Active weather returns Friday and continues into next week with extended periods of moderate to heavy rain, mountain snow, and breezy southerly winds. A colder storm could bring heavy snow and travel impacts to the mountains next week.
Discussion
Today will be the last day for a while with dry weather, mostly clear skies, light winds and cold morning temperatures as the upper level ridging that brought this pattern shifts eastward. Some patchy morning fog has started to develop in the southern Sacramento Valley and this is expected to expand into portions of the northern San Joaquin Valley. Areas of dense fog area possible, which could cause travel delays for the morning commute. The fog is forecast to lift/disperse by late morning. Low temperatures are again going to fairly cold, dropping into the low to mid 30s for many Valley locations, the mid to upper 30s for the foothills and the 30s to 20s for the mountains. Highs will be very similar to those on Wednesday, near normal with upper 50s in the Valley and mid 40s to 50s in the foothills and mountains.
A major pattern change is expected for Friday into next week, with a persistent stream of subtropical moisture and a series of atmospherica waves bringing an extended period of precipitation across the region. Precipitation begins late tonight for the Coastal Range, then spreads east into the northern Sacramento Valley in the early morning and then much of the rest of the area by around daybreak.
The plume of moisture has recently trended farther south, with the The Extreme Forecast Index (EFI) signature which had favored the northern mountains now more centered over the I-80 corridor. As a result, forecast rainfall amounts for the weekend have shifted a little farther south, trending a little higher for areas along and just south of I-80, such as the Sacramento metro area, and a little lower for more northerly locations, like Shasta County.
This initial atmospheric river system is a fairly warm system. Snow levels will begin at around 5000 feet Friday morning for the Sierra, then rise to around 6000-7000+ feet Friday night, allowing for snow accumulations, mainly at the higher elevations of the mountains on Friday.
Areas at and above Donner Summit (elevation 7239 feet) could see significant amounts of snow through the weekend, with higher peaks seeing the potential for 2 feet or more. The National Blend of Models (NBM) shows a 60 to 70 percent probability of snow amounts of 4 inches or greater around Donner Pass from Thursday night through Saturday night, 75 to 95 percent over higher elevations in the Sierra. There is a 80 to 95 percent probability of 12 inches over Lassen Peak and the highest peaks of northwestern Shasta County. While the majority of snowfall accumulations still appear to be above pass level, mountain travel impacts will be possible Friday and over the weekend. Small fluctuation`s in snow levels could bring big changes in impacts at pass levels, so stay tuned to the latest forecasts.
Moderate to heavy rain will bring rises in area streams and rivers, though dry conditions over the past several weeks will at least initially limit impacts. The extended period of rain could cause ponding on roads and low lying areas.
Southerly winds over the weekend will be gusty, up to 20 to 30 mph in the Valley, 40 to 50 mph in the mountains.
Extended Discussion (Monday through Thursday)
Cluster analysis and ensembles guidance indicate the upper level trough continuing to influence our weather over the extended forecast period, allowing for the active weather pattern to remain in place through at least the middle of next week. While some uncertainty remains in exact precipitation amounts, timing and snow levels, confidence is increasing in the active weather persisting through much of next week. Latest NBM advertises around a 50 to 80 percent probability of 2 inches of rain for the foothills and mountains Monday through Wednesday AM, 30 to 60 percent for the Valley.
This second system for Monday through Wednesday will be colder and could bring significant impacts to mountain travel. Snowfall probabilities for 12 inches or more have trended higher and are currently around 80 to 100 percent for elevations above 6000 feet for the same time-frame. For 24 inches in that period the probability is 60 to 90, with even higher amounts possible for high mountain peaks.
Latest forecast guidance has further increased precipitation and snow amounts for the extended, and there is increasing potential for rain and mountain snow impacts Monday through Wednesday. Snow levels are currently trending around 7000 feet on Monday morning for the Sierra, then lowering to around 5500-6500 feet Monday afternoon through Tuesday, and then down to around 5000 feet on Wednesday.
Periods of breezy southerly winds are also in the forecast during the extended forecast period. Plan ahead for continued wet and wintry weather over the extended forecast period and be sure to check back for the latest forecast information as we move through the rest of the week and details become more clear.