textproduct: Eureka

SYNOPSIS

An approaching cutoff area of low pressure will bring showers and interior thunderstorms on Wednesday, with increasing chances on Thursday. Chance for showers and thunderstorms from the low will continue on Friday and possibly into the weekend. A second much colder low will bring additional precipitation chances through the later half of the weekend.

DISCUSSION

A weak frontal boundary brought overnight chances for coastal light drizzle and a lessening chance for widespread fog. A cutoff 500mb closed low is advancing closer to the California coast tuesday. Precipitation chances begin to increase late Tuesday night from the nearing low. A lone, weak shower or two may brush the North Coast late Tuesday, but shower chances will not significantly increase until Wednesday afternoon. The deepened marine stratus is forecast to remain fairly well embedded over the coastal zones to dampen down daytime highs.

Increasing instability will bring chances for thunderstorms starting Wednesday afternoon for the interior. Early analysis of the soundings shows a surge subtropical moisture to be brought in with the low, increasing PWATs to 200% of normal. Interior areas through Trinity County are most favored for thunderstorm development Wednesday (15 to 20%). There is typically a delay in surface saturation, evident by inverted V profiles early in the afternoon. This will support some stronger downdraft wind gusts with any initial isolated thunderstorms. There will be "thin" CAPE, but modestly high for the region at 300 to 400 and eventually locally over 500 J/kg.

The low brushes closer to the North Coast Thursday when higher chances for showers and thunderstorm (20-30%) develop as a result. Broad forcing maximizes late Thursday around the low with deep- layer shear increasing to 30 to 40 kts. Southeast steering flow will allow for interior, possibly organized storm clusters to attempt to propagate westward toward the coast. Precipitable water values look to near at least 0.9 inches, so gusty winds and locally heavy rainfall will be possible if destabilization is fully realized. The thermodynamic profile more classically supports hail growth, with steep lapse rates reaching 7 C/km. More shower and thunderstorm chances continue through Friday and possible Saturday (up to 20%).

Beyond Friday into next weekend, wet and cooler weather may return on Sat or Sat night as an upstream and potentially much colder trough comes barreling down from the NW. This trough could bring a chance for snow to the highest mountain peaks. There are considerable differences in the ensembles and other outcomes are possible, including a warmer and drier scenario. Weak instability and colder temperatures aloft may bring an environment for small hail development if the colder scenario occurs.

AVIATION

Marine influence has persisted over the area. That said, weakening heat over the interior combined with approaching low pressure have helped form a more lifted and expansive marine layer. This has brought IFR ceilings all along the coast. Some brief lowering to LIFR is possible (30% chance) around sunrise, but any such conditions will be short lived. Marine stratus will most likely stay robust along the coast tomorrow (80% chance) but will most likely lift at least to MVFR. Ceilings are expected to lower again Tuesday night. /JHW

MARINE

Very gentle to calm northwest winds will persist into Tuesday. Similarly calm seas will persist as well. Low pressure slowly passing to the south will turn weak winds easterly around Wednesday and then bring very slightly enhanced northerlies late week. There remains relatively high wind strength uncertainty depending on the track of the low. The ensemble mean shows gusts only to around 16 kts by Friday in the outer waters, but there is a 20% chance of gusts closer to 25 kts. Northerly winds will continue to strengthen into the weekend. Regardless there is little to no swell and sea will be dominated by whatever short period seas the wind can generate. /JHW

EKA WATCHES/WARNINGS/ADVISORIES

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