A deepening trough will position itself across the western United States to start off the week. Remnants from Hurricane Linda will track eastward along the base of the trough and right entrance quadrant of the weak jet streak allowing for an ideal upslope flow of moisture into southern California.
While a temporary drying of the midlevels is allowing cooler conditions and a drop in dew points over California this afternoon, increased moisture will begin advecting over the region this evening in addition to a lower strato-cumulus cloud deck. Temperatures for most of the week will be below normal, but overnight lows will be mild in response to warm SST’s and increased boundary layer moisture.
Showers will begin moving into the Los Angeles basin and San Bernardino Mountains during the day Monday and gradually sink southward by nightfall. PWATs will begin to increase to 1-2SD above normal and a westerly flow off the warm Pacific will aid in additional orographic lifting for the coastal mountains. Heavy rain will be possible at times toward Monday night across the higher elevations, particularly toward the Big Bear region. Slightly elevated instability may allow for a rumble of thunder at times and rain rates in excess of 0.3″/hr in any convection that forms. Showers will sink southward slowly overnight Monday.
Less forcing and some dry air around H925 along the coastal valleys will preclude heavier rain totals with QPF generally at or below 0.4″ for most locations. Even lesser rain totals can be expected near San Diego and on southward.
Global and mesoscale models have been steadily decreasing total QPF from this event over the last few runs, and therefore, maximum totals will likely stay around 1.0-1.25 inches. Nevertheless, given the moist sounding and upslope flow, most areas will receive some rain for areas west of the mountains in southern California.The most widespread rain is expected in Orange, western Riverside/San Bernardino, and southern Los Angeles counties.
Most of California remains in an “exceptional drought” according to the latest U.S. Drought Monitor. While this rainfall will barely put a dent in the seasonal rain deficits, it is falling at a time that is normally dry across southern California.
Average September Rainfall:
Irvine : 0.2″
Los Angeles : 0.2″
San Diego : 0.2″
The rain also falls on the heels of some last week courtesy of monsoon moisture and afternoon convection that even moved along the coast of Orange County. This is a good sign for the upcoming winter as strong El Ninos often produce an active atmospheric river/pineapple express flow bringing excess rainfall to southern California. Both the CFS and ECMWF weeklies suggest above average precipitation is expected for the late September/early October time frame. For now, after this week’s rain we can expected some increasing upper level heights and a return to a mild and slightly above normal temperature pattern by next weekend over southern California.