textproduct: Eureka

SYNOPSIS

Dry weather and above normal high temperatures are expected to continue in the interior through Tuesday. Chance for showers and interior thunderstorms will arrive on Wednesday and then increase on Thursday. Chance for showers and thunderstorms will continue on Friday and possibly into the weekend.

DISCUSSION

Massive field of stratus offshore has been banked up against the coastline today. Marine layer was shallow (<1 kft MSL) but has been deepening today. Layer should continue to deepen tonight and push farther into the adjacent coastal river valleys. The stratus forecast has been nudged toward persistence, based largely on high boundary layer humidity from the ARW and NAM12. Otherwise, dry weather and above normal high temperatures (70s to lower 80s) are forecast to continue for much of the interior on Tuesday as a ridge aloft springs up in advance of an approaching 500mb trough. Cool-damp marine air will continue to result in cooler weather for coastal locations and adjacent river valleys.

Rain shower chances increase on Wednesday as SWLY flow aloft moistens up in advance of a closed 500mb low that will approach 130W around 39N. Slight upper diffluent flow aloft (positively tilted) develops over the area through the day and the atmosphere does appear to destabilize some with long thin CAPE around 200-500J/kg, lifted indices near -1C and PWATS near 200% of normal. Thunderstorms are possible (15% chance) with daytime heating for mostly eastern Trinity, eastern Mendocino (over the Yolla Bolly's) and perhaps far northern Lake. Potential for interior showers and wet thunderstorms are forecast to increase on Thu as the cut-off low moves inside 130W and stronger 500mb S-SE flow develops while PWATS remain near 200% of normal. Forcing with a perturbation pin-wheeling around the offshore low coincident with max daytime heating should result in stronger multi-cell storm clusters with greater potential for heavy rainfall, small hail and perhaps gusty winds. Large scale 500mb flow is favorable for storms to propagate out over the coast and coastal waters as S-SE SFC-500mb bulk shear increases to 30-40kt. Most likely time period for coastal thunder would be Thu night into Fri morning if storms fire up with daytime heating over the interior. One caveat to all of this is boundary layer destabilization. If surface temperatures cool down too much due to cloud cover and widespread showers, there may not be enough boundary layer destabilization for storm initiation. Other than the potential for locally heavy rain with thunderstorms, not very much rain is expected over 3 days with the incoming cut-off low, generally less than 0.25 to 0.50 inches Wed-Fri. Higher amounts near 1 inch over 3 days (Wed-Fri) are certainly possible with strong/deep moist convection over the interior.

Wrap around moisture with above normal precipitable water over 200% of normal will keep the threat for showers and thunderstorms going into Friday. Beyond Friday into next weekend, wet and cooler weather may return on Sat 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 too this upcoming weekend. There are considerable differences in the ensembles and other outcomes are possible, including a warmer and drier scenario.

AVIATION

IFR and LIFR ceilings have been steady at KACV and KCEC through the day. A brief break up of the solid stratus deck is possible, but confidence is waning quickly based on satellite imagery and BUFKIT time-height sections which indicate the layer gradually deepening tonight. Mesoscale models, particularly the ARW, indicates near full saturation of the boundary layer (99% RH) this evening and fog with vsby 1/4SM or less may come charging back in with light onshore winds. Easterlies overnight may mitigate the fog threat overnight as the layer lifts or drizzles outs. Chances for IFR clouds pushing up from Sonoma County and into Ukiah Muni is not looking very likely at this point unless the marine layer deepens considerably. Marine air will trickle into interior Mendo from the west and low clouds and fog should form with longwave cooling north of UKI around Willits.

MARINE

Marine winds and seas are forecast to remain below advisory thresholds for small craft through the week (Tue-Fri). Northerlies and larger steep waves should begin to build over the weekend (Sat and Sun) after passage of a cut-off low to the SE and surface ridging offshore builds. Primary hazards to navigation will continue to be the "patchy fog" and perhaps thunderstorms latter in the week that could propagate over the waters in the wrap around east-southeast flow, specifically Thu night into Friday.

EKA WATCHES/WARNINGS/ADVISORIES

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