textproduct: Salt Lake City

KEY MESSAGES

- Widespread heavy rainfall influenced by a strong upper-level low and deep tropical moisture brings a heightened risk for flash flooding across central and southern Utah starting Monday morning, continuing into Tuesday.

- High pressure quickly moves in mid-week, with dry and mild conditions persisting through the weekend.

DISCUSSION

An upper trough continues to deepen of the California coast this evening, with Utah under a southwesterly flow aloft. Moisture has increased today, having been drawn up from what is now post-tropical cyclone Odalys, with PWATs ranging from around 0.6 inches across northern Utah to close to inch across the southwest. Meanwhile, hurricane Polo continues to slowly approach the Baja coast. This hurricane will contribute additional moisture which will continue spread north tonight through Monday. This moisture will interact with the upstream trough, which will evolve into a closed low across central California Monday before tracking into Arizona Tuesday, to produce widespread and locally heavy precipitation, especially across southern, central and eastern Utah. An area of diffluent flow will be maximized across southern and eastern Utah on Monday.

Models continue to show substantial precipitation amounts. The latest NBM has a 75-95% chance of 0.5 inches or greater across southern through east-central Utah for the Monday through Tuesday timeframe, with a 70-80% chance of 1 inch or greater across much of the southern quarter of Utah. Amounts decrease farther north, but the Wasatch Front is still likely to see at least 0.1 inches. Heavy rain will bring the potential for flooding/flash flooding, and a Flood Watch is in effect for the southern and central Utah mountains and southern through east-central valleys from 12z Monday through 12z Tuesday. There remains a Moderate Risk for Excessive Rainfall across southern Utah Monday, with a Slight Risk extending through central and eastern Utah. On Tuesday, there is a Slight Risk across southern Utah.

There are still some nuances to where the greatest threat for any significant flash flooding will be. On Monday, while the plume of deeper moisture will extend across southeast Utah, cloud cover may limit instability there resulting in potentially more stratiform rain with just some embedded convective elements. Greater instability is expected across southwest Utah, where stronger convection may result in higher rain rates more conducive to flash flooding, but precipitation coverage could be more scattered. Lingering moisture on Tuesday will maintain an elevated flood threat, but with the track of the low across central/southern Arizona, there is some question regarding how much dynamics is available to aid in the development of precipitation. Some models are now a bit less enthusiastic with the coverage of precipitation by Tuesday afternoon.

The storm system will exit the area on Wednesday, with high pressure building in behind it. Cooler temperature on Monday and Tuesday will start to gradually rebound mid to late week. Drier and more stable conditions are expected through the end of the week and potentially through the weekend.

AVIATION

KSLC...Cloud cover and shower chances gradually increase into Monday morning, particularly after around 12Z as remnant tropical moisture overspreads the region ahead of a cold front. Some development of embedded thunderstorms will also be possible Monday afternoon. Southerly winds are favored to continue, with occasional gusts during times of precipitation. Models suggest a switch to a northerly wind direction around 00Z-05Z Tuesday, though uncertainty is noted. Precipitation chances then decrease overnight into Tuesday morning.

REST OF UTAH AND SOUTHWEST WYOMING...Remnant tropical moisture will continue to overspread the area, increasing shower chances Monday morning on into the day. During the daytime, some embedded thunderstorms are also expected to develop. Winds will generally favor a southerly direction, with gusts (and some potential variability) possible during times of precipitation, and especially thunderstorms. A cold front will gradually shift southward, allowing winds to begin to favor a more northerly direction at northern terminals Monday night into early Tuesday. VFR conditions are expected to prevail, with some modest decrease to CIGS/VIS during periods of higher precipitation intensity.

FIRE WEATHER, Issued 114 PM MDT Sun Sep 27 2026

Elevated southerly flow aloft will result in gusty winds developing across the western half of Utah this afternoon and evening, with widespread gusts 25-35 mph. RH values remain above critical values given increasing cloud cover from tropical moisture beginning to spread into the region aloft, therefore limiting the overall fire weather risk. This increasing moisture will also result in scattered showers and thunderstorms developing late this afternoon and evening mainly along and east of the I-15 corridor, with potential to produce gusty and erratic outflow winds to 50 mph. Deeper moisture overspreading the region originating from two tropical systems will interact with increasing instability from an upper level low moving into the Desert Southwest beginning early Monday, resulting in widespread heavy rainfall especially across portions of southern and central Utah through Tuesday morning. Environmental conditions will be favorable for widespread wetting rains across these areas, in addition to widespread flash flooding mainly across southern and eastern Utah. A lull on Tuesday morning will be followed by additional showers and thunderstorms across far southern Utah during the afternoon hours, before drier air quickly cuts off any additional activity late Tuesday. Drier, more seasonable conditions will persist through the end of the work week.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Flood Watch from 6 AM MDT Monday through late Monday night for UTZ117-120-121-123>131.

WY...None.