Evanston, WY 82930 Weather

Key Messages

- Widespread precipitation will bring an elevated and prolonged threat of flash flooding across southern and central Utah through Thursday. There is a Slight Risk of excessive rainfall (level 2 of 4) across southern Utah on Thursday.

- There is a Marginal Risk (level 1 of 5) for severe thunderstorms is in place across southern and western Utah on Thursday.

- The late week storm will also bring accumulating snowfall to the high terrain across northern and central Utah on Friday, with 1-4" likely above 9000 feet. There is a chance of locally higher amounts for favored locations.

Discussion

The center of a broad closed low is approaching the California coast this evening, with Utah and southwest Wyoming under a southerly flow out ahead of it. This has allowed deep tropical moisture to move into the forecast area, with satellite derived PWs primarily in the 0.6 to 0.7 inch range, which is above the 90th percentile for PW for this time of year. The position of the upper jet on the front side of the system has brought a good amount of instability, particularly for southwest Utah, with enough shear to promote some organized convection. One earlier storm produced golf ball size hail, but flash flooding has been the primary threat, and will continue to be through tomorrow.

Convection is currently in a relative lull, though at least some showers and thunderstorms will continue over southern Utah overnight. Meanwhile, the main low will move onshore tomorrow, splitting as it does so. One piece is expected to eject northward across Utah and southwest Wyoming late Thursday afternoon through early Friday with the other piece moving into northern Mexico. By the early afternoon, convection is expected to become more intense again over southern Utah, with the moisture and instability spreading northward into northern portions of the area with the movement of the trough. Conditions will remain quite favorable for flash flooding over central and southern Utah, and thus the Flood Watch remains in effect through early Friday.

As the trough moves through, it will bring colder air along with it, resulting in highs 5-10F below seasonal normals Friday afternoon. The colder air will also bring snow levels down to around 9,000 feet, bringing accumulating snow to the highest terrain of northern Utah, though most likely not significant amounts. Drier conditions will follow beginning Friday as the north piece of the trough ejects out of the area, but the presence of the southern branch of the low will keep temperatures on the cool side through the weekend. The southern branch of the system is also likely to bring a few showers to at least southeast Utah Saturday into Sunday as it tracks east across the Desert Southwest.

Weak ridging is then expected to move over Utah and southwest Wyoming to start the upcoming work week, bringing a bit of warming to the area. However, with no blocking ridge in place off the coast, the storm track is expected to remain active in some fashion next week, though there remains significant ensemble spread regard to the timing and details of any systems, along with their potential impacts for the forecast area.

Aviation

KSLC...The SLC terminal will see VFR conditions through the overnight hours. Southerly winds will continue overnight and through the day tomorrow. Showers and storms will develop again tomorrow, with a 30 percent chance of impacts to the terminal in the afternoon, gusty winds being the main threat. Showers are then expected to become more widespread beginning early evening, bringing an increased potential for gusty winds.

REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions are expected to prevail across the airspace through the overnight hours. Showers will continue overnight and into tomorrow morning over southern Utah, with the focus overnight shifting to southeast Utah. Showers and storms will increase in coverage tomorrow, spreading northward during the afternoon through tomorrow evening. Any storms that develop will be capable of producing gusty and erratic winds, lightning, and brief MVFR conditions.

Slc Watches

UT...Flood Watch through Thursday evening for UTZ117-120-121-123>131.

WY...None.