draped
southwest through south central NE toward northwest KS. Can`t
entirely rule out isolated warm air advection showers east of I-29
Monday morning, but guidance continues to favor dry conditions for
most of the day. Breaks in the clouds with warm air capped aloft
brings a warmer day than Sunday with highs in the 70s and a breezy
west (turning northwest or north) wind.

MONDAY NIGHT/TUESDAY: The progressive upper trough digging into the
Rockies sends two mid level waves (one weak, one strong) northeast
along the stationary front, with the best broad ascent aloft to
trigger widespread showers and storms late Monday night through
early Tuesday morning. Isolated severe thunderstorms are possible
near and south of a line from Jackson to Sioux Falls to Lake Andes
with this activity Monday night as the upper jet slides in aloft
with 40+ kts deep layer shear and 1000-2000 J/kg MLCAPE nosing into
northwest IA. All hazards are possible near the Hwy 20 corridor near
the sfc front, but further northwest, the threat will be mainly
large hail and damaging wind gusts.

The greater impact Monday night and Tuesday morning will be areas of
heavy rainfall as precipitable water values exceed 1.0-1.5", at
the 90th to 95th percentile of ensemble climatology for this
time of year. Though pockets of 1-2+ inches will be possible
with strong to severe storms in northwest IA, the more
widespread heavy rainfall is expected near the 700mb low track
through eastern SD into west central MN, where deterministic
models highlight a swath of 2-3 inches of rain Tuesday morning.
This would likely cause ponding of water in low lying or poor
drainage areas and rises on local rivers, as well as perhaps
isolated flash flooding.

Showers and storms continue well into Tuesday with potential for
another round of severe weather that bears watching Tuesday
afternoon and early evening for eastern Nebraska and across Iowa.
Low level moisture pooling just ahead of the sfc low should bring
1000-2500 J/kg MLCAPE. Combined with strong deep layer shear (0-6km
at 50+ kts), this suggests if initial surface based supercells
can develop in northwest IA (vs further east), all severe
hazards are possible.

WEDNESDAY THROUGH FRIDAY: Drier, cooler air moves in behind the
strong departing system Tuesday night, but dry conditions are short
lived. An active zonal flow weather pattern continues for the second
half of the work week. We start off Wednesday morning with
temperatures in the 40s to lower 50s. Another weak shortwave brings
a low chance of showers Wednesday. By Thursday night, another potent
longwave trough slides into the Rockies and drags a cold front
through the Plains with potential for another round of showers and
storms Thursday night through Friday.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 640 AM CDT Sun May 19 2024

Scattered showers and isolated, high based thunderstorms will
spread through the region through this afternoon. Conditions
will be primarily VFR aside from occasional MVFR or IFR near
thunderstorms. Confidence in precise timing of shower/storm
impacts at TAF sites is low. Southeast winds gusting into the
20s and 30s will turn more southerly this afternoon.

Additional strong to severe thunderstorms may develop and/or
spread in tonight after 00z but confidence in the details is
low at this time.

&&

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

&&

$$

DISCUSSION...BP
AVIATION...BP