Area Forecast Discussion
Issued by NWS Eureka, CA
Issued by NWS Eureka, CA
889 FXUS66 KEKA 221222 AFDEKA AREA FORECAST DISCUSSION National Weather Service Eureka CA 511 AM PDT Wed May 22 2024 .SYNOPSIS...Slightly below normal temperatures are forecast through the weekend. Gusty north and northwest winds are forecast for portions of the area today through Thursday before more typical breezes for late May return this weekend. && .DISCUSSION...Northwesterly flow aloft with generally below normal 850mb temperatures (especially across the northern half of the forecast area) will result in below normal temperatures for the next several days (til at least Saturday). A decaying front associated with a shortwave trough over the Pacific NW this morning has induced a push of shallow moist air with low clouds and fog pushing into interior Del Norte and Humboldt Counties. High resolution ensemble continues to indicate drizzle possible this morning with a few hundredths of an inch (10-20% chance it will measure) for coastal northern Humboldt, north of Cape Mendo. HREF means indicate over a tenth in 12 hours for higher terrain of SW Humboldt, but this is probably way overdone. Surface ridge will build behind the front this afternoon and pressure gradients will tighten. This is a distinctly onshore flow and clouds may end up re- packing up against the coastal terrain. Ridge folding over into SW Oregon should promote drying and probably clearing late today into this evening. Granted time-height sections indicate RH`s over 90% and with northerly winds offshore over the waters, this is a good sign that low clouds will reform in the delta and spring back over over Humboldt Bay. The slight airmass cooling, drying offshore winds and clearer skies inland will probably result in cooler temperatures for the interior valleys tonight. Probabilities for 36F or less increase to 30% or more for valleys of Trinity and eastern Humboldt below 3000 feet and frost may form. Confidence is not high enough at this time to hoist a frost advisory with probabilities less than 50%. Diffuse trough aloft will remain over the area on Thu and Fri, and temperatures in the interior will probably remain near or below late May normals which are about 80F for Willow Creek, Weaverville and Ukiah. Another shortwave trough will drop down in NW flow aloft Fri-Sat and once again may result in light drizzle for the North Coast as the depth of the moist layer increases. GFS does indicate some light precip for the interior mountains - mainly Trinity - with a deeper shortwave trough scenario. NBM probabilities for 0.01in/6hr are 5% or less and not worth including in the forecast at this point. Above normal 500mb heights and above normal 850mb temperatures will likely result in above normal interior temperatures early to mid next week. How warm and the rate of warming are not certain. Temperatures will probably start to rebound on Sunday. National Blend of models indicates probabilities greater than 60% for 80F or more in many of the inland valleys by Monday. By Thu, probabilities for 90F or more increase to 20-40% for the inland valleys. Large scale flow pattern does not appear to be one that would promote deep offshore wind flow and suppression of the marine layer. NBM GUIDANCE indicates a distinct onshore wind regime at the surface. Thus it is impossible for coastal areas to warm up with N-NW breezes and chilly 48-55F air spreading onto coast. DB && .AVIATION...A weak cold front descending into NorCal is reinforcing a stratus layer along the coast north of Cape Mendocino. CEC and ACV ceilings and visibility`s quickly lowered to IFR/LIFR conditions amidst moist NW flow. Poor conditions expected to persist through the morning - stratus may struggle to lift until gusty NW winds resume this afternoon. Gusts 20-30 mph possible at coastal terminals through late this evening in the wake of the frontal boundary, although the jet core will remain south of the cape. Gusts 15-20 mph likely to develop at UKI in the afternoon as well beneath clear skies. Marine stratus expected to redevelop around the Humboldt Bay area this evening as winds diminish, while HREF indicates a weaker signature for low level ceilings at CEC. JM && .MARINE...A resurgence of near gale to gale force wind gusts will develop in the southern waters and downwind of Cape Mendocino this afternoon as a transient ridge strengthens the coastal jet. The broad wind field will reinforce elevated winds and short period seas in the northern and southern outer waters through Thursday, with the highest wind gusts confined to the lee of the Cape. Calm winds and mild seas in the northern inner waters will remain below advisory level through the end of the week. The sea state is forecast to diminish Friday before another upper trough digs into the region. JM && .EKA WATCHES/WARNINGS/ADVISORIES... CA... None. NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 9 PM PDT Thursday for PZZ455-475. Small Craft Advisory until 9 AM PDT Thursday for PZZ470. && $$ Visit us at https://www.weather.gov/eureka Follow us on Facebook and Twitter at: https://www.facebook.com/NWSEureka https://www.twitter.com/NWSEureka For forecast zone information see the forecast zone map online: https://www.weather.gov/images/eka/zonemap.png