Lonetree, WY 82936 Weather

Key messages

- Dry and seasonably cool conditions continue through Thursday.

- An influx of tropical moisture brings scattered showers and thunderstorms on Friday, with potential to produce locally heavy rainfall leading to an increased flash flood threat mainly across southwest and west-central Utah.

- A stronger system taps into deep moisture from an atmospheric river on Sunday, producing widespread light to moderate, locally heavy rainfall across the region.

- A cooler airmass moves in late Sunday, with snow levels dropping to 7000 feet early Monday morning, allowing for substantial high elevation mountain snow accumulation.

Discussion

Dry, southwesterly flow across the region prevails through Thursday on the leading edge of a longwave trough currently situated off the west coast. This will keep dry and seasonably cool conditions in place through this period.

A shift in the pattern is expected on Friday as moist, southerly flow strengthens across the region as a northern stream low pressure system begins to phase into a tropical low off the Baja Peninsula. Concurrently, a shortwave trough ejecting northeastward on the eastern periphery of the parent trough will traverse our region through the day, introducing more substantial instability especially across southwest and west- central Utah on Friday afternoon. The combination of this tropical moisture influx with increasing instability will allow scattered showers and thunderstorms to develop by late Friday morning through the evening, with the main threat for heavy rainfall and resultant flash flooding focused where instability is maximized across southwest and west-central Utah. There is a considerable forecast bust scenario that still exists in models, as around 33% of members suggest the northern stream trough remaining decoupled from the tropical low, which would keep dry, southwesterly flow in place through this period. This lower end scenario would essentially squash the flash flood risk on Friday, so keep an eye on the forecast as we continue to monitor this potential.

A brief period of subsidence is expected behind the exiting shortwave on Saturday, with shower activity gradually tapering off Sunday morning. A relative lull is expected through the day, but will be brief as our next, likely wetter, system begins to move into the region. The northern stream trough and tropical low are expected to shift inland late Saturday. This will transport more substantial amounts of moisture into the region given the tropical low tapping into a strong atmospheric river, allowing widespread precipitation to begin filling in from the south as early as late Saturday. Models have started to converge on northern Utah tapping into more considerable precipitation, mainly due to a strong cold front passage as the northern stream trough digs into the Great Basin on Sunday. With northwesterly flow developing behind the front and much colder temperatures aloft, potential for lake enhanced precipitation will also come into the mix late Sunday into Monday morning. While widespread heavy precipitation is likely late Saturday through Monday morning, there still is a notable spread in potential accumulation amounts. For most areas, the 25th percentile indicates widespread totals in the 0.2-0.5" range, while the 75th percentile indicates totals between 0.75-1.25". For southwest Utah, the Wasatch Front, and all Utah mountains, higher totals are favored across the percentile ranges, with the 25th percentile indicating accumulations between 0.5-1", and 75th percentile between 1.5-2.5".

Behind the cold front late Sunday, snow levels are expected to drop as low as 6500-7000 feet by early Monday morning. With lingering showers expected to continue through this period, especially downstream of the GSL from lake enhancement, precipitation is expected to transition to snow and bring notable accumulations especially to areas above 8000 feet. Current forecast shows generally 3-6 inches above 8000 feet, with higher totals between 8-14 inches above 9000 feet.

Precipitation looks to taper off through Monday morning, with drier conditions prevailing through the remainder of the day. Low temperatures will be some of the coolest of the season so far early next week.

Aviation

KSLC: Minimal operational concerns are forecast for the KSLC terminal through the valid TAF period as VFR conditions prevail. Light northwesterly winds will remain in place through the day under limited cloud cover. A return to southerly flow is expected around 03Z this evening.

REST OF UTAH AND SOUTHWEST WYOMING: VFR conditions will prevail across Utah and southwest Wyoming through the valid TAF period. Breezy southwest winds with gusts to around 20kts are expected across southwest Utah (north of KSGU) and southwest Wyoming through the daylight hours, with most other terminals expected to see light terrain driven flows.

Watches, warnings and advisories

UT: None. WY: None.