textproduct: Salt Lake City

DISCUSSION

With a mid-level ridge centered over New Mexico, Utah is currently under increasing southerly to southwesterly flow, with overall hot and dry conditions. Temperatures will hover near 5-10 degrees above normal through Thursday statewide, though this will not last as a strong storm system arrives this weekend. Today, breezy westerly winds within a very dry air mass has resulted in critical fire weather conditions across southern Utah (see Fire Weather section for more details).

Early Wednesday morning, moisture will surge in from the south, with showers and thunderstorms crossing the UT-AZ border after 4AM or so. These showers, limited to the southern third of the state through the morning, will be high-based, at least at first, with the main threat being dry lightning and gusty outflow winds up to 50 mph. As the boundary layer moistens, the threat will transition to a mix of outflow winds and isolated flash flooding if a stronger storm moves over a sensitive area...at least across southern Utah.

By the afternoon, building instability across northern areas will result in scattered thunderstorms after noon-1PM. Mid-level moisture, steep lapse rates, and modest instability <500 J/kg will promote a dry microburst threat (with near textbook inverted-V soundings), with winds in excess of 60 mph possible with collapsing showers. The HREF max wind gusts currently highlight northern Utah and eastern Utah in particularly. There is currently a Marginal Risk (Level 1 out of 5) for severe thunderstorms across northern Utah and southwest Wyoming on Wednesday afternoon and evening.

On Thursday, drier air will attempt to push into the area as a shortwave trough crosses N-NV and S-ID. There is still some question with how quickly the moisture is eroded, which will ultimately determine thunderstorm chances and associated outflow winds and/or flash flooding. The highest precipitation chances will be across eastern Utah, with the lowest chances across southwestern Utah.

By Friday, as a potent longwave trough strengthens and pushes into the PacNW, southwesterly flow will quickly increase across our area. Coupled with an increasingly dry air mass, critical fire weather conditions, potentially significant, will develop, lasting until the main frontal passage late Saturday. Wind gusts of 40-50 mph can be expected, with a reasonable worst case scenario (90th percentile) of widespread gusts up to 55 mph. It's worth noting that a weaker, very shallow boundary will initially push through northern Utah late Friday night, briefly pausing the strong southwesterly winds. However, this front will dissipate before reaching too far, allowing the southwest winds to pick back up. A much stronger cold front will enter the northwest corner of Utah early Saturday evening, quickly pushing across much of the state during the overnight hours. With this frontal passage, some moisture will also briefly move into northern Utah, producing some showers overnight. There is still plenty of uncertainty with how long winds hold on across southeastern Utah, with elevated winds likely continuing into Sunday and potentially even Monday, too.

Aside from the winds, this seasonally-abnormal trough will feature impressive temperatures, too, as high temperatures drop to 20 degrees below normal across northern areas and 10 degrees below normal across southern Utah. Temperatures at 700-mb will likely dip below the climatological 10th percentile at KSLC. Low temperatures Monday morning may be quite chilly across higher elevations, reaching the mid- to low-30s across the Wasatch Back and southwest Wyoming, and upper-30s in the Cache Valley and Sanpete Valley. Cooler and drier conditions will likely last for at least a few days as a broad trough settles over the Intermountain West.

AVIATION

KSLC...VFR conditions expected to prevail, though elevated smoke will result in hazy skies. Southerly winds return by around 03Z Wed, with potential for modest gusts noted after around 12Z. Winds then expected to shift more W to NW between ~18-20Z. However, also anticipate development of afternoon showers and thunderstorms capable of strong and gusty outflow winds which may result in periods of variability.

REST OF UTAH AND SOUTHWEST WYOMING...For Tuesday into Tuesday night, winds will favor a diurnally normal pattern. VFR conditions are expected to prevail, though area wildfires will result in areas of smoke downwind, generally more elevated at areas further from the fires. Westerly to southwesterly winds become increasingly favored Wednesday. Additionally, moisture lifting into the area will result in scattered afternoon convective development. With most of this activity high-based in nature, expect the primary threat to be lightning and strong gusty outflow winds.

FIRE WEATHER

Critical fire weather conditions will continue today across southern Utah given wind and low humidity, with a hot and dry air mass sitting over the state. Red Flag Warnings are in effect for zones 492, 494, 496, and 498. Early Wednesday morning, scattered showers and isolated thunderstorms will move into far southern Utah. While initially dry, storms will become more of a mix of wet and dry, with the highest chances for wetting rains across southwestern Utah. Gusty outflow winds up to 50 mph will be possible. By the afternoon, thunderstorms will develop across northern Utah, with a threat of strong outflow winds up to 65 mph. Overnight RH recoveries will improve quite a bit Wednesday night.

Drier air will push into Utah on Thursday, however models have trended towards a bit more lingering moisture, particularly across eastern Utah where minimum RH will largely be in the 20-30% range. Gusty outflow winds will once again be possible with scattered thunderstorms that develop.

As a larger storm system strengthens across the western US, strong southwesterly winds will develop on Friday and Saturday statewide, with widespread gusts of 40-50 mph expected. A Fire Weather Watch has been issued for nearly all of Utah on these days. The air mass will become increasingly dry ahead of the cold front, with critical minimum RH on Friday and Saturday. A weaker, initial cold front will pass through northern Utah on Friday night, bringing lighter winds and a wind shift, however southwesterly winds will quickly redevelop on Saturday. A stronger cold front will move through Saturday night, bringing northwesterly winds and much cooler temperatures. There is some uncertainty with how long the gusty southwesterly winds last into Sunday across southern Utah.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Fire Weather Watch from Friday afternoon through late Saturday night for UTZ478-482-484-488-489-492>498.

Red Flag Warning until 10 PM MDT this evening for UTZ493-494-496- 498.

WY...None.