Wednesday, March 25, 2020

Complexities of Great Basin Weather in Spring

It was a wet night and morning in Salt Lake City, with a bit of the white stuff dusting the ground earlier this morning in the upper Avenues.

Snowfall during the 24-hour period ending at noon today appears to be about 11 inches at Alta Collins.  Thusfar, it's a pretty good event, with more on the way.

It's really interesting to look at what has been happening and what will happen from yesterday through tomorrow morning.  During this period, a large-scale upper-level trough is digging southward off the Pacific coast, resuting in southwesterly large-scale flow across the Great Basin.

At 2100 UTC (1500 MDT) yesterday, the 700-mb analysis showed evidence of flow splitting around the southern high Sierra, with confluent flow over central Nevada where there was a weak wind shift from SW flow to the south to west-southwest flow to the north.

Precipitation was not organized along that wind shift, but instead was widely scattered as convection developed due to heating of the weak stability airmass.

In the absence of surface heating overnight, however, the precipitation became more continuous along the wind shift, and this morning the resulting precipitation band was centered over the Salt Lake Valley and central Wasatch.

This band weakened as one moved southwestward toward Nevada.  There were some showers near Elko, but otherwise, the action was along the wind shift where there was also a weak temperature gradient.  One might call this a front, but a meteorologist might instead call it a baroclinic trough.

However, with surface heating today, we are seeing a breakup of the precipitation band and a transition to scattered convective showers.  This is a nice example of the diurnal modulation of precipitation processes in the presence of weak stability and daytime surface heating.

Throughout this period, the confluent wind pattern remained over the Great Basin.  In fact, it has moved perhaps 100 km southward.

Overnight, forecasts show that it remains stationary through 0000 UTC (1800 MDT) this afternoon, although it begins to move southward near the Sierra Nevada by 0600 UTC (0000 MDT) tonight.

Note how the NAM forecast above calls for the precipitation to redevelop along/near the wind shift overnight in the absence of surface heating, as happened last night.

Even by 1800 UTC tomorrow, the 700 mb wind shift remains pretty much over the Salt Lake Valley.

Thus, this slow moving feature will continue to dominate our weather through tomorrow, with transitions in the characteristics of the precipitation.  With temperatures slowly decreasing during the period, I suspect ski conditions tomorrow will be quite good, if it doesn't get too deep as it appears that another 8 to 16 inches are likely in upper Little Cottonwood through tomorrow afternoon.  Respect uphill closures at the result, make adjustments for avy conditions, and, as noted in the video below, "this is not the time to get sendy."

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