Area Forecast Discussion

KEY MESSAGES

- Increasing moisture over southern Utah will bring a round of high based showers and dry thunderstorms Saturday afternoon. By Sunday this moisture will allow for a better chance of showers and thunderstorms capable of wetting rainfall across southern Utah, with isolated high based showers and dry thunderstorms further north.

- Temperatures will warm steadily to start the upcoming week, bringing a return to hot and very dry conditions, with widespread valley HeatRisk values in the moderate range by the middle of the week.

- Gusty northwest winds combined with these hot and very dry conditions will bring a threat for critical fire weather across much of the area during the middle portion of next week.

DISCUSSION

A broad ridge is centered off the Pacific coast this evening, with a Canadian trough moving over the front of the ridge into Idaho and Montana. This is bringing a southerly component to the flow over southern Utah, which is allowing moisture to draw northward. Satellite derived PW values, which were in the 0.2 to 0.4 inch range this morning, are now predominately in the 0.5 to 0.6 inch range over southern Utah, indicating that moisture is starting to move in. By tomorrow afternoon, HREF mean PW values are above 0.7 inch for much of southern Utah, and in excess of 0.8 inch near the Utah/Arizona border.

Given how dry conditions have been over the past several days, lower levels will initially remain dry as the moisture moves into the mid and upper levels. By the afternoon, this moisture will combine with instability from daytime heating to produce scattered high based showers and thunderstorms over southern Utah, with little if any rainfall hitting the ground from the storms, this increases the potential for dry lightning and gusty outflow winds. Confidence remains relatively low regarding the coverage of potential dry lightning, so have made no changes to the Fire Weather Watch at this time. By Saturday evening, anticipating the moisture will start to make its way into the lower levels as well, with some of the hi-res guidance continuing to indicate another round of convection spreading out from Nevada at this time into southwest Utah, bringing a small potential for flash flooding.

The Canadian trough is expected to graze northeast Utah and southwest Wyoming during the day Sunday, though guidance is continuing to track farther east with the track of this system, which would limit impacts for the forecast area. The resultant northwest flow for northern and central Utah with the associated boundary would act to keep the moisture from southern Utah from spreading into northern Utah. By this time, with moisture in the lower levels, the potential for isolated flash flooding becomes more widespread over southern Utah.

As that trough exits, there is high confidence that the Pacific ridge will build eastward into the Great Basin, bringing the return of dry and increasingly hot conditions to Utah and southwest Wyoming. By midweek, anticipating maxes up to 15F above seasonal normals, resulting in orange (moderate) HeatRisk values for most Utah valleys. Winds will be generally light to start the week, but with some guidance showing another trough moving over the top of the ridge for the latter half of next week, the resultant increase in northwest flow could produce gusty enough winds to increase fire danger once again.

AVIATION...KSLC

Southerly flow will gradually establish itself over the next hour or two, though a few instances of variable winds may occur. An earlier shift to northwest is expected around 14-16z this morning with gusts up to 20kts after 16z. Otherwise, VFR conditions persist at the KSLC terminal.

REST OF UTAH AND SOUTHWEST WYOMING

VFR conditions will persist across all terminals with breezy afternoon winds. Dry thunderstorms are possible at KCDC and KSGU after 18z with the highest confidence of these storms impacting the terminals residing within the 20z-02z timeframe.

FIRE WEATHER

High based moisture is starting to spread into southern Utah this evening, and will continue to do so throughout Saturday. Widely scattered high based showers and dry thunderstorms are expected by early Saturday afternoon across the higher terrain of southern Utah, with storms spreading south and east during the afternoon and early evening hours. Dry lightning and strong outflow winds in excess of 40 mph can be expected with these storms. A Fire Weather Watch is in effect for Zones 494, 496 and 498, with lightning coverage expected to be more isolated further west across zones 495 and 497. This moisture will increase and spread into the lower levels on Sunday maintaining the potential for afternoon showers and thunderstorms with a better chance for wetting rains across southern Utah. The leading edge of this moisture will spread into central and perhaps portions of northern Utah Sunday afternoon accompanied by a threat for isolated dry thunderstorms.

High pressure will bring a warming and drying trend heading into next week, with the hottest temperatures of the year thus far across southern Utah and well above normal temperatures across northern and central Utah. Very low daytime RH and poor overnight recovery will be common across much of the state through at least the middle of next week. Winds will initially be generally light to start the week, becoming increasingly breezy for the midweek period.

SLC WATCHES / WARNINGS / ADVISORIES

UT...Fire Weather Watch from Saturday afternoon through Saturday evening for UTZ494-496-498.

WY...None.