Bonneville Fire Meteorology

Yesterday (Sunday) morning in Innsbruck I woke to find a number of text and e-mail alerts concerning a wildfire near the University of Utah Campus.  As of 1000 CEDT here in Innsbruck and 0200 MDT in Salt Lake City, Utah Fire Info is reporting that the fire is human caused, has burned 495 acres, and is 5% contained.  Fire perimeter mapping shows the fire so far has burned between Dry Fork to the north and Red Butte Canyon to the south, including the Mount Van Cott summit and environs.  

Source: https://firms.modaps.eosdis.nasa.gov/usfs/map/

The fire was "discovered" on Saturday afternoon.  Two aspects of the meteorology that afternoon and evening likely affected fire spread.  As shown in the meteograms below from the Mountain Meteorology lab at the mouth of Red Butte Canyon, the relative humidity that afternoon was only 15%. During that period, the winds were relatively weak and variable, with directions mainly from the west to north, but at about 1930 MDT, the winds increased and became more consistently north to northeasterly.  The peak gust of 25 mph was at 2044 MDT.  Although temperatures lowered and relative humidity increased, the winds likely led to rapid fire spread.  

Source: https://mesowest.utah.edu/

The increase in winds was caused by outflow from convective storms in northern Utah.  Evaporative cooling from precipitation produced by these storms created cold pools that spread southward.  The leading edge of these cold pools is known as a "gust front" and is often is associated with a wind shift and rapid increase in wind speed.  Gust fronts are often dry, but can be detected in radar as a line of enhanced radar reflectivity, caused by greater concentrations of bugs and other non-meteorological targets, as was the case on Saturday.  

Radar image from 0040 UTC 21 July/1840 MDT Saturday 20 July. Radar image source: ral.ucar.edu.

A few hours after the strong winds produced by the gust front, the winds settled into a cycle that is typical of what happens near the University of Utah when there are relatively quiescent large-scale conditions.  At night, the flow becomes steady out of the northeast.  These are downslope and down valley (in the case of Red Butte Canyon) drainage winds.  The mountain meteorology lab probably observes some of the stronger winds in this area because it is at the mouth of Red Butte Canyon where there is a valley exit jet.  During the day, the flow becomes lighter and more variable, although the tendency is for the winds to initially become southwesterly to westerly in the late morning, consistent with upslope flow, and then shift to northerly with time as the larger-scale up valley flow in the Salt Lake Valley and the lake-breeze strengthen during the day.  These transitions are very apparent in the meteogram for the mountain meteorology lab.  

Meteogram source: mesowest.utah.edu

As I write this, it is nighttime in Salt Lake City, and one can see the predominant NE flow in the foothills around the University of Utah in the surface-map from roughly 0200 MDT Monday 22 June. The strongest winds are at the Mountain Meteorology Lab at the mouth of Red Butte Canyon and also near the mouth of Emigration Canyon.   

Source https://www.weather.gov/wrh/hazards

Forecasts for today (Monday) suggest such a wind cycle will predominate near the fire through Tuesday morning.  The NE flow will persist overnight tonight and into tomorrow morning, and then shift to southwesterly to westerly with surface heating as the upslope flow develops.  The HRRR forecast valid 1600 UTC (1000 MDT) Monday shows the development of W-SW winds near the fire area by mid morning.  Note though that there will be some fluctuation in wind direction during this light-wind period.

Then, in the afternoon, the winds veer to NW-N as indicated by the HRRR forecast valid 2100 UTC (1500 MST) Monday.  

Finally, Monday night, the NE drainage flow predominates, as illustrated by the HRRR forecast valid 0800 UTC (0200 MDT) Tuesday.  Note that the HRRR though cannot produce the local valley winds and exit jets (the background terrain in this plot is the actual terrain...the HRRR does not resolve the canyons in the Wasatch Range).  Adding such details is an important consideration for the incident meteorologists who may be working on the fire.  

From a wind perspective, this is a best-case scenario in the sense that these local flows are what you get when the large-scale flow is quiescent.  Whether or not we might see some flies in the ointment after today (Monday) is TBD.  There are a couple of weak troughs advertised by the models mid week that might provide some minor wrinkles and possibly a stronger trough over the weekend that could more strongly affect the fire-weather conditions.  Continue to monitor official forecasts and notices and hopefully fire crews can make progress on containment in the next couple of days.