Ok, it’s been long enough without snow on the West Coast of the USA. California in particular is really getting the shaft from the snow gods this year so far. We understand that it hasn’t snowed, but why? NOAA gives us a glimpse into why the West Coast is staying dry right now and they allow a glimpse of the light at the end of the tunnel.
OFFSHORE HIGH KEEPS THE WEST DRY:
|This background image (above image) is a screenshot of water vapor satellite imagery. The warmer colors indicate dry air while the cooler blues to grey show areas of upper level moisture. Overlaid on the image are locations of High pressure in blue, Low pressure in red, and the heights of the 500 millibar pressure surface in yellow. The heavy black line shows the location of the ridge axis associated with a blocking high pressure commonly called an Omega High.
The large-scale pattern of a ridge off the west coast and broad troughing over the eastern United States is not particularly unusual. However, this pattern usually sets up over January to February rather than mid-December as it did this season. Under this flow regime, northerly low pressure systems are shunted well into Alaska and northern Canada. Southerly low pressure stalls on the westerly periphery of the ridge, unable to progress through the center of the ridge of high pressure; these systems typically weaken and fizzle out. The surface high pressure effectively blocks west to east flow. The result for the Sierra is a prolonged period of dry conditions with only weak and mostly dry systems dropping out of the north. Low pressures then intensify east of the Rocky Mountains drawing moisture from the Gulf of Mexico and jet stream energy from cold polar airmasses out of the north. The eastern United States often experiences wetter and cooler than average conditions under this pattern.
While persistent, this pattern will eventually break down. This can occur under several scenarios. First, high pressure could shift northward and southerly lows could move south of the surface high pressure bringing moisture to southern California and the Southern to Central Sierra. Last season, this happened and brought a few weaker storms to the Southern and Central Sierra. A more permanent mode of breaking the ridge down would occur with the amplification of upper level flow in the central Pacific which requires a strong jet stream to pull colder Polar air southward.
We usually look for an initial wave to weaken the northern portion of the ridge, then stronger low pressure drops southeastward out of the Gulf of Alaska as the blocking ridge retreats into the tropics or begins to move eastward. Long range models do show the potential for a low pressure to weaken the northern portion the ridge by next week. However, these same models are notorious about breaking these persistent blocking patterns too soon. A more likely scenario will be that the northern portion of the ridge flattens a little allowing some moisture into the Pacific Northwest but not into the Sierra.