Antimony, UT Weather

Key messages

- Remnant moisture from Tropical Depression Vance off the west coast of Mexico will interact with a shortwave on Friday to produce scattered convection, particularly across southern Utah, that will increase the risk of flash flooding.

- A more substantial storm arrives on Sunday as a broad trough in the PNW swings into the Great Basin and phases with the southern jet stream that will be bringing more tropical moisture from a disturbance near Hawaii.

- Cooler air moves in behind a strong cold front that will bring snow levels as low as 6000 feet with accumulating mountain snow.

Discussion

Zonal flow in place is keeping conditions dry with near normal temperatures across the region. This will continue through Thursday before a wetter and cooler pattern starts to take shape.

By Friday a broad trough off of the PNW coast will start to dig south and eject a shortwave into the southwest. Tropical depression Vance will be making landfall north of Mazatlan, Mexico on Friday morning. Most of this moisture will move into west Texas, but some of the remnant moisture will be drawn up into the southwest and interact with the aforementioned shortwave. Guidance is split about 50/50 on exactly where this shortwave tracks. 50% of the guidance brings this shortwave into southern Utah with the 75th percentile QPF totals up to 1" across the terrain of southern Utah. However, the other 50% of ensemble members keep the shortwave to our east with much lower QPF as noted by the 25th percentile that keeps totals <0.2". If this shortwave does take the westerly track then it will have some decent CAPE (500-1000 J/kg) to work with along with steep lapse rates that will increase the risk for heavy rain rates and flash flooding. The shear will be limited so this will limit the strength of the storms.

There will be a relative lull in activity on Halloween as this shortwave ejects out and the main storm remains to our west. A few spotty showers will be possible, but chances are good for a dry trick-or-treating window at some point on Saturday night.

By Sunday the trough will be moving into the Great Basin, and as it does so it will phase with the southern jet stream that will be pulling in tropical moisture from a low near Hawaii. This will result in widespread precipitation for most of the Great Basin and southwestern CONUS. There are still some timing difference on exactly when that low arrives. Some guidance brings it in slowly with a split in precipitation between a round of precipitation across northern Utah and a more distinct areas across Arizona and southern Utah. Other guidance phases the low in more seamlessly with the incoming trough resulting in a broad swatch of precipitation extending from the southwest up through the Intermountain West. These difference have resulted in a rather large spread in potential QPF amounts with the low end (25th percentile) generally 0.5"-1.0" and the high end (75th percentile) 1.0-2.5" areawide.

A strong cold front will usher in much lower snow levels Sunday night as low as 6000 feet. This will bring accumulating snow to most mountain locations with 3-6 inches the most likely scenario and upwards of 12"+ in areas of favored flow. Northerly flow currently looks like the most likely scenario as the trough splits overhead.

Conditions become drier with a return to near normal temperatures by the middle of next week as high pressure starts to build back in behind the exiting low.

Aviation

KSLC: The SLC terminal will see VFR conditions through the morning with increasing high clouds. Southerly winds are expected to shift to the northwest tomorrow afternoon between 19Z and 21Z.

REST OF UTAH AND SOUTHWEST WYOMING: The airspace will see VFR conditions through the morning with increasing high clouds. Generally light and terrain driven flows are expected overnight, with increasing southerly flow bringing isolated breezy conditions across the airspace Thursday afternoon.

Watches, warnings and advisories

UT: None. WY: None.