textproduct: Salt Lake City

KEY MESSAGES

- It is increasingly likely that deep tropical moisture from either Hurricane Odalys or Hurricane Polo (or both) will be drawn northward ahead of a strong disturbance Monday into early Tuesday morning.

- A slight risk (level 2/4) for excessive rainfall has been issued by the Weather Prediction Center for southern Utah between 12Z Monday and 12Z Tuesday.

- This will bring an increased risk for flash flooding across many southern national parks and recreation areas Monday including Zion National Park, Capitol Reef National Park, Goblin Valley State Park, the San Rafael Swell, the Grand Staircase-Escalante National Monument area as well as Glen Canyon National Recreation Area.

DISCUSSION

As September winds toward a close, the pattern remains quite active. Two major hurricanes, category 4 Hurricane Odalys and category 5 Hurricane Polo, remain across the eastern Pacific. A strong Pacific trough continues to push into the Pacific Northwest, while a strong upper level ridge remains centered near the Gulf Coast. The interplay between this Pacific trough and the two Hurricanes is the biggest forecast challenge for the next 7 days.

Current National Hurricane Center track forecasts bring Hurricane Polo into Baja Sur Monday, whereas Odalys is forecast to initially move to the north before weakening and shifting to the south and east well off the Mexican coast. Integral to how these hurricanes may bring heavy rain to the Great Basin is the exact track and timing of the upper level Pacific trough and the tracks of these hurricanes...and how they interplay to advect moisture into the Southwest.

A portion of the guidance envelope would suggest deep tropical moisture from the further west storm (Odalys) would advect north and east into the Great Basin ahead of the upper level trough, bringing a period of moderate to heavy rain Monday into Tuesday morning across southern and eastern Utah.

Another portion of the guidance envelope would have the trough in a slow enough position to pull some portion of the tropical moisture from the plume associated with Polo and again, bring a period of moderate to heavy rain to southern and eastern Utah Monday into early Tuesday.

Around 25% of ensemble members from the 00Z runs, however, largely rejects any tropical influx of moisture ahead of the Pacific trough and just brings a typical strong cold front and associated period of precipitation through the state.

The contrast between the tropical moisture plume favored ensemble members and the upper level trough only ensemble members is wide for precipitation. For those with a strong tropical moisture plume, the 25th/75th percentile QPF ranges are around 0.50-1.50 inches of rain across a broad area of southern and eastern Utah, with locally higher amounts up to 2 inches. For those without, Utah is looking at more of a general 0.25-0.50 inch event.

These types of events are notoriously difficult to capture for most ensemble members, when the interplay of Hurricane tracks across the eastern Pacific and a deepening Pacific trough will be the main driver of the forecast. Subtle differences in tracks, speed and depth of the trough can cause big differences in expected precipitation, particularly at the higher end of the ranges.

For now, will continue to message the potential for an extended period of moderate to heavy rain across southern and eastern Utah Monday, but will not issue any flood watches yet. This is something to monitor moving forward. Those with potential outdoor plans on Monday in these areas should consider alternate plans outside of slot canyons, normally dry washes, and other flood prone areas. Those near recent burn scars such as Widemouth 2, Cottonwood, Forsyth, among others should continue to monitor forecasts.

AVIATION

KSLC...The SLC terminal will see VFR conditions throughout the day with increasing high clouds. Southerly winds are expected to shift to the northwest between 21Z and 22Z, with a 20 percent chance this shift will not occur at all.

REST OF UTAH AND SOUTHWEST WYOMING...Isolated showers and thunderstorms will continue over the higher terrain of southern Utah and adjacent valleys this afternoon and evening

FIRE WEATHER

Afternoon convection will continue to develop across the higher terrain and adjacent valleys of southern Utah this afternoon and evening, bringing gusty and erratic winds near thunderstorms. As a strong Pacific disturbance approaches the region, gusty southerly winds will develop across western Utah Saturday, with gusts up to 30 mph expected. These winds will combine with humidities near 15% to bring elevated to locally critical fire weather conditions to western Utah Saturday afternoon and evening. These winds will redevelop Sunday afternoon and evening, with gusts to 35 mph...and humidities again near 15%, bringing locally critical fire conditions to a similar area Sunday afternoon and evening.

There is an increasing potential for tropical moisture from either/both Hurricanes Polo and Odalys to spread northeast into the Great Basin ahead of the previously mentioned upper level system. This would bring an extended period of moderate to heavy rain across southern and eastern Utah Monday into early Tuesday morning. While the other 25% of guidance keeps any tropical moisture away from the region, it still brings widespread wetting rains as the upper level system crosses the area.

SLC WATCHES/WARNINGS/ADVISORIES

UT...None. WY...None.