Wednesday, November 12, 2014

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Small storm with high elevation snow on the way

The weather in the Sierra has been quiet, with above average temperatures and no precipitation since the Halloween storm that brought several inches of snow to Tahoe ski resorts.  During the fall and winter, when the weather is quiet in the west, you could usually bet the pattern is more active in the mid-section of the country or the northeast. 

 This is exactly the case, as shown in Figure 1.  The ridge of high pressure that is over currently over CA is impressive to say the least, and extends all the way into AK and northern Canada (Figure 1).  In fact, at the 500 mb level (~18,000 ft.) the temperatures Tuesday morning were colder over MT and WY than the temps over parts of the Arctic Circle!  The blocking ridge of high pressure has allowed a deep trough to dig into the north-central US, which has already brought frigid temperatures and the first significant snowfall for many places.

Figure 1. 500 mb heights (black contours), valid 12Z Tuesday, show the massive ridge being pumped up into AK and Canada. The 500 mb temperatures (dotted colored lines) show a deep trough and arctic air over the central US. Source: RAP/UCAR.

This mega-ridge (aka Ridgeasaurus) isn't going to break down, but rather it will become nearly closed off over AK, which will allow for the southern branch of the split jet stream to make its way into CA, and bring a rather weak system to the Sierra by late Wednesday night.  Figure 2 shows the jet stream level winds, and you could see the nearly closed high in AK, with easterly winds off the coast of British Columbia, and a jet streak with westerly flow moving into central and southern CA.

Figure 2.  Upper level winds (250 mb; shading indicates wind speed), valid 12Z Thursday, showing a jet streak moving into CA.  Source: RAP/UCAR.

The dynamics are not very impressive with this system, and the cold front that will come through is extremely weak.  Looking at the 700 mb (~10,000 ft.) temperatures for early Thursday morning, you could see that the temperature contours (colored lines) over CA have a lot of space in between them (Figure 3).  A much more dynamic front can be seen moving through the mid-west into the northeast, where the temperature contours are very close together, indicating a strong temperature gradient.  

Figure 3. 700 mb temperatures indicate the freezing level at ~10,000 ft. to be just south of Tahoe during the heaviest precipitation. Source: RAP/UCAR.

The 700 mb temperatures in Figure 3 can also give us a good indication of what the snow level will be.  At the onset of the storm, snow levels will likely be high (above 9000 ft.), and without a deep, digging trough, and cold air advection behind the front, the snow levels may only fall briefly to the 7000-7500 ft. level. The blue contour in Figure 3 shows the freezing level at 700 mb to be just south of Tahoe during the heaviest precipitation.

Ok, so on to the precipitation and snow fall amounts.  The high resolution (2-km) WRF simulation shows about 0.5-0.75 inches of liquid over most of the Sierra crest ski resorts, with slightly higher amounts for the southern areas down to Kirkwood (Figure 4).  

Figure 4. Storm total liquid precipitation.  Notice the slightly higher values just to the south of Tahoe. Source: CEFA/DRI.

With the high snow levels, this should translate into 4-6 inches of snow possible above 8000 ft., with slightly higher totals near the crest and around Kirkwood.  I would expect little to no snow accumulation below 7000 ft.  Sorry, but this will not be the storm to get the ski resorts open (but every bit helps).

The area should stay dry through the weekend, but forecast models are indicating that a stronger (and slightly colder) storm could bring more high elevation snow to the region next week. A lot could change by then, and we will have an update with more details over the weekend.

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