Storm Updates: Idaho–Utah Marginal, Dakotas Flood Slight — Thursday, September 17, 2026

This is a live Thursday afternoon snapshot of U.S. local-storm activity as of Thursday, September 17, 2026. The Storm Prediction Center 2000 UTC Day 1 Convective Outlook holds a Marginal Risk of isolated severe wind gusts and hail this afternoon and evening over parts of Idaho and Utah and a small part of the central High Plains. No convective watches are valid. Mesoscale Discussion 2304 covers southern and central Idaho with a 20 percent chance of a watch. The Weather Prediction Center Day 1 Excessive Rainfall Outlook remains the organized ticket: Slight Risk over portions of the north-central Plains and Midwest and over the southwestern U.S. There is no named tropical cyclone in the Atlantic, eastern Pacific, or central Pacific.

These are county- and ZIP-scale thunderstorms and flash-flood pulses, not a mainland hurricane landfall. Restoration exposure this afternoon is isolated hail and wind if cells verify over the Snake River Plain and Wasatch Front, plus water from the Wyoming Front Range across South Dakota into southwest Minnesota and leftover Southwest terrain. Conditions change fast. Verify with the National Weather Service for the specific county or ZIP before you roll.

What is live this afternoon

SPC issued the 2000 UTC Day 1 Convective Outlook at 0247 PM CDT, valid 172000Z–181200Z. Forecasters Chalmers and Weinman made no changes from the midday package. Headline: thunderstorms with isolated severe wind gusts and hail will be possible over parts of the Intermountain West this afternoon and evening. A few strong to severe gusts may also occur across portions of the central High Plains. Risk-area population centers listed by SPC include Salt Lake City, West Valley City, Provo, West Jordan, and Orem. Categorical risk is Marginal only. No Slight Risk. No tornado probabilities in the 2000 UTC package. The Ohio Valley Marginal from Wednesday remains off the map.

Current convective watches: none. SPC watch page last updated 2059 UTC Thursday: no watches currently valid. Most recently issued watch remains Severe Thunderstorm Watch 669, already expired. Do not treat 669 as a live product.

MCD 2304, issued 0246 PM CDT and valid 1946Z–2215Z, covers parts of southern and central Idaho. Concerning: severe potential, watch unlikely. Probability of watch issuance: 20 percent. Summary: the risk of a few severe storms capable of producing large hail and severe wind gusts will increase through the remainder of the afternoon. Most probable peak wind gust 55–70 mph. Most probable peak hail size 1.00–1.75 inches. Offices tagged: Pocatello, Missoula, Boise. By late afternoon the live MD list showed no mesoscale discussions currently in effect; treat 2304 as the latest issued discussion, not a standing watch.

WPC Day 1 excessive-rainfall language valid 12Z Thursday through 12Z Friday still has two Slight Risk corridors. One is a high-end Slight across much of South Dakota, expanding toward Sioux Falls and into southwestern Minnesota and northwest Iowa, and west-southwest to the Cheyenne, Wyoming region east of the Laramie Range. The other holds Slight Risk over New Mexico and eastern Arizona into the San Juans and western Colorado, with Marginal coverage into the Salt Lake corridor and southeastern Idaho. Wednesday’s Moderate Risk over southern and central Arizona is aftermath, not a live upgrade.

Hotspot 1 — Southern and central Idaho into northern Utah

Morning water vapor showed an upper low over northern California with a band of strong flow aloft and embedded shortwave troughs across Nevada into Utah and Idaho. Strong heating was expected to aid scattered thunderstorms from southern Idaho into northern Utah by mid-afternoon. Sufficient vertical shear is present for rotating updrafts capable of large hail and gusty or damaging winds. Coverage is limited. SPC still expects one or two severe cells later today, mainly over the eastern Snake River Plain. The 18Z Boise sounding in MCD 2304 sampled steep deep-layer lapse rates and an elongating, mostly straight hodograph with about 30–40 kt of cloud-layer shear. Cells first. Small clusters later if outflow organizes.

Overnight LSRs already put quarter-size hail at Huntington in Emery County and at East Carbon and Sunnyside in Carbon County. That is the same Salt Lake City forecast office that owns this afternoon’s Utah cells.

Restoration read: hail first if discrete cells fire over the Snake River Plain, Boise, Pocatello, or the Wasatch Front. Wind second if storms cluster. Stage roof and auto-glass survey capacity before the first warning, not after. Flash-flood risk on this corridor is Marginal to isolated, not the organized Dakotas ticket.

Hotspot 2 — Dakotas water corridor and the High Plains wind overlay

Remnants of a weak tropical low tracked northeast across the Four Corners this morning. Ample moisture and weak ascent support considerable clouds and scattered thunderstorms over the Colorado mountains and southern Wyoming, spreading east into eastern Colorado and western Nebraska and Kansas. High-based convection poses a risk of locally damaging wind gusts. Daytime heating is the limiter because of lingering cloud cover. That is the SPC Marginal overlay: isolated gusts, not a Slight Risk wind event.

WPC is more organized here than SPC. A tropical-origin shortwave ejecting out of Arizona pivots through the central Rockies and into the northern Plains tonight. Precipitable-water anomalies approach the 95th–99th percentile from South Dakota into southern Minnesota by the end of the period. Discussion language includes 2–4 inches, locally higher, with HREF neighborhood probabilities of greater than 2 inches at 50–80 percent across a large High Plains swath. The centroid of greater-than-3-inch probabilities sits just east and southeast of Rapid City into Badlands National Park. The Slight Risk was expanded east toward Sioux Falls for the last 6–8 hours of the forecast.

Restoration read: water first from the Wyoming Front Range across western and central South Dakota into southwest Minnesota and northwest Iowa tonight into Friday morning. Wind second if High Plains clusters organize. Do not send one crew to both a Cheyenne gust and a Rapid City wash without checking which county has a warning now.

Hotspot 3 — Southwest terrain leftover plus Ohio just-hit

The shortwave that drove Wednesday’s Arizona Moderate flood risk is leaving. WPC still keeps a Slight Risk over New Mexico and eastern Arizona into the San Juans and Sacramentos. Guidance still prints 1–3 inches on the Sangre de Cristos, San Juans, and interior New Mexico terrain. Sacramento Mountains burn-scar sensitivity remains in the discussion as locally Moderate impacts embedded inside a higher-end Slight. That is runoff on ground that already took water Wednesday, not a new Moderate upgrade.

Ohio is off today’s SPC Marginal. It is the just-hit desk. Overnight LSRs from the 16/1200Z–17/1159Z window include multiple trees down in Versailles and Piqua, a snapped hardwood onto a home awning in Darke County, barn sheet-metal loss and a blown silo near West Alexandria in Preble County, and trees and wires southeast of Mansfield in Richland County. NWS Cleveland had severe thunderstorm warnings into mid-evening Wednesday for Richland, Ashland, Holmes, and Knox counties. Those products are expired. The trees are still on the lines.

Just hit

SPC preliminary storm reports for the current 17/1200Z–18/1159Z cycle show no tornado, hail, or wind reports as of this afternoon desk. That is expected. The Intermountain and High Plains threats are still an afternoon and evening problem.

The last 24-hour ticket pile sits on the prior cycle (16/1200Z–17/1159Z): no tornadoes. Hail: 1.00 inch at Huntington, Emery County, Utah; 1.00 inch at East Carbon and Sunnyside, Carbon County, Utah; 1.00 to 1.25 inches near Linn Creek, Camden County, Missouri, with two-foot-diameter limbs down. Wind: downed power lines in Mesa, Maricopa County, Arizona; trees in Versailles, Darke County, and Piqua and Lockington, Miami County, Ohio; a snapped pole and large limb near Broken Arrow, Tulsa County, Oklahoma; a power line and a condo-garage roof near Osage Beach and Linn Creek, Missouri; barn roofing and a grain silo near West Alexandria, Preble County, Ohio; trees and wires southeast of Mansfield.

This morning’s Storm Tracker already covered the same outlook family at 1300 UTC. This afternoon desk updates to the 2000 UTC package, adds MCD 2304, and confirms still-zero watches and still-zero LSRs on today’s cycle.

Pulse table

What this means for restoration teams

Isolated severe hail on the Snake River Plain and Wasatch Front is a roof and auto-glass survey ticket if cells verify over Boise, Pocatello, Salt Lake, or Provo. Isolated High Plains gusts are tree, siding, and gutter work in a thin corridor. The organized ticket today is still water from the Wyoming Front Range across South Dakota into southwest Minnesota and northwest Iowa tonight. Flash flooding on leftover Arizona–New Mexico terrain is wet drywall and a same-night extraction clock if water is inside a structure that already took rain Wednesday. Ohio trees-on-wires from last night are a separate staffing problem from this afternoon’s West.

For teams near the Intermountain Marginal: stage before mid-afternoon local. Property owners should photograph damage in daylight if it is safe, call the carrier, and start extraction the same night if water is inside. Turn around, don’t drown on High Plains and Southwest washes.

What this is not

This is not a Slight Risk severe day, not a live convective watch, and not a U.S. mainland hurricane landfall. MCD 2304 is watch-unlikely, not a watch. NHC reports no tropical cyclones in the Atlantic, eastern Pacific, or central Pacific as of this afternoon. The work, if it verifies, will be scattered: a hail swath through an Idaho or Utah county, a gust cluster on the High Plains, and heavier rain over South Dakota terrain tonight.

Looking ahead

SPC Day 2 (Friday, 1730 UTC package) holds Marginal Risk of isolated severe storms across parts of Idaho and Utah, Nebraska and Iowa, and the Piedmont of the Carolinas into north Georgia. Population centers listed on that outlook include Charlotte, Atlanta, Omaha, Lincoln, and Greensboro. WPC’s Day 1 water corridor over the Dakotas is the bridge into the Plains Marginal. The next Storm Tracker update should re-check the 0100 UTC Day 1 outlook and any new watches rather than assuming this afternoon’s Marginal stays the last word.

Sources and how to verify

Primary sources: NOAA Storm Prediction Center Sep 17, 2026 2000 UTC Day 1 Convective Outlook (Chalmers/Weinman); Mesoscale Discussion 2304 (Weinman/Hart); current convective watches (none valid; last issued Watch 669); SPC preliminary storm reports for the 20260917 cycle and 20260916 cycle; Weather Prediction Center Day 1 Excessive Rainfall Outlook (Kleebauer); National Hurricane Center Tropical Weather Outlook, September 17, 2026; NWS warning text archive including Cleveland OH overnight products now expired.

Check weather.gov for the county warning, spc.noaa.gov for the outlook, watch, MD, and LSRs, and WPC excessive rainfall for the flood overlay. Morning desk for this calendar day: Storm Updates: Intermountain West + High Plains Marginal, Dakotas Flood Slight — Thursday, September 17, 2026 (Morning).

Informational only — not an official weather warning. Follow local authorities and the National Weather Service. Turn around, don’t drown.