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.
Damn you Ridgeasaurus!
ReplyDeleteGood job!
ReplyDeleteKim