Sunrise 6:02 AM   Sunset 8:50 PM Moonrise 5:14 PM   Moonset 3:24 AM

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FXUS63 KFSD 201130 AFDFSD

Area Forecast Discussion National Weather Service Sioux Falls SD 630 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 HRRR 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, HRRR 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.

HRRR 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 /12Z TAFS THROUGH 12Z TUESDAY/
Issued at 626 AM CDT Mon May 20 2024

Early this morning, patchy fog has developed near the I-29 corridor from Sioux Falls to Brookings and two areas of IFR to MVFR stratus have developed, first along the MO River in central SD and second near and east of the I-29 corridor. Low confidence in how long these will linger, but have kept TAF more pessimistic with MVFR stratus lingering near all 3 TAF sites until mid day when mixing starts lifting according to model profiles. Additionally, may see showers near Huron late this morning.

Winds are mostly light and variable early this morning but will swing to the northwest with gusts in the teens and 20s following a frontal passage this morning.

This afternoon and evening, expect mainly VFR conditions though there may be isolated showers or storms at times. Conditions deteriorate to IFR to MVFR after 06z tonight through mid Tuesday morning as widespread showers and storms spread through the region. Scattered severe thunderstorms with heavy downpours, large hail, and damaging and/or gusty erratic winds are possible overnight.

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

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