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December 12, 2020 – Light showers continuing south of Highway 50 through the early morning. Otherwise dry weather today before another weather system will bring rain and mountain snow tomorrow, causing travel difficulties. Dry early next week before unsettled weather returns mid to late week.

Discussion

Early morning radar is showing the main band of precipitation is moving through the area a bit quicker than expected, with most of the precipitation currently south of Highway 50. However, periods of light to moderate rain and snowfall were observed last evening and overnight and are expected to continue for areas south of Highway 50 through around 8am. A few lingering showers are possible through the morning hours as the upper level trough moves south of the area. Otherwise, the area will have a brief break in precipitation today in as shortwave ridging builds over the area before the next weather system moves.

The next Pacific trough will slowly push onshore overnight tonight. Precipitation will spread north to south throughout the forecast area tomorrow morning along and ahead of a surface front, becoming widespread by late morning. This is expected to be a wetter storm as the Pacific trough will likely be deep enough to tap into some tropical moisture. Liquid precipitation forecasts are around 0.3 to 0.75 inch for the Valley and around 0.75 to 2 inches in the foothills and mountains. Snow levels will generally be around 4500 to 5500 feet, with the heaviest snow above 5500 feet. Snow amounts of 12 to 18 inches are forecast, most of which will fall in the morning through Sunday evening. The heaviest precipitation is expected late morning through the evening, especially over the mountains. Periods of moderate to heavy snowfall could cause limited visibilities and hazardous driving conditions. Mountain travel is highly discouraged on Sunday.

Dry weather returns by Monday morning as the upper level trough moves east of the forecast area and upper level ridging takes over. Models indicate the possibility of a weak shortwave bringing slight precipitation chances to the Coastal Range and Southern Cascades on Tuesday, though confidence is low due to diverging model solutions.

Yesterday’s cold front dropped daytime temperatures back down to normal for mid December. Temperatures will remain near normal through Tuesday, generally in the low to mid 50s across the Valley and 30s to 50s across the mountains and foothills. Mild overnight temperatures are expected through Sunday night due to cloud cover, dropping down to the 30s in the Valley and foothills and teens to 20s in the mountains Monday and Tuesday nights due to radiational cooling.

Extended discussion (Wednesday through Saturday)

Reasonable certainty at the beginning of the EFP (on Wed) that a Pacific/Wrn NOAM ridge will prevail over the region. This ridging has resulted in timing issues with incoming wx systems, but this evening’s runs show much more agreement. The ECMWF now follows the GFS more closely (after being the outlier 24 hrs ago) in a progressive pattern over the mid latitudes of the Ern Pac and bringing a trof through NorCal on Thu.

This is basically in line with 4 out of 5 of the 5H Clusters for the 3-7 Day period. With higher certainty in a wetter period for NorCal on Thu, we increased PoPs/QPFs accordingly, blending to the WPC guidance available at the time. The drier forecasts will will likely trend wetter that follow today. So the forecast was heavily weighted to Cluster 4 (Thu) on Day 5 today, and following Cluster 4 on Day 6 (Thu) yesterday. The GFS Prob of IVT tool indicates there is plume of moisture then, with the GEFS IVT Plume Forecast indicating a weak AR, or Not an AR, along 38N, thus not the wettest system.

However, we would mention that the deterministic runs indicate more digging with the Thu system than what we thought yesterday. This is hinted by Clusters 3 and 4 with the system forecast to dig into the Great Basin/4-Corners area Fri. This forecast of digging will keep a chance of precip in our forecasts into Fri with a break in the precip into the beginning of next weekend. For 12/20, a classic La Nina pattern sets up over the Ern Pac with a strong westerly Pacific jet stream slamming into the Pac NW. This pattern keeps the bulk of precip for NorCal along or N of 40N.