2007-01-31 Wx Discussion and Forecast
The 500 mb cut-off low has moved inland to a position over San Diego and appears to have opened. This has occurred in response to the shortwave trough positioned over Wyoming that is digging southward fast.
Winds aloft of ARB have increased and turned to the north/northeast. At the surface, a stagnant situation continues with fog in the Valley. Temperature in Sacramento is in the upper 30s; Blue Canyon is in the mid 30s, and upper-teens to low-20s are reported at higher elevation.
Snow depth at higher elevations ranges 6" at Blue Creek (5280') to 25.8" at Slate Creek (5700') to 48.7" at Sand Flat (6750). The snow pack at the Blue Canyon radar site is 0".
00-72 Hour forecast
The weather features of interest are the shortwave trough that will kick the cut-off low eastward and another shortwave trough barelling out of Canada into the central US around 72 hours. The GFS and NAM disagree on the westward extent of the trajectory of the second shortwave trough, with the NAM bringing the core of the jet streak into a position farther west. The location of this jet streak will have a bearing on whether the finger ridge moves inland or remains offshore. The GFS, with the more eastern position of the jet streak, brings the finger ridge inland in advance of a Pacific shortwave trough.
The bottom line for the ARB is that model guidance suggests the chances of precipitation are very slim over the next 72 hours. The only chance is for flurries at higher elevation locations, depending on the cloudiness associated with the jet streak in the 72 hour period.
Two members of operational GFS 00 UTC ensemble have precipitation in the ARB at 00 UTC Feb 6; Six members of Candian 00 UTC ensemble. PSD mean precipitation anomaly has precipitation in the ARB. This all hinges on a couple of factors. First, a potent shortwave trough over the central-north Pacific is forecasted to move the position of the large, broad trough eastward, while the jet stream becomes more zonal and intensifies over the central-north Pacifica. The intensification of the jet stream may be related to Tropical convection, as the Tropical ridge moves westward and forms a very nice wave train with a ridge over the Arabian Sea, a trough over the Bay of Bengal, and a ridge over the western Tropical Pacific at 144 hours into the forecast. It's this ridge over the western Tropical Pacific that would permit the Tropical convection to flare up and a northward flux of high IPW air into the subtropical and extratropical jet stream.