textproduct: Eureka

SYNOPSIS

Another strong storm system will bring additional moderate to heavy rain, mountain snow, and strong winds Thursday into Friday. Shower and isolated thunderstorm activity will gradually to diminish through the day on Friday. Drier and colder weather is forecast for the weekend and should last into early next week.

KEY MESSAGES

* Strong southerly winds, periods of moderate to heavy rain, and embedded thunderstorms expected through Thursday.

* Gusty winds, showers and isolated thunderstorms with locally heavy rain expected on Friday. * Flood Watch remains in effect through Friday evening for Humboldt, Trinity, Mendocino and Lake counties.

* Winter Storm Warning for elevations above 4000 ft through Friday evening for Trinity County.

* Hazardous marine and coastal conditions expected through Thursday.

DISCUSSION

A deepening surface low (978-982mb) inside 130W north of Cape Mendo will impact NW California with strong and gusty winds, moderate to heavy rain and high mountain snow (above 4000 feet) on Thu and Fri. There will probably be some isolated to scattered thunderstorms as cooling aloft and low level moist air transport creates greater instability. We have already had multiple lightning strikes detected offshore associated with transient shortwave trough. A coastal barrier jet best represented by the 925mb surface is forecast to develop Wed evening and early Thu morning with top speeds around 65 to 70 kts. The atmosphere is much more unstable and well mixed and the chance of downward momentum transport is greater compared to the last storm. It appears the window of greatest threat for strong and damaging wind gusts down to the coastal plain (mean sea level) will be with the passage of the frontal occlusion by early Thu morning. It is a sure bet, winds will gusts to 70 mph or more over the ridges. The surface low will deepen slightly offshore Del Norte and S-SE winds will remain strong into at least mid or late morning. Del Norte coast stands the best chance of gusts to 60 mph more on Thu. Rate of pressure falls may not support winds that strong for the Humbodlt Bay area and Eel delta. Another secondary spoke of energy or trough line will pin-wheel around the low in the afternoon and head toward the Mendocino coast Thu afternoon. This line will likely be convective and may have a few strong thunderstorms over the waters. The surface low will slowly meander offshore and eastward toward the coast Thu night into Friday morning with additional rounds of gusty winds. Winds look to be borderline for an advisory. Expect more showers, isolated thunderstorms with locally heavy rain into at least the morning hours on Friday.

Cold air moving in from the deep trough will support lower snow levels late tonight into Thursday. Snow levels should drop to 4000-4500 ft tonight and then to 3500-4000 ft Thu night and Fri. 48-hour total snow amounts of 2-3 feet are expected through midday Friday. 6-hourly rates fall off during the afternoon and evening, but travel over Scott Mtn Pass will be very difficult or impossible if roads are now plowed. Even south Fork Mtn Summit on highway 36 (around 4000 ft) may get a dumping of snow with NBM chances for 4" in 48 hours around 70%.

The models remain in general agreement that the upper trough complex responsible for multiple days of rain will finally head eastward into the Great Basin as high pressure rebuilds offshore and toward the West coast.

High pressure is expected to start to move over the area bringing a break in the prolonged wet spell. Northerly may be a bit breezy and gusty over ridges this weekend weekend. The valleys will likely see widespread fog and low clouds each night and morning. Frost and freezing temperatures will also be possible for the North Coast, especially Sunday morning. Cooler and drier weather should last into early next week. This may be too optimistic this time of year. Chances for more than 4 to 5 days in a row of no possibility of precip is not high. In fact CPC 8-14 day outlook already has us back in a slight risk for heavy rain and high mountain snow.

AVIATION

Latest METARs show -RA and VFR/nearly MVFR conditions as a powerful frontal system approaches. High confidence exists for the arrival of a 60-70 kt LLJ at 925mb after 08Z, resulting in severe Low-Level Wind Shear (LLWS) and high-end mechanical turbulence at all terminals. Surface wind gusts are expected to peak between 40-50 kt at KACV and KCEC between 09Z and 12Z. Despite the heavy rain and embedded thunderstorms, ceilings at the coastal terminals are forecast to hold in the MVFR range at OVC035, while visibilities drop to 2-3SM in heavier convective cores.

At KUKI, the primary concern is strong southerly flow channeled through the Russian River Valley, resulting in gusts up to 45 kt through late Thursday morning. TAF ceilings for the inland terminal remain in the MVFR category, holding at OVC025-OVC030. HRRR and NBM guidance suggest that while deep moisture and mechanical lift will persist, cloud decks should remain above IFR thresholds through the 24-hour period. Conditions will trend toward VFR ceilings (OVC050) late in the period as the region transitions to post-frontal showers. /MH

MARINE

Storm Warnings remain in effect for all coastal waters. Observations and high-resolution models indicate the LLJ will produce frequent storm-force gusts to 55 kt through Thursday morning. These winds will build extremely steep, life-threatening seas peaking between 20-24 ft, particularly for the waters from Pt St George to Cape Mendocino.

The environment will remain highly unstable, with frequent thunderstorms and the potential for waterspouts across all zones through Thursday afternoon. While the pressure gradient will slowly relax on Friday, chaotic and hazardous seas will persist before a moderate northerly flow develops over the weekend. /MH

HYDROLOGY

Once again rivers and streams have been receding through this evening after a period mostly moderate rain. This will likely begin to reverse overnight into Thursday as the next surge of moderate to locally heavy rain spreads into the area overnight into Thu morning. Period of highest rain rates over 0.25in/hr occurs after midnight and during the morning on Thu. HREF banded maxima over 0.50in/hr is forecast over SW and SRN Humboldt, much of Mendocino and Lake, and into the higher terrain of NE Trinity where it should be all snow. Minor flooding of smaller rivers, creeks and streams with poor drainage will be possible with grounds saturated and all the water going into run-off. Mad River at Arcata and Eel River at Fernbridge could touch Monitor Stages Thu-Fri. Hopland could also touch or exceed slightly minor flood stage (15% chance) Thu-Fri. Otherwise, all main stem river stages are forecast to remain below Monitor or Action stages through Friday. Another period of organized and widespread rain arrive Thu night into Friday morning. Duration and moisture flux will drop off and the convection will be shallow and short-lived.

EKA WATCHES/WARNINGS/ADVISORIES

CA... Flood Watch through Friday evening for CAZ101>115.

High Wind Warning until 8 PM PST this evening for CAZ101.

Wind Advisory until 8 PM PST this evening for CAZ102.

Wind Advisory until 2 PM PST this afternoon for CAZ103.

High Wind Warning until 2 PM PST this afternoon for CAZ104>106.

Winter Storm Warning until 10 PM PST Friday for CAZ107-108.

High Wind Warning until 10 AM PST Friday for CAZ109.

Wind Advisory until 10 AM PST this morning for CAZ110>115.

NORTHWEST CALIFORNIA COASTAL WATERS... Storm Warning until 3 PM PST this afternoon for PZZ450-455- 475.

Storm Warning until 11 PM PST this evening for PZZ470.