textproduct: Salt Lake City

KEY MESSAGES

- Wind gusts in excess of 45mph are expected across southern and central UT valleys as well as Tooele Valley this afternoon and evening ahead of an approaching cold front. Elevated winds across the forecast area are expected as well, though just short of advisory criteria.

- Another round of strong to severe storms is expected to occur ahead of the cold front this afternoon across northern UT. Currently, the Storm Prediction Center has northern UT and SW WY outlined in a Marginal Risk (1/5) for severe weather with wind as the main threat.

- A system looks to impact the area as early as tonight through Wednesday bringing periods of valley rainfall and mountain snowfall to the region. The highest mountain snow accumulations are expected to occur across our southern mountains.

- An unseasonably cold airmass looks to move into the region Thursday resulting in anomalously high probabilities for a hard freeze (>80%) across the majority of the forecast area, excluding SE/SW UT and portions of the Wasatch Front.

- Guidance continues to trend toward a much colder storm across northern UT Wednesday through Friday. While mountain accumulations continue to trend up, the chance for valley snow across much of UT and SW WY also continues to increase.

DISCUSSION

Current satellite imagery is indicative of a low churning off the northern CA coast with ample mid-level water vapor in place across northern UT. This low will eventually open into a longwave trough as it dives south across the Sierra before traversing east across the Southwest. While this occurs, H7 southwesterly flow will increase markedly to around 40-50kts and mix to the surface bringing blustery conditions to much of the forecast area this afternoon. While most locations can expect gusts capping around 40mph, portions of southwest UT, central UT, Castle Country, and the Tooele Valley can expect gusts in excess of 45mph this afternoon.

Additionally, a cold front will gradually progress across the forecast the forecast area this afternoon as it moves in from the northwest. With strengthening H7 flow ahead of the frontal boundary and ample mid-level moisture in place, strong to severe storms are forecast to develop ahead of the frontal boundary across northern UT and SW WY this afternoon. Instability is forecast to remain modest this afternoon (roughly 200-300J/kg MLCAPE), precluding a severe hail threat. Given strong mixing throughout the morning and afternoon, DCAPE is forecast to reach roughly 500-800J/kg suggesting strong to severe downbursts are possible with any storm that establishes itself. Lastly, effective shear values ranging from 30- 40kts suggest that some storms may sustain themselves for a while. If any storm remains long-lived, a more robust severe wind threat may develop, though exact location and certainty of this solution remains quite uncertain at this time.

Across our higher terrain across the forecast area, snowfall is expected as cooler H7 temperatures begin to move in tonight through Tuesday bringing snow levels down to 6000-7000ft areawide. Favorable forcing will gradually move south with time, keeping snowfall totals across our northern UT mountains more modest with most locations showing near zero probabilities of exceeding 6", other than the Upper Cottonwoods where probabilities increase to ~70%. However, chances for 12" of snow or greater across the Upper Cottonwoods is near zero for this event. The bulk of the snowfall is expected to occur across our southern UT mountains where stronger ascent will favor persistent showers across the higher terrain. Chances for 6" of snow or greater will generally be confined to the Tushars and Brian Head, ranging from 60-80%. However, chances for 12" of snow or greater is confined to ~10% at Brian Head and 40-50% across the Tushars.

Most of the forecast will be characterized by temperatures below seasonal normals (roughly 5-10F) following the passage of the cold front later this evening. This will persist through the majority of the forecast through D7 other than Wednesday when transient ridging is expected to build in following the departure of the first system. Overnight lows tonight through Wednesday morning look to drop below freezing locally across northern and central UT valleys. Be sure to protect gardens or other sensitive agriculture each evening, if needed.

Ensemble guidance remains in great agreement regarding an anomalously cold system progressing into the Great Basin Wednesday evening following a brief warmup on Wednesday. A longwave trough is expected to dive south from the Gulf of Alaska into the PNW, deepening as it approaches the Great Basin Wednesday evening into Thursday morning. Ensemble H7 temperatures (25th-75th percentile) fall to roughly -9C to -13C following the passage of a cold front some time Thursday evening into Friday morning. As such, ample snowfall accumulations across our northern UT mountains is expected with this system, and potentially some valley snow as well across northern and central UT. While specifics need fine-tuned as valley snow is quite difficult to forecast at this range, ensemble consistency does raise some eyebrows regarding the potential for accumulating valley snow with this system. For example, NBM probabilities for 1" of snow or greater across much of the I-15 corridor through Friday evening ranges from 40-60%. This will absolutely change as the event draws near, particularly due to global model grid spacing not resolving valleys nearly as well, though this is quite an interesting signal and will need to continue monitoring trends with this upcoming storm.

Lastly, this system will be accompanied by an anomalously cold airmass at the surface following the cold front. A very widespread signal for temperatures less than 32F develops Thursday night into Friday morning with NBM indicating greater than 90% probabilities of this occurring across nearly the entire forecast area, other then the I-15 corridor ranging from Provo to Ogden and portions of southeast UT where probabilities cap around 60%. Chances along the aforementioned I-15 corridor remain similar Friday evening into Saturday morning, though probabilities increase substantially across southeast UT as the cold airmass lingers. If this fully materializes, it would result in a hard-freeze and be much more impactful to gardens and other sensitive agriculture. Trends will continue to be monitored, though this system has trended colder across the most recent forecast cycles preceding this one.

REST OF UTAH AND SOUTHWEST WYOMING

Southwesterly winds will prevail today with gusts to 35-45kts across much of the area. Blowing dust is possible in prone areas in western UT, reducing VIS to MVFR at times. Although a few isolated showers/thunderstorms may develop over higher terrain after 18z, higher coverage showers will arrive along a cold front that is expected to push into northwestern UT after 20z. Thunderstorms may produce isolated severe wind gusts and small hail across northern Utah and southwest Wyoming this afternoon through early evening. A stalled boundary across central Utah overnight will bring periods of showers there, with showers and thunderstorm reinvigorating along and south of the boundary Monday afternoon into early Monday evening.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Wind Advisory until 8 PM MDT this evening for UTZ102-115-116- 118>120-122-126>128.

WY...None.