Light weekend precipitation, trending wetter next week

January 10, 2019 – Isolated showers today then dry tonight and early Friday. Light precipitation spreading over the north state late Friday and Saturday. More light precipitation Sunday Monday but again not significantly impactful. Heavier precipitation middle of next week with mountain travel issues likely again.

Discussion

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Upper trough axis over the Sierra Cascade range with just a few isolated light showers showing up on radar at this time. Slight upper ridging over the coast brought some partial clearing over night allowing central valley fog to form from about Marysville southward. Some locations have even shown brief dense fog but varying cloud cover should prevent widespread dense fog. Should see some clearing from the south today as upper ridge builds except the threat of isolated showers far north over the top of the ridge.

Upper ridge amplifies quickly tonight so valley fog is likely again Friday morning under partly cloudy mainly stable conditions. The upper ridge shifts quickly east into the Great Basin on Friday as a Pacific storm system approaches the coast. By Friday afternoon, models fairly consistent in spreading precipitation into the coast range with a threat of precipitation over the entire forecast area by Saturday morning. Precipitation amounts are expected to remain fairly light as models have remained consistent in sending bulk of moisture and energy with this system southeast into the southern half of the state and south of the forecast area. Isolated showers will remain possible through Saturday with enough clearing for the return of valley fog again Sunday morning. Next upstream system forecast to start spreading precipitation into the coast range by late Sunday morning. Rain and higher elevation snow spread eastward Sunday afternoon and night but again precipitation amounts are expected to remain on the light side as main moisture and energy with this system is again expected to move inland south of the forecast area.

Extended Discussion (Monday through Thursday)

An active pattern next week as a series of systems possibly impact the region. The initial feature which currently lurks out over the Central Pacific near the International Dateline is primed to approach the California coast by early Monday. It will come in the form of a slow moving, broad upper low accompanied by a “weak” Atmospheric River/moisture plume. The 06Z/00Z GFS and 00Z ECMWF diverge with the upper low track with the former being closest to what was seen this past weekend. Ultimately the GFS solutions would favor heavier precipitation given the strong jet dynamics would move into Northern California. It is difficult to say which solution will verify as ensemble forecasts depict some spread with timing and strength of the trough.

Once the initial upper low becomes absorbed, the offshore longwave trough will likely re-load allowing for the persistence of mid- level southwesterly flow across the state. Additional embedded shortwaves should keep precipitation chances in the forecast for the following days. The 06Z/00Z GFS depict a more defined closed low reaching the San Francisco Bay Area by Wednesday morning. As such, the GFS solution is rather robust with regards to precipitation. Strong moist, upslope flow against the Sierra and Southern Cascades would promote moderate to locally heavy precipitation Tuesday night into Wednesday. However, the ECMWF depicts a much flatter wave with less impacts over the region. Given this is a Day 5-6 forecast, much will likely change during future model cycles.

Finally, the last system in the chain is surprisingly well agreed on in the guidance with the upper low reaching the northwestern California coast by next Thursday, January 17, morning. After dropping more widespread precipitation to the region, an upper ridge will finally settle over the West Coast by next Friday, January 18, allowing for a shift to drier weather.