Dust Front Over Northwest Utah

Mark Struthwolf, WSFO Salt Lake City, UT

July 3, 1997

Introduction -

On June 30 1997, a strong, but dry, cold front moved across the Great Basin. Due to dry antecedent conditions across northwest Utah, strong winds in excess of 50 mph associated with the front were able to stir up and suspend hugh quantities of dust, salt, and sand from over the Great Salt Lake desert and adjacent terrain. Visible satellite imagery captured this dust storm as it partially masked the normally discernible terrain of northwest Utah; i.e. Great Salt Lake (GSL) and salt flats in the west desert, enabling WSFOSLC forecasters to accurately time the passage of the front along the populous Wasatch Front; including Ogden and Salt Lake City.

Data -

An impressive cold front spread across northwest Utah during the late afternoon and evening of Monday, June 30 1997. Visible satellite images captured this event clearly as a well-defined dust cloud spread southeast across the GSL and Bonneville Salt Flats and points farther south and east. Earlier this day locating the exact position of the cold front was difficult as pressures across eastern Nevada were lowering in response to the deepening low pressure system while simultaneously a thermal trough was developing over northwest Utah creating a broad area of low pressure across the tri-state region. Wind shift reports from Elko, NV and metar sites in south central Idaho were more gradual than one would expect with such a vigorous front. Isolated convection also caused erratic winds in southern Idaho and northeast Nevada creating even more uncertainty.

Between 2030-2100Z convection occurred near the tri-state point with at least one lightning strike observed. This storm died rapidly but an apparent gust front (which may have enhanced the winds where it intersected or coincided with the cold front) can be seen moving southward in the form of an arc cloud by 2130Z (Fig. 1) in northeast NV. Although this gust front is ill defined at the northwest tip of Utah at this time, its becomes a little more noticeable by 2300Z (Fig. 2) as an arc-shaped broken line of clouds from near the northern tip of the GSL toward the southwest across the northern 1/5 of the salt flats.

FIGURE 1 - 2130Z Jun 30, 1997. Convection near the tri-state area of ID-UT and NV created an outflow boundary in the form of an arc cloud. FIGURE 2 - 2300Z Jun 30, 1997. Arc cloud better defined in northwest UT near the northern tip of the GSL.

While the portion of the arc-cloud near the northern GSL was likely closely linked with the actual cold front, the portion of the arc cloud tailing back across the salt flats was lagging behind the front since frontal passage at Wendover, UT (located on the UT-NV border due west of Salt Lake City) occurred at 2230Z. Also, imagery supports a farther eastward position of the front as suggested by small cumulus clouds moving southeast since 2200Z (Fig. 3) in extreme east central NV (just southeast of the 40N and 115W intersection), well ahead of any gust front. By 2345Z (Fig. 4) the fuzzyness of the suspended dust, salt, and sand can be seen partially obscuring the northern 1/3 of the GSL and 3/4 of the salt flats. The 'dust front' advances steadily east southeast for the next couple of hours easily viewed by the satellite imagery (Figs. 5, 6, 7, and 8). Notice that the eastern portions of the salt flats appear somewhat more fuzzy, which is likely due to higher winds resulting from a combination of gust front and cold front interaction.

FIGURE 3 - 2200Z Jun 30, 1997. Small cumulus over east central Nevada are moving southeast after 2200Z.	FIGURE 4 - 2345Z Jun 30, 1997. Dust storm developing northwest UT with dust cloud partially obscuring northern tip of GSL.

FIGURE 5 - 0000Z Jul 01, 1997.	FIGURE 6 - 0030Z Jul 01, 1997.

FIGURE 7 - 0100Z Jul 01, 1997.	FIGURE 8 - 0130Z Jul 01, 1997.

Summary

Careful examination of the satellite images revealed the leading edge of a dust cloud associated with the cold front across northwest Utah. With the combination of doppler radar's base velocity and WFO Advance's distance-speed application for satellite imagery, forecasters at WSFOSLC were able to precisely time the passage of the cold front across the populous Wasatch Front. The following NOWCAST was issued that evening...

"High wind warning Great Salt Lake desert until 10 pm tonight...
Wind Advisory Wasatch Front until midnight tonight...
Doppler radar indicated a cold front moving southeast at 35 mph across northwest Utah. The front will reach Ogden about 715-730pm and Salt Lake City 730-745pm. Winds behind the front will be northwest at 20-40 mph sustained with some gusts in excess of 60 mph. Visibilities will be briefly below one-half mile in dust."

The front moved past SLC airport at 739pm with sustained winds of 25-30 kts and gusts to 43 kts. Gusts of 50 mph plus were observed from locations 25 miles west of Salt Lake City westward to Wendover and south southeast across Dugway Proving Ground and Rush Valley. These higher wind gusts probably resulted from a combination of generally flatter unobstructed terrain and the coexistence of the gust front and cold front interacting with each other across this region.