textproduct: Eureka

SYNOPSIS

Wet and unsettled weather with showers and a chance of thunderstorms is expected this afternoon through Friday. Significant cooling, more rain, possible thunderstorms and high mountain snow are anticipated for this upcoming weekend.

DISCUSSION

A closed low continues to approach the California coast. Instability is likely to increase as cooler temperatures move in aloft. Daytime heating will allow the cap to break, bringing scattered showers and isolated thunderstorms, mainly for the interior areas. These are already starting to develop early this afternoon. The highest thunder chances are in eastern and northern Trinity County. Soundings show an "inverted V" shape, with dry low- levels and moist upper-levels. This signature is supportive of gusty outflow winds in the strongest storms, perhaps as high as 30 to 40 mph. Localized heavy downpours and small hail are also possible within the strongest storms.

Thursday, the low moves closer to the area. This will bring a higher chance for showers and thunderstorms. Thunder chances also move westward into much of the interior, and perhaps the coast could even see a stray thunderstorm, as well. High instability with CAPE of over 800 J/kg, steep lapse rates, and low level shear will support some of these thunderstorms to become strong to nearly severe. Gusty winds of up to 50 mph or higher, small hail, and heavy downpours are likely with the strongest of these storms. Some minor rotation is possible with some of these storms, which will need to be watched. The pattern Friday looks very similar, with the storms shifting further west, closer to the coast. Total rain amounts are minor with maybe up to a half inch for the coastal areas, and 1-2 inches in isolated areas in Trinity County within heavier downpours.

A closed low moving through Saturday is likely bring more widespread rain to the area. The position of this low is highly variable, and rain totals are likely to vary over the coming days as the track becomes more established. Precipitation amounts have trended higher than previous days. NBM shows about a 50% chance or higher for over most areas outside of southern Mendocino and Lake Counties. Chances for over 2 inches are high in the King Range and the mountains of Del Norte and southern Trinity. Snow levels drop to around 3500 to 4000 ft by Saturday night. NBM is showing around a 50% chance for over 4" of snow on Highway 3 at Scott Mountain Pass and a 25% chance on Highway 36 at South Fork Mountain. Cold temperatures aloft could support instability near the coast, bringing showers with thunderstorms and small hail. This will need to be watched as we get closer to this event. If these colder upper-level temperatures do develop, snow levels may be even lower - perhaps as low as 2000-2500 ft.

Much colder temperatures are likely during and and behind this frontal passage. Near-freezing to sub-freezing temperatures are possible, especially Sunday and Monday mornings. Stay tuned to our latest forecast for more details and possible watches, warnings and advisories. JB

AVIATION

A frontal system off the California Coast will bring variable conditions through the TAF period. While VFR remains midday through the early afternoon, low cumulus clouds are forecasted to form as the possibility of showers increases this evening through Thursday. Ceilings are forecasted to remain VFR/MVFR with very slight change of IFR developing around 12-15Z (30% probability). If rainshowers to flow over terminals, degraded visibilities are possible. The majority of showers/thunderstorm threat remains for Thursday, especially for Trinity, Humboldt, and Del Norte Co. The main hazard with these storms that could develop today and Thursday seems to be sporadic, gusty winds travelling through river valleys. Until then however, VFR remains at all terminals with light N-NW winds. DS

MARINE

A series of surface low pressure centers will arrive to the area today through the weekend. While the swell forecast looks to be low, variable winds could cause short period seas. These variable winds are forecasted to remain relatively calm (<=10kts) through Thursday as the first low pressure center sets up off the coast of San Francisco. Come Friday afternoon through the evening however, a second low pressure center arrives and will be farther north. This second system is forecasted to cause southerly winds to develop. Most of the models show a slight increase in winds, yet the NAM12km model shows southerly winds developing to breezy to potentially sub-gale force. While this speed max is in the forecast now, a change in the center of the 2nd system could drastically increase or decrease its impacts here along the coastal waters. DS

EKA WATCHES/WARNINGS/ADVISORIES

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