374
FXUS63 KFSD 190933
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
433 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
  low chance (20-30%) 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 1-1.5", 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
@ 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 /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1039 PM CDT Sat May 18 2024

VFR through the period. Showers and isolated thunderstorms will
be possible Sunday morning into the afternoon with some chance
for stronger thunderstorms later Sunday afternoon into Sunday
night. Confidence is fairly low on this activity.

&&

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

&&

$$

DISCUSSION...BP
AVIATION...08