textproduct: Eureka

KEY MESSAGES

* Probable rainfall totals trend higher Wednesday through Thursday afternoon, mainly for the interior.

* Seasonable temperatures, with a higher chance for frost and subfreezing temperatures for the interior late in the week and beyond.

* A drier scenario is becoming more likely for the expected weekend disturbance.

SYNOPSIS

Chance for rainfall increases throughout Wednesday, with higher chances overnight and into Wednesday. A cold front will then quickly dry out the environment and bring broad areas of frost and some interior freezes late in the week.

DISCUSSION

An area of low pressure is observed on mid level water vapor imagery off the coast of central and southern California. The low is sending wraparound moisture into N California from the SE. Ongoing shower activity will increase through the day for areas initially generally south of Cape Mendocino. This moisture will then push farther north through Humboldt and likely to Del Norte County later Wednesday through Thursday.

The low is forecast to tap into some deeper subtropical moisture south of southern California and send it our way. Now to complications in the forecast. There remains question in how much moisture wraps into our area from over the higher terrain to the east. The current forecast of total rainfall is on the lower end, with areas east through Lake, Southern Mendocino, and Trinty County in the 0.25 to 0.5 in inch total through Thursday. These forecast amounts have increased, and align closer to the mean estimate. There is very large differences from the high resolution REFS ensemble max, which has a broad area over 1 inch, and locally 1.75 inches that expands farther west into Humboldt for the extreme scenario.

We've discussed the argument for the lower end amounts, but a scenario for the higher end is plausible. The low will usher in modest elevated instability, with a potential for some thunderstorms. The mentioned subtropical moisture will yield some precipitable water values over 1 inch. Given the nature of cutoff, stalled lows, small deviations in the track are possible, and this can yield much large changes in the rainfall amount. Anytime there is instability with anomalously high precipitable waters amounts, models can underestimate what is actually produced.

Northerly winds will turn gusty Friday, followed by a strong NE wind surge Friday night. Any valleys that do clear out Thursday night will likely see frost or freezing temperatures, and if we get the forecast coverage of rainfall, interior fog would suppress the chances for frost and freezes. The dry offshore winds Friday night should allow for things to clear and and get cold, particularly Saturday morning when frost conditions may expand to the coast. JJW

AVIATION...6Z TAFs

Low level moisture is increasing under a shallow inversion. There is a potential for shallow marine stratus advancing down the coast, with at least tempo fog visibility. Probability of LIFR CIGS are around 40 to 50% at CEC and up to 64% for ACV. Chances for visibly under 1sm is near 30%. An area of low pressure offshore will deliver instability, which has been know to weaken the inversion and allow for scattering. This continues to give uncertainty in the forecast, though the instability is elevated which may keep to TAF forecast on track.

MARINE

A mid period westerly swell has peaked and will gradually subside over the coming days. An upper low off the coast, which is generating this swell, will move southeastward closer to the central CA coast. This is bringing some uncertainty on the winds on Wednesday, with some models pushing stronger northerlies in the outer waters. A few models are also showing a southerly wind surge near the Humboldt and Del Norte Coasts Wednesday morning. Most models show this as fairly weak, peaking around 10 to 15 kts, however the CANSAC model does show locally stronger winds over 20 kts from Cape Mendocino to the Del Norte Coast. This scenario is unlikely and very dependent on the track of the low, but it is something to watch.

As this low exits the area Wednesday night into Thursday, stronger and more widespread northerly winds return across the waters. Gale force gusts are likely for the outer waters and the lee of Cape Mendocino by Thursday evening. Strong winds continue through Friday evening. Much lighter winds are expected this weekend. /JB

FIRE WEATHER

Higher instability will arrive Wednesday when moisture from an area of low pressure wraps in. Coverage and totals of rainfall has increased. A much larger potential for a wetting rain exists over most of the interior (broadly 40-60%, with up to 70% over Lake County. There is a potential for some thunderstorms, up to 10% chance. The higher instability and arrival of subtropical moisture has the potential to bring higher rainfall amounts. A cold front will sweep the moisture away Thursday afternoon, followed by plummeting RH back to the teens Thursday night. A Surge of offshore winds overnight Friday will reinforce the dry conditions Saturday.

EKA WATCHES/WARNINGS/ADVISORIES

CA... None. NORTHWEST CALIFORNIA COASTAL WATERS... None.