Thursday, April 26, 2012

Valley Rain, Mountain Snow

Things are starting to get interesting to our west.  The regional radar shows scattered showers, some fairly heavy, over Nevada and Idaho.

A close up from the Boise radar shows the linear nature of some of the stronger precipitation features.

At the moment, however, there is no well defined surface front in this area, although one may develop during the day today over eastern Nevada.  I think we'll see a better organized front with a sharper airmass transition and wind shift develop later today.

Looks like we will pick up a decent amount of valley rain with the frontal passage, and it is badly needed.  There's a chance we may get some showers ahead of the front as well if we can bubble up some pre-frontal convection.  Thunder and lightning?  Perhaps.  

Mountain snow is also in the cards, but snow levels look to be quite high.  The forecast 700-mb (crest-level) temperature is around -1ºC as the front comes in this evening, and falls to about -6ºC late tonight as the frontal precipitation tapers off.

Thus, snow levels will probably start out near 9000 feet just ahead of the front and fall to perhaps 5500 feet later tonight (if the precipitation hangs).  Given how productive the storm is upstream, water totals in the mountains tonight might be in the range of .4–.8".  Snow accumulations will depend very strongly on elevation due to the warmth of the storm.  Given that the snow will probably be high density, perhaps 4–8" will fall above 9000 feet in Little Cottonwood.

There is some potential that we will see lake-effect late tonight and tomorrow morning, which would add to these anticipated totals.  The NAM forecast time-height section shows a layer of moist, potentially unstable flow below about 600 mb at 1200 UTC (0600 MST).

Further, the lake is quite warm given our recent "heat wave."  I don't have a recent lake-temperature estimate, but the mean satellite-derived lake temperature analysis for April 15–21 showed a range of about 11–14ºC.

It is probably warmer than that after a few more days of warm temperatures.  The expected temperatures, humidities, and lake temperatures yield a 50–70% likelihood of lake effect forming somewhere downstream late tonight and early tomorrow morning.  Note that is just the likelihood that we see some lake-effect precipitation features develop during that period.  Whether or not these features are persistent and produce accumulations is anyone's guess.  Most lake-effect events do not produce large accumulations, but occasionally they do.  Unfortunately, we have little skill at predicting lake-effect intensity, duration, coverage, and location at these long lead times.

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