textproduct: Eureka

SYNOPSIS

A frontal system will continue to produce moderate to heavy rain and a threat for minor flooding through this evening. Showers tonight are forecast to diminish on Wednesday.

KEY MESSAGES

* Increased risk for urban and small stream flooding through this evening as a moderate atmospheric river brings widespread moderate to heavy rain.

* Minor road flooding around Humboldt Bay early Tuesday morning around 4 AM with high tide.

* The Russian River at Hopland is forecast to reach or exceed flood stage Tuesday afternoon.

* Wet and unsettled weather possible this weekend.

DISCUSSION

Long southwesterly fetch of subtropical moisture intersecting the coastal terrain has been resulting in moderate to heavy rain overnight for Del Norte and Humboldt. Strong southerly winds offshore and over the coastal mountains has also been observed through the night, though not quite to the magnitude of the mesoscale models (at least over land areas). Overnight rainfall has ranged from 0.8 to 1.5 inches for many stations in Del Norte and Humboldt. A few stations reported 2 to 2.5 inches in 12 hours. River gauges were starting to respond to the rainfall run-off with the steepest rises noted on the Mattole river.

Slow moving frontal boundary offshore will continue guide the influx of deep subtropical moisture into the area - mostly Humboldt and Del Norte this morning. Another 1 to 2 inches, locally up to 4 inches, are forecast. Rainfall intensity will increase for Mendocino and Lake Counties later this morning into this afternoon as this steady stream of subtropical moisture proceeding the surface front edges southward. Mesoscale models from the HREF continue to indicate hourly rates from 0.25 to 0.75 inches for the mostly SW facing higher terrain. The most extreme ensemble members approach 1 inch per hour. These heavy rain rates are forecast to spread south and east and expand in coverage for Mendocino and much of Trinity. Southeastern Lake County may not get any heavy rain with 24 hour storm totals no more than 1". We will need to keep an eye on the potential for flooding due to the rapid run-off and high snow levels above 9,000-10,000 feet. Widespread moderate to heavy rain will likely decrease to showers this evening. A flood watch remains in effect until 10 PM Tuesday as it will take at least that long for water to drain and rivers to recede. Forecast from the river forecast center has nearly all stages below minor flood stage. The exception is the Russian river at Hopland which is forecast to reach minor flood stage (15 feet). There is a chance other rivers could get close or exceed minor flood stage. Stay tuned.

Conditions will begin to settle down on Wed. Considerable low level moisture will linger in the wake of the front and with warm air aloft, expect considerable low cloud cover and fog in the interior valleys and along the coast. Otherwise, we are looking dry until Fri or Sat when the stalled out upper low over the central Pacific edges closer to the coast. Timing and location of trough passage remains uncertain. Moisture transport under 250kg/m/s is forecast while PWATS increase. It not looking like any major precip at this time. There is a subtle hint for some convection or thunder as a mid level speed max noses toward the North Coast on Sat in advance of the 500mb cold core. Surface temperatures should remain mild and above normal as we head into the last week of February and the first few days of March.

AVIATION...06Z TAFs

Steady moderate, with periods of heavy, rain will move through Northwest California this morning and into late this afternoon. Low level windshear will be an issue this morning as a coastal jet moves overhead at up to 50 to 55 kts. Gusty southerly winds are likely to mix down to the surface affecting area terminals early this morning. Gusts from 30 to 40 knots are likely around Del Norte, Humboldt, northern Mendocino, and Trinity counties and potentially 20 to 25 kt gusts for terminals in southern Mendocino and Lake counties. MVFR to IFR conditions can be expected with heavier showers and also toward the end of the TAF period as the rain wraps up. /JLW

MARINE

Gale conditions continue into early this morning from strong southerly winds in all zones as a strong area of low pressure moves on shore. Near Storm force gusts are expected very early this morning for the northern outer zone (470) beyond 10 nm, and around Cape Mendocino. Steep and hazardous seas of 12 to 15 ft will develop in response, largest in zone 470. Winds and seas slowly subside throughout the day Tuesday.

Seas are forecast to drop below 10 ft late this afternoon/evening as winds drop to 8-12 kts, which will begin an extended stretch of much calmer conditions. A weak northerly wind surge will develop Wednesday, up to around 17 kts, otherwise mostly gentle to moderate northerly winds are forecast for the remainder of the week. Some fresh breezes of at least 20 kts will develop Friday in the far western portion of the northern outer zone.

There is a great deal of uncertainty for winds through the weekend and early next week. Winds are currently forecast to increase from the north with a strengthening pressure gradient. Strong southerly winds will also be possible if the area of low pressure pushes by the coast. In either scenario, Gale conditions may be met. JJW

HYDROLOGY

A moderate atmospheric river (AR3/AR4) is forecast to intersect the coastal terrain through Tuesday bringing widespread moderate to heavy rain. Duration of the heaviest rain rates are forecast to last about 24 hours. Rates could locally exceed 0.75 in/hr for an hour or two per the most extreme HREF members. This bout of heavy rain will result in rapid rises on rivers, streams and creeks. So far most all the river gauges were flat and not show quick responses to the run-off. This will change early this morning and stages will likely start to rapidly increase. Flooding of low-lying areas with poor drainage appears likely. Snow levels will pump up above 9,000-10,000 feet, so all the precip in the water sheds will be rain and go directly to run-off. The CNRFC is forecasting the Russian River at Hopland to barely reach minor flood stage on Tuesday afternoon. As result, a River Flood Watch is now in effect for the Russian River at Hopland from 2 PM to 10 PM Tuesday. The reminder of the main stem rivers are expected to remain below minor flood stage through this event. Our prolonged dry spells this wet season and low flows will reduce the threat for major or severe flooding. Stay tuned for updates and possible warnings and advisories.

COASTAL FLOODING

Persistent southerly winds have created over 1 foot of storm surge and tidal anomaly for the North Spit tide gauge. A high tide of 7.4 ft is forecast at 4:10 AM Tuesday. This high tide will coincide with much stronger southerly winds and heavy rainfall. The storm surge model forecasts a high tide up to 8.6 ft at this time, but given the described conditions, advisory level values of 8.8 ft or higher are possible for Humboldt Bay early Tuesday morning. A tide this high would bring up to half foot of saltwater inundation and minor road flooding around Humboldt Bay such as King Salmon and Jackson Ranch Road. A Coastal Flood Advisory is in effect from 3 AM to 5 AM PST Tuesday for low-lying areas around Humboldt Bay.

EKA WATCHES/WARNINGS/ADVISORIES

CA... Flood Watch through this evening for CAZ101>111.

Wind Advisory until 4 AM PST early this morning for CAZ101- 102-105.

Coastal Flood Advisory until 5 AM PST early this morning for CAZ103.

Wind Advisory until 1 AM PST early this morning for CAZ104- 106.

Flood Watch from 4 AM PST early this morning through this evening for CAZ112>115.

NORTHWEST CALIFORNIA COASTAL WATERS... Gale Warning until 6 AM PST early this morning for PZZ450- 470-475.

Gale Warning until 7 AM PST this morning for PZZ455.