435
FXUS63 KFSD 200930
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
430 AM CDT Mon May 20 2024

.KEY MESSAGES...

- Low (<30%) chance of isolated showers and storms today.
  Isolated strong to severe storms remain possible this evening with
  threat increasing overnight.

- High confidence in widespread moderate to heavy rain overnight
  into Tuesday morning. By Tuesday evening, high chance (>90%)
  of at least 1" and moderate (40-70%) chance of at least 2" for
  a swath including much of southwest MN, far southeast SD, and
  far northwest IA. A few sites may receive 3+" of rain after
  multiple rounds of storms.

- Isolated to scattered strong to severe storms remain possible
  overnight with the greatest threat from 3 AM to 9 AM Tuesday
  morning and again 12 PM to 6 PM Tuesday afternoon, mainly near
  and east of a Marshall to Sioux Falls to Yankton line. All
  threats are possible, though large hail up to ping pong ball
  size, isolated flash flooding, and damaging wind gusts up to
  70mph are the main threats.

&&

.DISCUSSION...
Issued at 413 AM CDT Mon May 20 2024

TODAY: As of 3 AM, remaining showers and storms are just exiting
eastward into central MN/IA with skies clearing behind the cirrus
shield. Upstream in central SD, GOES-16 satellite images show
scattered stratus developing with light winds and cooling just
behind the sfc trough. NAM/NAM3km profiles and HREF guidance suggest
stratus will spread east to the James River Valley this morning with
occasional showers and will lift to more of a cumulus field by early
afternoon.

The strong inverted sfc trough slides eastward today, with the most
unstable air oriented near and east of a line from Marshall MN to
Sioux Falls to Yankton SD by 4 PM. Lower confidence in exactly how
far east the sfc trough will get today and how much cloud cover will
linger but most guidance continues to produce isolated showers and
weak storms mid day to mid afternoon so have maintained a low rain
chance this afternoon. Highs are forecast to range from near 70
degrees (south central SD) to the upper 70s to lower 80s further
east near the sfc trough.

TONIGHT/TUESDAY: Most guidance favors isolated to scattered storms
developing near or east of the I-29 corridor this evening 6pm or
later. A "worst case scenario" for the evening, 00z/06z HRRR and GFS
are the most bullish of solutions with evening storm coverage
(scattered vs isolated) of storms southeast of a Marshall to Sioux
Falls to Yankton line. Deep layer shear (0-6km at 35-50 kts) and
1000+ J/kg MUCAPE should allow isolated supercells with hail up to
ping pong ball size and heavy downpours as the main threats. Further
south (likely near and south of the Hwy 75 corridor), damaging wind
gusts up to 70 mph and perhaps even a brief tornado or two become
more likely as the likelihood of surface based storms increases.

Despite this early evening scenario, the more likely solution is
still only isolated severe storms (if any) in the evening with the
main severe threat being with more widespread showers and storms
until after midnight (likely 3am-9am) when coverage of storms
increases with broad synoptic scale lift from coinciding upper jet
streaks, strong warm air advection, and the sfc low ejecting into
the Central Plains. Coverage of storms increases in the early
morning hours with one or more linear bowing segments tracking into
the region. Isolated to scattered severe storms remain more likely
the further south you go (especially south of I-90) where more
unstable air lingers and the main severe threat transitions to wind
and heavy rain/flash flooding with secondary threats of large hail
and a few tornadoes.

The more widespread impact through Tuesday morning still looks to be
areas of heavy rainfall as precipitable water values exceed 1.5" at
times. By 7 AM Tuesday, HREF probability matched mean rainfall
favors a widespread 1-1.5" of rain with a few locations seeing 2-3".

May see a few hours break in storms late Tuesday morning before it
looks likely that another round of strong to severe storms develops
with the anomalously strong, deepening sfc low tracking northeast
through northwest IA Tuesday afternoon. If the system continues to
slow and storms can develop in northwest IA, this looks to be the
more impactful round of severe weather likely thanks to a strong EML
nosing in with mid level lapse rates of 8+ C/km and sfc moisture
pooling right ahead of the cold front. Strong deep layer shear and
2000+ J/kg suggests storms would rapidly merge and grow upscale with
damaging wind gusts and large hail being the main threats. Tornado
threat is less likely with south or southwest sfc winds but cannot
be ruled out.

HREF guidance suggests Tuesday afternoon`s storms would bring a
widespread 1+" of rain east of a Brookings to Yankton line with
pockets of 2-3+" of additional rain during the 6-hour period.
Multiple rounds of heavy rain would easily cause ponding of water in
low lying or poor drainage areas and rises on local rivers, as well
as perhaps pockets of flash flooding. The axis of the heaviest
rainfall has shifted slightly further east than Sunday`s forecast,
now looking more likely to be centered from northeast NE
northeastward into southwest MN and the IA Lakes. Will continue to
monitor these shifts in the high-resolution models as the event
approaches.

Outside of storms, Tuesday will be cloudy, relatively cool, and
windy for areas west of the severe weather risk area (south central
and east central SD). Gradient winds mixing into the low level jet
should bring gusts in the 20s and 30s throughout the day.

WEDNESDAY THROUGH FRIDAY: Drier, cooler air moves in behind the
strong departing system Tuesday night with temperatures returning to
near normal values, but 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. A weak clipper
system brings a low chance of showers Wednesday. Expect a dry period
Wednesday night through Thursday, but another northern stream wave
tracks through the Northern Plains inducing another round of shower
and storm chances for Thursday night and Friday.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1037 PM CDT Sun May 19 2024

Late tonight into Monday morning some MVFR and possibly IFR
ceilings are possible but confidence is fairly low in this
occurring. Showers and thunderstorms will increase in coverage
on Monday night and continue into Tuesday. A few strong to
severe storms will be possible in mainly northeast NE and
northwest IA late Monday afternoon and Monday evening.

&&

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

&&

$$

DISCUSSION...BP
AVIATION...08