textproduct: Salt Lake City

DISCUSSION

A very active and very impactful week shaping up for the Beehive State, with a wide array of weather impacts. Currently, several large wildfires are creating a significant amount of smoke across much of the state, with widespread haze, decreased air quality and reduced visibility, particularly in association with the Iron, Cottonwood and Grapevine Wildfires. Please see air.utah.gov for more information and tips on how to handle air quality issues related to smoke.

Increasing southwest flow will continue to shift a significant moisture surge into the region tonight into Wednesday. Precipitable water values will exceed the 90th percentile by 1-2 AM this morning near St. George, and by 4 PM to the Salt Lake City area. This will combine with a lifting shortwave trough to bring convection to the region.

With the approach of the shortwave trough and edge of the deeper moisture advection reaching southwestern Utah overnight, expect isolated to widely scattered showers to develop in this area after midnight. With better forcing reaching southern Utah around 15-18Z, an area of showers and thunderstorms...likely high-based will shift north and east across Washington, Iron and Beaver Counties through 18Z. With forcing then shifting north, this initial area of convection will weaken and shift east around 20-21Z, though some members of the HREF hold on to convection a bit longer, through 23Z or so. For these areas, the 25th to 75th percentile precipitation ranges from around a trace to 0.15" or so. Peak rates around 0.50" are possible in areas where convection is able to train across the same locations, especially in areas prone to flash flooding like slot canyons and normally dry washes. For this reason, the Weather Prediction Center has outlooked portions of southern Utah in a marginal risk for excessive rainfall and the flash flood potential rating is possible for most locations.

Further north, with afternoon heating, expect higher instability and deeper shear to bring the potential for more organized convection. HREF SBCAPE (25th to 75th percentile) ranges from around 250-750 J/kg by late afternoon. This will combine with deep layer shear around 20-35 kts to bring the threat of organized convection to northern and central Utah.

Looking at the 00Z HREF, the 25km neighborhood probability across northern and central Utah is around 20-30% for wind gusts in excess of 55 mph with stronger thunderstorms. Further coordination will continue with the Storm Prediction Center for a possible upgrade from a marginal to a slight risk for severe thunderstorms Wednesday afternoon and evening. For the Wasatch Front, the highest risk of severe convection will be between 3 and 8 PM.

Another round of convection will be possible across eastern Utah Thursday afternoon and evening as dry southwest flow begins to advect into western Utah. Again, peak rainfall rates around 0.50-0.70" will bring an elevated flash flood threat to places like the San Rafael Swell, Goblin Valley State Park and the Capitol Reef area. The Flash Flood Potential Rating has been increased to Probable for these eastern parks and the Weather Prediction Center has outlooked this area with another marginal risk for excessive rainfall.

As an atypically strong (for June) upper level trough crosses into the Pacific Coast Friday, strong southwest winds will develop across the entire state, with wind gusts in the 40 to 50 mph range. These wind gusts will combine with very low humidities to bring widespread and significant critical fire weather conditions to all areas with critical fuels. A weak cold front may bring a period of lower winds to portions of northern Utah Friday night while the remainder of the state continues to see strong, gusty winds through the overnight hours. These strong winds will redevelop in all areas by early Saturday afternoon ahead of a stronger cold front. In some locations across southern and eastern Utah, critical fire weather conditions could continue for 36 hours straight between Friday and Saturday. This is shaping up to be a very significant event...and a Fire Weather Watch has been issued for Friday and Saturday. Wind advisories are likely to be needed as well.

It is incredibly important that everyone be extremely careful with any potential ignition sources during this period of critical fire weather conditions. Even in areas where campfires are allowed, consider skipping them for this time. If you must have a campfire (and they are allowed by fire restrictions), make sure to thoroughly dose the fire, feel it with your hand, and douse again til there is no heat. Consider avoiding the use of exploding targets and fireworks. Make sure you aren't dragging chains on your RVs and other vehicles. Don't park on high grass.

Much colder temperatures behind the cold front late Saturday will bring the threat of frost and may even freeze conditions to the Cache Valley, Wasatch Back and other normally colder valleys. Those with gardens in these areas should pay attention to the forecast and consider protecting their plants if temperatures continue to trend this cold.

AVIATION...SLC

VFR conditions expected to prevail, though elevated smoke will result in hazy skies. Southerly winds will prevail through the night into Wednesday morning, with potential for modest gusts noted after around 12Z. Winds are then expected to shift more W to NW between ~18-20Z. However, also anticipate development of showers and thunderstorms, largely between 21z and 02z, which will be capable of strong and gusty outflow winds which may result in periods of variability.

REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions are expected to prevail, though wildfires will result in areas of smoke downwind, with local MVFR conditions possible near sunrise and sunset. Typical diurnal winds are expected tonight, then westerly to southwesterly winds become increasingly favored Wednesday. Additionally, moisture lifting into the area from the south will result in scattered morning and 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

Issued at 201 PM Tuesday

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.

WY...None.