Riverside, UT 84334 Weather

Key messages

- Tropical moisture will move into the region on Friday, leading to scattered showers and thunderstorms capable of producing heavy rainfall that could lead to flash flooding, particularly over southwest Utah.

- After a relative lull in precipitation on Saturday, a strong 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 once again and bring accumulating snow to the mountains.

- Dry and unseasonably cool conditions are then expected through about midweek before a more active pattern returns.

Discussion

Utah and southwest Wyoming is under an increasing southwest flow this evening ahead of a series of troughs along the Pacific coast. This has brought pockets of mid and high level cloudiness over the area and is starting to draw tropical moisture northward. Satellite derived PWs, which were predominately in the 0.2 to 0.3 inch range eight hours ago are now in the 0.3 to 0.5 inch range, with these values only expected to increase through Friday morning, particularly over southern Utah. The approach of the weakening southern trough will also provide some favorable instability, primarily focused over southwest Utah. Thus, anticipating the potential of scattered showers to develop over much of the area tomorrow afternoon, though chance of precip is much less over northern portions of the area. Southwest Utah is expected to be the focused, however, with the threat of heavy rain that could lead to flash flooding increasing for areas like lower Washington county and Zion National Park. Elsewhere, flash flooding is expected to be more isolated in nature.

This initial push of tropical moisture is expected to lift out of the area during the day Saturday, providing a relative lull in precipitation for much of the area, though at least some showers are expected to linger. Then attention turns to the stronger trough still expected to be parked off the Pacific coast. This system is expected to track across Utah and southwest Wyoming during the day Sunday into Monday morning, with the persistent southerly flow tapping into moisture from a potent atmospheric river. This combination of deep moisture and strong instability will result in widespread coverage of valley rain and mountain snow across the area, with the threat of flash flooding heightened, particularly over southern Utah. Flows remain high from other recent rain events, so flows remain high, increasing the threat. This will also bring accumulating snow to all of Utah's mountains. Snow levels will start on the high side, in the 8,000 to 9,000 foot range, but could drop as low as 6,500 feet by Monday morning with a good amount of cold air moving in. Will need to monitor for the potential for some significant snow accumulations in the northern mountains that could impact travel. The one limiting factor to the Sunday/Monday system is that guidance has been consistent in showing it to be quite progressive in nature, with most precipitation ending Monday morning.

As the trough exits, strong high pressure is expected to move in beginning late Monday, though temperatures will stay on the cold side (up to 10F below seasonal normals) through midweek. Long range guidance continues to show a lack of a blocking ridge off the Pacific coast which, as the ridge tracks east for the latter half of the week, will open up the region to additional storm systems through next weekend, which will likely result in another wetter than normal period.

.HYDROLOGY, Issued 1131 AM MDT Thu Oct 29 2026: Setting up for another active late week/weekend as another round of significant rainfall is expected to impact portions of central and all of southern Utah. Two primary periods of heightened rainfall potential exist, with the first materializing across namely southwestern Utah Friday afternoon and evening. Noting increasing PWAT trends streaming north across western AZ attm, with peak values reaching some 2-3 standard deviations above normal by this time tomorrow. Driving this is an amplifying upper level trough off the Pac coast, and an ejecting short wave translating into SoCal northeastward across Utah Friday afternoon. Although available PWAT won't be quite to the extent of points south (AZ) across southwestern Utah tomorrow, values are still expected to push near .80" which, coupled with MUCAPE values peaking between 400-700 J/KG will drive scattered convection capable of heavy rainfall and the potential for flash flooding in prone areas such as slot canyons, normally dry washes, and within/downstream of recent burnscars. Near to record high near surface soil moisture values for this time of year will further aid the efficiency of runoff and exacerbate impacts where and downstream of where the stronger cells form.

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" 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 vsby's are possible tomorrow afternoon as showers move into the airspace. Additionally, ceilings will gradually lower through the afternoon, with mountain obscurations possible as early as 00z.

REST OF UTAH AND SOUTHWEST WYOMING: Showers and thunderstorms are forecast to develop across southern UT at first, with coverage increasing further north throughout the day tomorrow. While ceilings will remain VFR, mountain obscurations are possible across our southern UT sites around 15-18z with chances increasing across northern UT and southwest WY after 00z. Additionally, showers and thunderstorms may yield brief MVFR vsbys, particularly at KSGU and KCDC tomorrow afternoon.

Watches, warnings and advisories

UT: None. WY: None.