textproduct: Salt Lake City

KEY MESSAGES

- A prolonged inversion period will set up this weekend across the northern valleys and persist through at least Thursday. This will result in an increase in haze, nocturnal fog, and a degradation in air quality.

DISCUSSION

As expected, high pressure continues to build into the region this afternoon. Looking at the change in 700mb temperatures between the 00Z and 12Z KSLC sounding, significant mid-level warm air advection has strengthened the valley inversion markedly. Aircraft soundings through early afternoon indicate the inversion has remained quite similar to the 12Z sounding.

While stratus kept temperatures elevated overnight across the northern valleys, any lingering stratus has largely diminished. This will increase the threat of valley fog after sunset. Looking at the HREF and NBM visibility forecasts, there is around a 40% chance of dense fog in the Cache Valley and Bear River Valley between 09Z-16Z. With very cold temperatures, patchy freezing fog is also a threat in the Cache and Bear River Valley, with the potential for slick roads, overpasses, and other elevated surfaces. For now, given the probability of dense fog development is less than 50%, did not issue a dense fog or freezing fog advisory with this package...but this will need to be monitored moving forward over the evening shift.

Other locations that have a 20% chance of dense fog development include areas near the Great Salt Lake in Weber and Davis County, and Heber City near the reservoir.

As the inversions strengthen and particulate concentrations increase, the threat of dense fog and/or visibility restrictions in haze will increase Sunday through most of the next week. Currently, the Utah Department of Air Quality is forecasting Moderate/Yellow air quality conditions to develop Sunday across most monitored northern Utah valleys. Those in sensitive groups should continue to monitor air quality forecasts and consider precautions once conditions are forecast to shift to orange/unhealthy for sensitive groups.

Looking at the latest guidance, a portion of the ensemble members suggest the potential for a backdoor cold front with mid-level cold air advection sufficient for most northern valleys to ventilate Thursday into Friday. The mean from the LREF (including EPS, GEFS and Canadian ensemble members) for 700mb temperatures less than -2C is Thursday night. This is our next hope for improvement in northern valley inversion. Something to monitor moving forward but given broad and significant variations across the ensemble suite, confidence is low. Either way, this will be a prolonged period of inversion conditions.

REST OF UTAH AND SOUTHWEST WYOMING

Light, terrain driven winds prevail for all regional terminals today. Clear skies across the airspace continue for the remainder of the day, with mid-to-high level clouds increasing across the northern half of the airspace overnight into Sunday morning. There is a chance for fog or low stratus to develop across the Cache Valley between 07z-16z, with a ~30% chance to impact KLGU with IFR/LIFR CIGS/VIS through this period.

SLC WATCHES/WARNINGS/ADVISORIES

UT...None. WY...None.