textproduct: Eureka

SYNOPSIS

Moderate to locally heavy rain is forecast tonight through Tuesday night for mostly Del Norte, Humboldt, Trinity and northern Mendocino Counties. A break in the rain is forecast on Wednesday, followed another high chance for heavy rain either on Thursday or Friday. Another chance for heavy rain and strong winds will arrive for the latter portion of the weekend.

DISCUSSION

A strong low pressure over the NERN Pacific, centered in the Gulf of Alaska, will send a series of frontal systems into NW California through Wednesday. Expect moderate to heavy rain and gusty southerly winds. Frontal passage, cooler and drier air will arrive during the day on Wednesday.

Satellite and radar data showed a band of moisture extending southwestward from the Pacific NW coast to just offshore the northern California coast this afternoon. Embedded convection, a few lightning strikes and heavy rain rates (0.30 in/hr) with this stream of deep layer moisture (PWATS 250% above normal) has been moving over Del Norte County this afternoon. HREF continues to depict highest chance for heavy rain > 0.25in/hr over Del Norte and northern Humboldt this afternoon and evening. A secondary precip max spikes up later tonight into early Tue across SW Humboldt as the axis of moisture shifts southward. Coverage of heavier rain rates over 0.25in/hr will expand during the afternoon Tue into Tue night. This will need to be watched for possible urban and small stream flooding and possible rock/mud slides. Much lighter rain rates are expected for southern Mendocino and Lake Counties with storm total rain around 0.10 inches or less ending 4 AM WED.

Southeast winds will also be gusty through the week. Peak wind gusts at lower elevations will most likely be 20 to 35 mph, with a 30% chance of impactful gusts over 45 mph on high coastal ridges. Winds gusts will most likely be strongest with the second round of rain Tuesday into Wednesday. ECMWF ensemble indicates potential for strongest winds Thursday or Thursday night, though there continues to be large spreads. Above average warmth and fairly high snow levels will severely limit all prospects for significant snowfall except over the highest mountain peaks.

There remains generally high confidence (80% chance) that a wet pattern will continue through the end of the week and into the weekend. GFS and ECMWF continues to depict another moist plume impacting the northern most portion of the area on Thursday. Eventually this boundary moves across the area by Friday, though it could speed up or slow down. High pressure and generally drier conditions are expected for Saturday, however another storm in the line up will begin to spin up offshore and may begin to impact the area as early as Sat night or Sun. This low pressure system has the potential to generate strong and damaging winds as well as more heavy rain. Timing is still uncertain. Looking at the chance of IVT > 250 kg/m/s the following week Dec 22-26, confidence is moderate to high in wet weather continuing for latter portion of Dec. CPC's 8-14 day outlook also has NW CA in a high risk for heavy precip and a moderate risk for high winds. Stay tuned.

AVIATION...18Z TAFs)

A frontal passage this morning has caused MVFR conditions along the North Coast today due to passing rainshowers with sporadic lightning strikes and moderately low cloud bases. Overcast ceilings around 1500-2000ft are to be expected all over NW CA today. The strongest winds are expected in Del Norte and Humboldt Co. and near coastal plains (i.e. KCEC and KACV). Winds aloft forecasted to decrease significantly through the evening decreasing the threat of LLT and mountain wave formation. Ceilings lower to potential LIFR tonight, yet most of the model data shows IFR is more probable. Rain tomorrow to the North Coast beginning around 15/16Z, with a 30% probability of reaching as far south as KUKI.

MARINE

Southerly winds present in the northern waters as a warm front moves over the areas. Winds will peak in the northern outer waters while other zones can expect moderate southerly wind gusts but will generally remain calmer. Wind waves creating short period seas over 6 feet in the northern outer waters through tonight.

Winds will generally weaken for much of tomorrow, but a mid period westerly swell building to over 10 feet will maintain moderately steep seas at least for the the northern waters. Another storm system will cross the area late tomorrow into early Wednesday. Southerly winds will again, only briefly, increase over 20 kts and be mostly confined to the northern waters.

Calmer conditions form again Wednesday as a mid period westerly swell up to 10 feet will likely maintain very moderately steep seas in the northern waters. A stronger storm system is expected later in the week around Friday. This will force the southerly winds into the southern waters with stronger gusts in the norther waters, though the potential for proper gales remains moderate to low (20-40 percent). /JHW/DES

HYDROLOGY

All main stem rivers are forecast to remain below Monitor or Action Stage through mid week. Minor flooding of smaller rivers, creeks and streams with poor drainage will be possible. Soil moisture is expected to continue increase or saturate late in the week into the weekend with additional rain, increasing the risk of rockslides and mudslides, and minor flooding. Chance of main stem river stages exceeding monitor stage will also increase as we head into the following week, Dec 22-28.

EKA WATCHES/WARNINGS/ADVISORIES

CA... None. NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory from 1 AM to 11 AM PST Tuesday for PZZ450.

Small Craft Advisory until 11 AM PST Tuesday for PZZ470.