The GFS has done an excellent job so far in forecasting the coupling of a short wave now over the Northern Rockies that will ultimately diverge to the Rt and pick up the old cut-off low, now located at about 139W and just north of the Tropic of Cancer. All models have this scenario now over the next 48 hours.

Current Weather as of 5-3-2013: From the 12Z Friday WRF Model

The Subject short wave is located From Coeur D’Alene, SW to the CA/OR border. As the upper ridge off the NW coast builds north due to upstream amplification, it becomes positive tilt allowing the short wave to diverge to the Rt, forcing it to spin up a closed center over Northern Ca Saturday afternoon. Saturday PM will be the first opportunity for showers over the Sierra from the northern system only. At the same time, all this SW digging has picked up the Old Cut off as it begins to move toward the Southern Ca coast Saturday. This old cut off is the system that carries the moisture. By Saturday/night,  the cut off opens up and spins up a 70….then 90knot upper jet that transports moisture into Southern Ca. During the day Sunday…..Moisture and UVV increase for Southern Ca and so does the rain and possible thunder, especially Sunday afternoon and night.

Mammoth WX:

By Sunday evening, the cooler system to the north is well off the San Fran Coast and the short wave that was once the old cut off is moving on shore early Sunday evening. Rain should be in progress by Sunday afternoon and this is great news for the fire fighters that are battling fires in the mountains NE of LA.  As the upper low at 500mb continues to spin up off San Fran coast, subtropical moisture gets pulled up through the Owens Valley, Southern Sierra, reaching the Mammoth Area then late Sunday night into Monday morning. The moisture then continues to flow up into the Northern Sierra and remains until the offshore cut off low makes its move inland….Monday night into Tuesday.  

 

QPF: The GFS put more precipitation in the Sierra then LA…the WRF has more in the LA area.  WRF has .75 inches over next 72 hours with the Sierra .50 

The GFS has a lot more in the Sierra. At this time, the Dweebs are looking at light snowfall for the Mammoth area with any where from 1 to 6 inches over the higher elevations above 9000 feet.  The snow level will be as low as 8000 during the Monday/Tuesday time frame.  As the upper low ejects east between Tuesday and Wednesday…..Shower chances diminish mid week, lending to a fair warm following weekend with above normal temps….

 

This is the Dweebs last update until Mothers Day…….Adios!!!

Dr Howard and the Dweebs…………………………..:-)