textproduct: Eureka

SYNOPSIS

Moderate to heavy rainfall will taper off to the south into the afternoon. More heavy rainfall returns Saturday night into Sunday. An active storm track will bring additional heavy rainfall and a flooding threat, strong winds, and lowering snow levels much of next week.

KEY MESSAGES

- Pacific storm track remains active with additional heavy rainfall and chances for strong southerly winds this weekend and through much of next week.

- Flooding risk trends higher into this weekend and early next week as more rounds of heavy rainfall occur over saturated grounds.

- Snow levels gradually drop by early next week, and could be as low as 3500 to 4500 ft by Tuesday.

DISCUSSION

The frontal passage moving through today is gradually moving to the south and will exit the area by this afternoon. Generally, 2-5 inches have been reported outside the lower elevations along the coast, where 1-2 inches have been reported. Minor flooding from creeks and streams will ease this afternoon as moderate to heavy stratiform rain transitions to light to locally moderate showers.

This lull will be short lived as another round of rain arrives to the area Saturday afternoon and evening. This round of rain is more focused on the southern half of the area, Mendocino and Lake Counties. CAMs are showing mostly light to locally moderate rain to start with in the afternoon and evening hours with precipitation enhancing overnight into early Sunday. Probabilities of rain rates over 0.25 inches an hour are above 70% for the higher elevations of Mendocino and Lake Counties. Chances for rates above 0.5 inches an hour are also high in the highest terrain of Mendocino and Lake Counties. These will elevate creeks and streams further as soils are already saturated. As a result, a Flood Watch has been issued for Mendocino and Lake Counties. Areas north of Cape Mendocino will also see rain, however impacts are expected to be more limited. Total amounts from Saturday afternoon to Monday afternoon south of Cape Mendocino range from 3-6 inches with locally higher amounts in higher elevations. North of Cape Mendocino, 1-2 additional inches of rain are forecast for the same time period.

Going into early next week, ensemble guidance continues to keep the storm door open, but the position of the systems are uncertain. Some ensembles keep the bullseye to the south of the area towards the Bay and some keep the bullseye in Mendocino and Lake Counties. A potent shortwave trough or perhaps a compact closed low with heavy rainfall and strong winds has been modeled to arrive around Tuesday or into Wednesday. NBM is showing near 50% probabilities for peak gusts over 40 mph for much of Mendocino and Lake Counties. The exact location and timing of this feature yields the ensemble uncertainty. There is then the potential for a strong Pacific cyclone to impact the area late next week. Eventually, main stem river flooding is possible with additional storm systems. See the Hydrology section below for more information.

Snow levels remain high (above 7000 ft) through the weekend. Snow levels will gradually fall going into early to mid next week. Monday night to Tuesday morning and again Tuesday night and Wednesday morning, snow levels may drop for the northern portions of the area to 3500-4500 ft. Depending on precipitation amounts, these may bring us our first impactful snowfall of the season to the high mountain passes.

Outside a few brief lulls in the next week, precipitation is forecast somewhere in northwest California each day through at least Friday of next week. NBM probabilities drop to below 20% for a 1/2 inch of rain or more in 24 hours by Saturday. CPC does maintain our area as being more likely to receive above normal precipitation in their 8-14 day outlook, which goes through January 1st.

AVIATION...(18Z TAFs)

Rapidly evolving conditions immanent for all terminals. Degraded visibilities and rainshowers with IFR/LIFR ceilings are forming midday along the North Coast. The last bit of the surface front is also bringing gusty northerlies to terminals along the coast. After this passage, clearing to scattered MVFR/VFR skies is expected this afternoon with calm winds. Inland terminals can expect continued cloud coverage as a orographic lift cloud coverage with MVFR ceilings with IFR conditions under rainshowers. Westerly winds aloft can create some Low Level Turbulence and a continues mountain wave threat, especially in the wake of the frontal boundary. TOnight into tomorrow conditions are to remain MVFR and above along the coast whereas inland areas could experience valley fog due to increased moisture in the area. DS

MARINE

Winds are quickly become northwesterly behind the frontal passage from early this morning. The wind waves have already been recorded as dropping steadily in the areas where the front has past through, leaving a swell dominated sea state. A west to northwest swell is has been building today, and is forecasted to reach around 10 feet at 11 seconds. This will slowly diminish tomorrow.

Another weather system is expected on Sunday yet there is wide variability in the model solutions. The models are currently struggling to resolve the location of a strong area of low pressure and where it will exactly track. The locations of this storm will dramatically impact the strength of the coastal winds, so looking at multiple models can give a better idea of the potential winds formation. The NAM is now showing weaker winds more like the GFS, yet the ECMWF seems to capture the influence of the coastal terrain the best so a 2:1 blend has been made with the previous forecast. This yielded winds of around 20 kt. This seems reasonable for now, as confidence remains low, yet there is still the potential for a stronger winds with only a slight variation in the forecasted strength and/or track of the low.

Monday and Tuesday the winds look to generally remain fairly light with the next storm system likely to bring gales on Wednesday or Thursday. MKK

HYDROLOGY

Creeks and streams are responding to today's pulse of heavy rain. In Del Norte, the Smith River at Dr. Fine Bridge came just a couple inches short of reaching monitor stage before starting to recede. Minor flood impacts will ease this afternoon as rain moves to the south of the area and creeks and streams peak and recede.

Additional rain starting Saturday night and continuing into early next week has the potential to be more impactful. There is still some uncertainty on the positioning and how long the heaviest rain will last, but the heaviest rain will start around Cape Mendocino and southward. This will again pose a threat for quick rises in streams and creeks along with rock and mudslides. Risk for rock and mudslides will again increase and could impact travel on area highways.

At this time, none of the main stem rivers are forecast to exceed action/monitor stages. Ensembles are showing slightly higher probabilities for main stem rivers to reach flood stage within the next 10 days. The highest chances are mid to late next week, but some potential is there Sunday-Tuesday. Over the next ten days the RFC gives the Russian River at Hopland a 47% chance to reach minor flood stage. The Navarro River at Navarro has a 40% chance to reach minor flood stage and an 18% chance to reach moderate flood stage. The Eel River at Fernbridge has a 31% chance to reach minor flood stage and a 60% chance to reach action/monitor stage. The Mad River at Arcata has a 56% chance to reach action/monitor stage with only a 4% chance to reach minor flood stage. No other rivers have more than a 10% chance to reach flood stage at this time. This can change if the narrow bands of moisture deviate from the forecast, so these probabilities are likely to change.

EKA WATCHES/WARNINGS/ADVISORIES

CA... Flood Watch from Saturday afternoon through Monday afternoon for CAZ109>115.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 9 PM PST this evening for PZZ450- 475.

Small Craft Advisory until 3 AM PST Saturday for PZZ470.