textproduct: Eureka

SYNOPSIS

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

DISCUSSION

An approaching closed 500mb low is tracking east nearing the California coast Wednesday. The low has tapped into some subtropical moisture, bringing precipitable water values 200% of normal. With surface CAPE ramping up in the interior (some models near 1000J/kg), and surface based lifted indices near -3C to -2C, stronger and more widespread thunderstorm activity appears possible Wed afternoon into early evening in the interior - mostly in Trinity County. There is typically a delay in surface saturation, evident by inverted V profiles early in the afternoon over Trinity County. This will support some strong to marginally severe downdraft wind gusts. Soundings depict DCAPE values up to 600 J/kg, further emphasizing the threat, which includes small hail and eventual locally heavy rainfall after the lower- levels saturate and precipitable water values reach up to 0.9 inches.

The low brushes closer to the North Coast Thursday when higher chances for showers and thunderstorm (25-35%) develop as a result. Broad forcing maximizes late Thursday around the low with additional mesoscale vorticity maximums rotating in, and surface- to- 500mb S-SE bulk shear (25-35kt) shear increasing to 25 to 35 kts. Southeast steering flow will allow for interior, possibly organized storm clusters to attempt to propagate westward toward the coast. The thermodynamic profile supports hail growth, with steep mid-level lapse rates reaching 7.3 C/km. Hodographs on occasion show some rounding, especially late Thursday afternoon and evening through far northern interior Humboldt through Del Norte and through the Oregon border. This potential lower level rotation is also captured with 0-3km SRH, which coincides with some potent looking vorticity maximums. This period will have to be watched closely for some stronger thunderstorms capable of at lease some brief rotation. The action continues Friday with a similar storm track favorable to westward coastal propagation. Beyond Friday into next weekend, wet and cooler weather returns on Sat or Sat night as an upstream and much colder trough and closed low comes barreling down from the NW. Most all the models have trended much wetter for this storm system, which will include high mountain snow and a period of gusty winds. There is high uncertainty on where the closed low ends up coming ashore, thus there is a wide range in ensemble members for total rainfall amounts. Over half of the ensemble members hold 1 to 2.4 inches for 24 hour rainfall totals Saturday through Sunday for the North Coast. NBM 90% shows 0.50 to 1.50 inches falling in 12 hours. This would be plausible if the low takes a more southern track as it quickly moves into a saturated airmass left from the kicked previous low. Higher amounts may be possible for the interior, and total rainfall would be lower with a more northern track. Snow levels currently look to drop to 4000 feet through this period, but lower levels are possible.

Thunderstorms are possible with this cold system Saturday through Saturday night. 700 to 850 mb temperatures look nearly cold enough to support small hail and possible travel impacts if low level temperatures are cool enough overnight Saturday. Much colder surface temperatures will arrive behind the passing of the low. Late season coastal frosts and interior frosts and freezes will be possible Sunday morning and into early next week. Stay tuned to our latest forecast for more details and possible watches, warnings and advisories.

AVIATION

The marine layer is in place along the coast bringing IFR to MVFR conditions. An upper level low approaching the area this morning is expected to lift ceilings at some point during the morning to VFR. The timing is uncertain on when this will occur. Later this afternoon and evening as the upper low approaches the area there is the potential for showers and thunderstorms. These are mainly expected to be over Trinity county, but a few could make it as far west as the coast. So this will need to be monitored. These showers will bring lower conditions, but these are expected to be fairly limited in coverage. MKK

MARINE

Northerly winds are around 5 to 10 kt across the waters. The waves remain small as well with several swells, but all of them less than 3 feet. An area of low pressure is expected to approach Cape Mendocino Wednesday afternoon. This is expected to switch the southern waters to southerly winds while farther north they remain northerly. This generally generally continues into Friday with the low moving very slowly. Winds are generally expected to remain below 10 kt.

For the weekend a stronger area of low pressure is expected and this has the potential to bring stronger winds. The strength, timing and location remain highly uncertain due to low confidence on the track of the low. MKK

EKA WATCHES/WARNINGS/ADVISORIES

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