Thursday, November 13, 2014

Another Difficult Forecast

With regards to the forecast through the weekend, we're in a tough spot.  The pattern in the higher latitudes is completely wacked out with a high-amplitude ridge centered currently over the Yukon Terrritory and a deep trough over the eastern United States.  Meanwhile, southerly flow is cutting underneath the high-amplitude ridge and penetrating over the southwest U.S.

Downstream of the col in the height pattern, Utah is in an area of confluence between westerly flow from over the Pacific and flow originating over western Canada, as illustrated above.   In terms of the timing and intensity of precipitation features moving through northern Utah, there's a lot of sensitivity in a pattern like this and I've been watching the models struggle coming to a consensus over the past couple of days.  Look at the difference, for example, in the GFS (top) and NAM (bottom) forecasts for 11 PM MST Saturday night.  The GFS has the upper-level trough axis over northern Utah with a cold front and associated precipitation over central Utah.  In contrast, the NAM has the trough axis over southern Idaho (extending all the way to Portland, OR), a less well defined front, and a plume of moisture remaining over northern Utah.

Since the gold-standard ECMWF is similar to the GFS, I'll lean toward the GFS on this one.  The GFS time height-section shows a decent plume of moisture and an upper-level front moving through today, a bit of a break tonight and early tomorrow, and then another pulse of moisture later tomorrow through Saturday morning.  After that, cold, dry air moves in.

I'm still not sold on the idea that the models have a great handle on all these modulations in precipitation, so periods of mountain snow through Saturday morning is perhaps the most reasonable forecast.  I'm leaning toward a total accumulation by noon Saturday of 8–14 inches in the upper Cottonwoods, but confess that the error bars on this one are fairly wide.

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