Castle Dale, UT 84513 Weather

Key messages

- Tropical moisture will fuel showers and thunderstorms through this evening across much of Utah. Locally heavy rainfall may lead to flash flooding, particularly across southwest Utah.

- After a relative lull in precipitation on Saturday, an atmospheric river will interact with a strong cold front Sunday into Monday, resulting in widespread moderate to heavy precipitation across the region. This will increase the potential for flash flooding on Sunday across southern and eastern Utah.

- Accumulating mountain snow is expected Sunday night through early Monday as colder air spreads into the region. Local snowfall totals above 8000 feet may exceed 12" across areas favored in northwest flow.

- Cool, dry and stable conditions are expected Tuesday into Wednesday, before a more active pattern returns later next week.

Discussion

A longwave trough resides along the Pacific Coast late this morning, anchored around an upper low spinning off the WA/OR coastline near 130W. A shortwave trough is noted ejecting through the lower Colorado River Valley, and is pulling an anomalously moist airmass into the Desert Southwest/southern Great Basin. Meanwhile an Atmospheric River (AR) is noted along the base of the longwave trough near 20N. This initial shortwave passage today, followed by the main upper trough late Saturday night through Sunday, will bring 2 rounds of heavy precipitation across southern and eastern Utah. The first round this afternoon/evening will be more convective in nature, while the second round late Saturday night into Sunday associated with the inland-penetrating AR will be predominantly stratiform. See the Hydrology section below for more details on the heavy rain and flood potential through Sunday.

As the initial shortwave ejects northward, ongoing convection across southwest Utah into northern AZ will lift into central and eventually northern Utah later this afternoon through this evening. In the wake of this feature, lingering moisture will maintain a chance of showers and isolated thunderstorms Saturday, however with the forecast area remaining between today's ejecting shortwave and the approaching longwave trough axis, large scale forcing will remain limited which will keep convective coverage isolated to perhaps scattered in nature.

As the upper low off the Pacific NW coastline begins its eastward progression late Saturday, the aforementioned AR will spread inland around the southern end of the Sierra, and into the southern Great Basin/Desert Southwest Saturday night, reaching southern Utah by early Sunday. The approaching upper low will translate through the Great Basin Sunday through Sunday night, with the combination of the two bringing widespread and potentially heavy precipitation across the forecast area Sunday through Sunday night. Colder air in the wake of the surface-700mb trough axis will spread into the area late Sunday through Sunday night, with snow levels falling from 9000+ ft Sunday afternoon to below 7000 ft by early Monday. This will result in accumulating high elevation snowfall across the forecast area Sunday night through Monday morning. Assuming the post-frontal environment evolves as currently forecast, a favorable orographic setup Sunday night into Monday morning looks likely across the upper Cottonwoods and other northern/central mountains favored in NW flow, where snow totals in excess of 12" are currently forecast.

In the wake of this system, mid level ridging will bring a cool and stable airmass into the area through mid-week, followed by another series of troughs beginning Thursday. Considerable uncertainty exists regarding how this pattern evolves beyond Wednesday, thus confidence is low regarding the probability, timing and coverage of precipitation later next week.

Hydrology

Through Sunday night: additional rainfall, locally heavy across southern Utah, will once again bring elevated potential for localized flash flooding and widespread rises to area waterways in southern and portions of central Utah. The most significant period of precipitation is expected on Sunday, but convection will bring a localized flash flooding potential to southwestern Utah this afternoon and evening.

A tongue of elevated PWAT advecting north out of western AZ has pushed values to near .80" across southwestern Utah this morning. Aiding this is the approach of a progressive short wave trough ejecting from the broad upper level trough off the Pac Coast, which will begin crossing the region from west to east around mid afternoon. Ahead of this, noting scattered convective development across far southwestern Utah where modest MUCAPE of 400-600 J/KG exists, with stronger cells packing 1-2+"/hr instantaneous rainfall rates. For much of Washington County, and western Kane County, localized flash flooding will remain a potential through at least late afternoon where this convection will be most focused.

A relative downtick in heavy rain potential exists for Saturday, but the primary precipitation producer will be moving into the area Sunday. The aforementioned broad upper trough off the Pac Coast is expected to phase with a strong short wave embedded within a strengthening southern tier jet. Anticipated set-up will allow a land falling atmospheric river (AR) to penetrate inland into southern/central Utah producing widespread moderate to locally heavy rainfall across the Arizona strip, Grand Staircase, and southern/central mountain regions, including National Parks and Recreation Areas within these areas. Rainfall amounts of 1-2.5" are likely with this, and given antecedent conditions, will efficiently runoff the landscape promoting widespread rises for all waterways, including streams, creeks, normally dry washes, slot canyons, and rivers across the region.

Aviation

KSLC: VFR conditions are forecast to prevail through much of the TAF period, though brief MVFR visbys are possible this afternoon as showers move into the airspace after 1830-1900z. Chances for thunderstorms remain around 15% through 01z. Additionally, ceilings will gradually lower through the afternoon, with CIGs approaching 6000ft AGL as early as 20z. South winds are expected throughout the day, with occasionally erratic winds near showers. While the bulk of the shower activity will wind down by 03-04z, isolated showers will remain possible overnight.

REST OF UTAH AND SOUTHWEST WYOMING: Widespread showers and isolated thunderstorms will continue to overspread the state from southwest to northeast through the afternoon, with activity winding down after 04z or so. Still, isolated showers will likely last into the early morning hours. VFR conditions will largely prevail, though MVFR VIS/CIGs are likely with heavier rainfall, primarily before 03z. Winds may be briefly gusty and erratic near thunderstorms this afternoon, but will otherwise generally follow a diurnal pattern.

Watches, warnings and advisories

UT: Flood Watch from late Saturday night through Sunday evening for UTZ120-121-123>125-127>131.

WY: None.