textproduct: Eureka

SYNOPSIS

Drier and colder weather is expected to last today. Light rain returns from south to north Wednesday into Thursday, followed by a frontal system with moderate to heavy rain, high mountain snow and gusty winds on Friday. Bouts of rain and gusty winds are forecast to continue into the weekend.

DISCUSSION

High pressure remains over the area today bringing mainly clear skies except for fog and low overcast in some interior valleys. Tonight into Wed multi-layer clouds are forecast to spread over the area from the south in advance of an upper level low approaching the area from the south. This is expected to keep overnight temperatures warmer. Shallow humid layer for coastal areas will likely keep temps warmer and may even result in thicker fog and/or low clouds.

Light rain is expected to start in the afternoon on Wednesday in the southern portion of the area and spread north Wednesday evening and overnight. PWATS increase to over 250% of normal. Precip rates are quite light and after multiple days of drying and receding waters levels, hydro concerns are little to none. Rain may taper off in the afternoon on Thursday, but the timing is still in question. Some of the faster solutions show it ending by late morning. Generally amounts are expected to around a half inch to 1 inch with locally higher amounts in the mountains. There may be a short break in the rain Thu night-Fri AM before the next front arrives. Snow levels are expected to be fairly high with this first system, likely over 6,000 feet.

The next system is expected to bring heavier rain starting Friday midday or afternoon. It may even take longer to arrive, Friday evening. This is a more typical trough approaching from the west. It is more dynamic and negatively tilted. The GFS is hinting at a secondary area of low pressure developing along the front and closer to the coast. This may bring some higher winds to the coastal ridges, the gfs shows 60 kt winds at 925mb just off Cape Mendocino. There is still quite a bit of uncertainty on how much these will mix down to the low elevations around Humboldt Bay and other areas along the coast. Ridges will gusts to 60 mph or more for sure with 65+ kt at 925mb. Rainfall rates are expected to be higher with this front as well and the threat for urban and small stream flooding will increase. Granted we have had multiple days of no rain and receding water levels which may mitigate tee risk for rapid run-off. The current forecast has around 1 inch in the valley and rain shadow locations with 1.5 to 2.5 inches in the higher terrain and windward slopes. The ensembles are still showing some differences in the IVT with low probabilities of over 500 kg/m/s. High tides and rainfall run-off may result in flooding around Humboldt Bay and along the coast. Snow levels are expected to stay fairly over 5,000 feet through Friday evening and then start to fall late Friday or Saturday depending on the timing of the system. It looks like the colder air will move in after the heavier precip, but there may be some light snow down to 3,000 or 4,000 feet. GFS and ECMWF models are showing cold 500mb temps around -25C, instability and steep lapse rates. Isolated low topped thunderstorms and some small hail will be possible in the colder air behind the front into Sat. Snow levels plunge on Sat and some light accumulations are possible down to 3500 ft. NBM snow levels will need to evaluated.

More rain and strong gusty winds are probable over the weekend into early next week.

AVIATION...(30/06z TAFs)

Mainly VFR conditions with mostly clear skies is observed across NW California this evening (30/06Z). Strong radiational cooling will allowed another round of interior valley fog developing tonight into Tuesday morning, including at UKI. Expect IFR to LIFR visibility reduction in mist FM 10Z-17Z at UKI, with a TEMPO of 1/2SM FG or less between 14Z-16Z. Other than the overnight interior and morning fog, VFR will prevail with light winds across Northwest California.

MARINE

No significant chances to the previous forecast. Light winds and relatively low seas will persist for the next several days. Winds turns southerly Wednesday as an area of low pressure off the Southern California Coast moves northward. These southerly winds will steadily increase on Thursday as the low pushes east across Central CA, and particularly Friday as a stronger trough from the Gulf of Alaska digs south and eastward. Gale conditions are forecast through that period, and models have been trending higher in the magnitude of winds. Forecast wave heights have been quite variable, and will be potentially higher than currently forecast for Friday and Saturday. JJW

HYDROLOGY

A series of atmospheric rivers will bring an increased risk for urban and small stream flooding late this week and into this weekend. Mainstem rivers will also likely rise sharply and will need to be monitored for possible flood stage exceedance. Be on the lookout for watches, advisories and warning as this next heavy rain event unfolds. Stay alert and remember to never attempt to drive through water covered roads. Water may be moving much faster than you think.

COASTAL FLOODING

Another round of very high tides is coming up the 31st through 4th. At the North Spit Tide gauge on Humboldt Bay these are expected to be peak around 8.7 above MLLW on January 2nd and 3rd. There is still a lot of uncertainty on the winds, bu there is the potential for gale force southerly winds during this time period. The storm surge models are already showing nearly a foot of surge, but higher amounts are possible if the winds line up with the highest tides. This could yield quite a bit of flooding in the low lying areas around Humboldt Bay. Rain may also be occurring which exacerbate flooding surrounding Humboldt as creeks and low areas may not be able to drain during high tide. There are still a lot of variables, but this could bring 1 to 1.5 feet of saltwater onto normally dry ground around Humboldt Bay. The impacts are less clear outside of Humboldt Bay and will be more driven by the wave size and running up much farther than normal. MKK

EKA WATCHES/WARNINGS/ADVISORIES

CA... Frost Advisory from 1 AM to 9 AM PST Tuesday for CAZ103.

NORTHWEST CALIFORNIA COASTAL WATERS... None.