textproduct: Eureka

SYNOPSIS

Few isolated showers continue to diminish as a cold upper low progresses south. Clearing and drying will allow for some colder overnight lows. A sharp warmup with building high pressure can be expected for the end of the week, followed by some rain chances.

DISCUSSION

A cold upper low continues to pass south of the forecast area and into the Four Corners Region this afternoon. Wrap- around moisture is generating some light showers over mainly Humboldt and Del Norte counties, and that activity will fade tonight. As with last night, lingering moisture, dissipating cloudcover, and some patchy fog formation will hold up overnight temperatures for some, particularly where additional light showers occurred today. Overall, the area has dried out more than the last 24 hours, and areas of frost and subfreezing temperatures are expected in portions of areas where the agricultural growing season has begun. Frost Advisories and Freeze Warnings are out in Southern Mendocino and Lake counties. Interior Humboldt will be watched closely, as the agricultural season has begun, but confidence is low on the coverage of frost potential at this time given the previously described conditions.

On Thursday, a ridge of high pressure will begin amplifying over the NE Pacific and into the Western CONUS. The pressure gradient will increase against the departing upper low and northerly winds will increase to breezy levels, with gusts of 18 to 25 mph in the exposed ridges and coastal headlands in response.

For Friday, the ridge progresses over W CONUS when abundant warmth will ensue. There are indications an inverted trough may form with favorable positioning for enhanced offshore winds and perhaps higher temperatures than forecast. Currently, daytime highs of around 5 degrees above climatological norms are forecast, with low 60s at the coast and low 70s for the interior valleys. Saturday may be the apex of warmth, with daytime highs nearing 10 degrees above average, with some mid 70s for interior valleys. Cloud cover arriving ahead of the short wave trough may inhibit the full potential of warmth for some.

The shortwave will be the first of two systems that will deliver light rainfall to the region late Saturday through Monday, and maybe into Tuesday. Snow levels look to be high, at 4500 to 5000 ft, and snowfall does should not be an issue. The proximity of the low to the low will determine the strength of the southerly winds, and NBM is still struggling to resolve meaningful probabilities of wind gusts over 25 mph through that period. There are some ensemble solutions that portray a stronger 925mb low level jet and heavier rainfall, and there is high uncertainty in 24 rainfall amounts from ensemble spread (1-2 inches difference). Probabilities for 24 rainfall over 0.5 inch are currently moderate to high (46 to 75%). /JJW

AVIATION

Improving weather, under drying northerly flow, rendered the TAF sites mostly VFR today. However, widely scattered, diminishing showers continued to lurk off the Humboldt Coast and interior areas through early afternoon. Infrequent MVFR Cigs have been associated with showers. Winds across the region shifted more to the WNW-N this afternoon. Due to a micro "bubble" high over CEC, the wind gradient was slower to develop until around 1300 LST (20Z). In the meantime, winds were already gusty at UKI and ACV airports. At the coast, winds will diminished somewhat tonight, but should stay relevant enough to prevent any marine layer development under a northerly gradient flow. Winds will be light overnight at UKI. Therefore light fog is possible for a short period of time Thursday morning, but seems unlikely at this time (Wed Afternoon). /TA

MARINE

Fresh to moderate northerlies will persist tonight through Thu evening. Strongest north winds are initially forecast to occur over the outer waters and in the lee of Cape Mendocino inside 10NM tonight. Generally, 15-25 kt is forecast with gusts up to 30 kt. The largest steep waves of 6-8 ft are also expected across the southern waters. Diurnal wind max is forecast to expand northward on Thu with HREF probabilities of 30-60% for gale force gusts to 35 kt around and offshore of Pt St George Thursday afternoon and evening. Coverage of these gale gusts based on the HREF does not appear sufficient at this time to warrant a gale watch. Later runs of the CAMS could expand.

Northern inner waters will be much more marginal for gusts to 21 to 24kt this afternoon and evening. Winds will likely lay down overnight and Thu morning before increasing again Thu afternoon in response to land mass heating. North winds should be stronger Thu afternoon and evening for the northern waters and a small craft advisory will be necessary as steep waves increase.

Northerly conditions should begin to relax and ease up on Friday as a thermal trough shifts over the coastal waters. Steep wind waves around 5-7 feet will probably persist in the outer waters, however. Southerly winds are forecast to develop over the weekend in advance of a frontal system. NBM guidance continues with probabilities not more than 20% for gusts over 25 kt, so right now winds do not look to be very strong and impactful. A long period westerly swell group will build on Sunday and peak to 9 to 11 ft near 18 seconds by mid to late morning according to GFS-waves. GFS wind fields could be wrong and the swell overdone. Stay tuned as this swell may pose a risk for sneaker waves on Sunday. DB

EKA WATCHES/WARNINGS/ADVISORIES

CA... Freeze Warning from 2 AM to 9 AM PDT Thursday for CAZ110- 111-114.

Frost Advisory from 2 AM to 9 AM PDT Thursday for CAZ112- 113-115.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 9 PM PDT this evening for PZZ450.

Small Craft Advisory until 9 AM PDT Thursday for PZZ455.

Small Craft Advisory until 3 AM PDT Friday for PZZ470-475.