Marine Weather and Tide Forecast for St. Helens, OR

DISCUSSION
Now through Saturday...Radar imagery as of early Sunday afternoon depicts light rain over the area. However, given that sounding profiles are showing a dry low level atmosphere and surface obs indicate dewpoint depressions of 15-20 degrees, most of this rain is coming down as virga (evaporating before hitting the ground). An upper level ridge is building offshore over the northeast Pacific Ocean, and moisture from an atmospheric river (AR) is riding the northern periphery of the ridge. This is what's bringing the overcast conditions and virga this afternoon. This trend continues through tonight, but as the low level atmosphere moistens up we could could see some light rain actually reaching the ground, with the highest chances (50-70%) along the north Oregon and south Washington coast.

The upper level ridge will move overhead by Monday, bringing dry weather and decreasing cloud cover. The further south you go, the more sunny skies you'll see. The moisture riding the ridge may keep some scattered/broken cloud cover around longer for areas north of Salem and Tillamook. Above-average 500 mb heights will also set-up over the area, resulting in a warmer air mass and high temperatures in the mid to upper 60s across interior valleys. Some places like the southern Willamette Valley may even exceed 70 degrees (60-80% chance) given the lack of cloud cover and greater magnitude of high pressure.

The ridge will gradually shift eastward on Tuesday, and as it does the aforementioned AR that's riding the ridge will shift a bit further south. While the bulk of the rain from this AR will be directed toward northwest Washington and western British Columbia, the southern periphery of the AR will return precipitation chances over our area. The highest chances (60-80%) for rain will be across southwest Washington and the coast from Tillamook northward. From Wednesday to Friday, the upper ridge remains settled over the Great Basin/Rockies with minimal break-down or eastward movement. This will maintain precipitation chances over southwest Washington and far northwest Oregon through the end of the week as moisture continues to trek along the backside of the ridge.

Precipitation amounts look light, resulting in negligible impacts to our area. Chances for 24 hour rain exceeding 1 inch each day this week are around 20-40% along the north Oregon and south Washington coasts, and less than 10% everywhere else.
Thus, chances for river flooding over the next 10 days remain under 5%. If the AR ends up pushing even further south, then we could receive even more rain. In this case, the wettest scenarios for 24 hour rain each day this week (10% chance of occurring) would be around 1.25-1.75 inches along the coast and 0.30-0.80 inches along the I-5 corridor, highest north of Salem.

Despite the renewed precipitation chances this week, spring-like temperatures in the 60s and potentially low 70s will continue across interior valleys through Friday, warmest further south.
Chances for Eugene exceeding 75 degrees are around 20-30% from Wednesday to the end of the week. Given the warmer, temperatures we could see some increased snowmelt across the Cascades. Snow levels will be above 7000-8000 feet, so any precipitation that falls over the Cascade passes will be rain. By Saturday, most ensemble guidance (80%) suggests the ridge will begin to break down, resulting in cooler temperatures. -10



AVIATION
A broad upper level ridge with mid-level moisture will maintain west to northwest flow aloft along with mid to high level clouds streaming across the area. Expect widespread VFR conditions to continue through the period at most terminals. MVFR conditions are expected to develop along the north Oregon coast after 06-10z Monday, with a 50-70% chance of MVFR cigs spreading south to the central Oregon coast after 10-12z Monday. Additionally, there's a 30- 45% chance of conditions deteriorating to IFR along the north Oregon coast after 11z Monday. Another round of light showers will move through the region, mainly north of KSLE, through 18z Monday.
Inland terminals may only observe virga or a trace of rain, and there's a 15-25% chance that cigs fall to MVFR for inland terminals with the showers between 10-18z. Generally light and variable winds expected through the TAF period.

KPDX AND APPROACHES...VFR with broken to overcast mid and high level clouds expected through the TAF period. There's a 15-25% chance of MVFR ceilings 10-18z Monday as another round of virga/light showers move over the area. Generally light and variable winds. -03/DH

MARINE
Offshore winds this afternoon turn more southerly later today and tonight as a warm front approaches the coast.
Fairly benign conditions continue through this evening with winds generally 15 kt or less and seas of around 3 to 5 ft.
Southerly winds are expected to increase on Monday, strongest across the northern coastal waters. Breezy southerly winds continue through at least mid-week as a quasi-stationary front remains offshore. Winds are likely to gust up to 20-25 kt, with Small Craft Advisory thresholds expected north of Cape Foulweather through at least Wednesday. A west-southwest swell pushing into the coastal waters along with increasing southerly wind waves will build combined seas to around 9 to 11 ft by Tuesday. The front will likely push inland late in the week, returning northerly winds by the weekend. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.
WA...None.
PZ...Small Craft Advisory from 5 PM Monday to 5 AM PDT Wednesday for PZZ210-251-252-272.

Small Craft Advisory from 11 AM Monday to 5 AM PDT Wednesday for PZZ271.