textproduct: Eureka

SYNOPSIS

Light rain expected tonight and Thursday. Moderate to locally heavy rain forecast to return late Thursday night into Friday for mostly Del Norte, Humboldt and Trinity Counties. Moderate to locally heavy rain is forecast to have greater impacts on Lake and Mendocino Counties this weekend and early next week.

KEY MESSAGES

- A 15% chance for minor flooding late Thursday night through Friday with another round of moderate to locally heavy rain.

- More rain with a 15% chance for minor flooding over the weekend.

- Strong gusty southerly winds from 25 to 40 mph over the ridges and coastal headlands tonight Del Norte and northern Humboldt.

- Stronger wind gusts from 45 to 60 mph expected Thursday night and Friday morning for coastal headlands and mountains of Del Norte and northern Humbodlt.

- Snow levels falling to 3500 to 4500 feet Wednesday with very little or no snow accumulations expected.

- Pacific storm track remains active with additional heavy rainfall and chances for strong southerly winds next week.

DISCUSSION

Warm front will bring light rain tonight and Thu, generally less than 0.10 to 0.25 inches in 6-12 hours for Del Norte, Humboldt and Trinity. Southern Mendocino and Lake may not get any rain at all tonight and Thu.

Main impact heading into Thursday will be strong and gusty southerly winds for Del Norte and Humboldt. Chance for wind gusts from 50-60 mph for the Del Norte coast increases Thu night into Fri morning as a 925mb speed max around 55-65kt develops in advance of the next surface front Thu. Lacking instability will limit stronger wind gusts through lower elevations. High terrain and ridges have a good chance (55 to 70%) to see gusts to 50-60 mph. Chances are high (65 to 80%) for gusts over 55 mph for coastal Del Norte. Snow levels will rapidly increase tonight and Thu in advance of the next IVT plume, above 7000-8000 ft.

Moderate to heavy rainfall will begin spreading into Del Norte County late Thursday as the next IVT plume sags south. Mean HREF and RRFS hourly rainfall rates are 0.3 to 0.5 inch per hour, with locally up to 0.6 inch per hour over the higher and windward facing terrain through a large portion of Del Norte County. 75th percentile hourly rates are similar but with greater coverage through the region.

The front and focus of moderate to heavy rainfall and chances for minor flooding will then progress southward through N CA throughout the day on Friday. South of Del Norte, the heaviest rainfall rates will be mostly along the windward slopes of the higher terrain. All main stem rivers are forecast to remain below Action or Monitor stages, however rivers in narrow basins will likely once again quickly rise. Creeks and streams will also quickly rise and could result in nuisance flooding. Ponding on roadways is also likely throughout the duration of heavy rainfall. Rainfall amounts of 2-4 inches are forecast for Del Norte County, with 1 to 3 south through Northern Mendocino County. Outside of Del Norte, there is up to a 70% chance for 24 hour rainfall over 3 inches for southwest facing slopes of Cape Mendocino and some higher interior ridges of Humboldt.

Rainfall will turn much lighter Friday afternoon with a transition to lighter rain or showers. Rainfall will then begin trending higher Saturday south through Lake and Mendocino counties as the flow turns zonal. The persistent ridge that has been south of the region will then eject east as a strong NE Pacific trough takes shape. Models diverge on development of a new frontal wave and northward surge of IVT ahead of the trough, but heavy rainfall looks to be focused in more of a narrow band, and could become more persistent over one area. It is likely that Lake and Mendocino will be in the bullseye for this focus of moderate to locally heavy rain. The precip max could easily shift northward Sunday as we have seen many times before. IVT values over 500 kg/m/s may persist for multiple days next week and the situation will need to be monitored closely as the grounds begin to saturate more.

AVIATION

VFR conditions prevail at the coast this morning while some of the valleys are seeing low clouds. These may reach into UKI this morning, but confidence is low on this. Around sunrise a weak warm front may start to bring some MVFR conditions to the area. These could easily end up being higher and conditions will be VFR. Southerly winds are expected to increase in Del Norte county KCEC this morning. Stronger southerly winds are expected to gradually shift south through the evening and overnight. Gusts to 40 kt or higher are possible tonight at KCEC. Winds are expected to be lighter at KACV with gusts to 20 or 25 kt possible. A strong low level jet around 925 MB will bring some low level wind shear to KACV and KCEC. MKK

MARINE

The northwest swell has built in to around 9 to 11 feet at 11 seconds. This is expected to slowly diminish through the day. Today winds are expected to become southerly again and increase to 15 to 30 kt midday Thursday through early Friday morning. The gale warning for the northern waters remains on track. Winds may diminish a few hours sooner, but will leave the end time the same for now.

Friday afternoon winds will become northerly again around 5 to 15 kt. This will be shortlived and Saturday afternoon winds will diminish and become southerly once again. Another weather system is expected on Sunday but there is wide variability in the model solutions on that. The models are struggling to resolve a strong area of low pressure and where it will exactly track. The NAM shows near gales with the low moving north of the area. The GFS shows a weaker lower moving into the CA waters. The NAM solution seems reasonable and there is certainly the potential for gales. So for now the wind forecast remains southerly near 20 kt, but this will likely increase as it gets closer. MKK

HYDROLOGY

A break in heavy rain is expected tonight and Thu. Light precip is expected with a warm front tonight and Thu. Rivers and streams will continue to drain and recede tonight through Thu. Next round of heavy rainfall is expected to edge into Del Norte County late Thu night. Once again localized minor urban and small stream flooding will be possible for Del Norte, Humboldt, Trinity and northern Mendocino. A prolonged period of heavy rainfall is forecast with rates of 0.3 to 0.5 inches per hour initially late Thursday into Friday morning for Del Norte County. Some rates over 0.6 inch per hour are possible over the windward side of some higher terrain. Widespread moderate to heavy rainfall will then spread south throughout the day on Friday. Rainfall amounts of 2-4 inches are forecast.

The duration and magnitude of the moisture flux is lower and hourly or 6 hourly precip rates initially seem lower compared to rainstorm last night. There will be over 24-36 hours before heavy rain develops again which should help to reduce the threat. Risk for rock and mudslides will increase and could impact travel on highways 299, 36, 101 and 199, especially on Friday.

All main stem rivers are forecast to remain below Action or Monitor stages through Thursday. At this time, none of the main stem rivers are forecast to exceed action/monitor stages. Rivers in narrow basins may once again quickly rise with heavy rain on Friday. This is not too uncommon for our rivers. Ensemble plumes continue indicate low probabilities (less than a 10% chance) for flood stage exceedance late in the weekend into early next week, particularly for the Eel river at Fernbridge, Navarro River at Navarro and Russian river at Hopland. WPC still has still has our forecast pegged with a 15% chance of flash flood guidance exceedance through the weekend, so this will need to be monitored.

EKA WATCHES/WARNINGS/ADVISORIES

CA... Wind Advisory from 5 PM Thursday to 8 AM PST Friday for CAZ101.

Wind Advisory from 8 AM Thursday to 8 AM PST Friday for CAZ102.

Wind Advisory from 10 PM Thursday to 8 AM PST Friday for CAZ103.

Wind Advisory from 10 PM Thursday to 10 AM PST Friday for CAZ104.

Wind Advisory from 7 PM Thursday to 8 AM PST Friday for CAZ105-106.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 7 PM PST Thursday for PZZ450.

Gale Watch from Thursday evening through Friday morning for PZZ450.

Small Craft Advisory until 6 AM PST Thursday for PZZ470.

Gale Watch from Thursday morning through Friday morning for PZZ470.

Small Craft Advisory from 6 PM this evening to 6 AM PST Thursday for PZZ475.