textproduct: Portland

SYNOPSIS

Mostly dry weather this week as an upper level ridge builds over the area. This ridge will result in spring- like temperatures this week with highs in the mid 60s to low 70s across interior valleys. However, Tuesday through the end of the week, moisture riding the northern periphery of the ridge will return chances for precipitation, with the highest chances across southwest Washington and the Oregon coast north of Tillamook, but the bulk of the rain is forecast to track toward western British Columbia and northwest Washington, with lighter amounts over our area.

DISCUSSION...Now through Saturday

An upper level ridge building over the eastern Pacific Ocean, and moisture from an atmospheric river (AR) are the two predominant synoptic features this week. The ridge building up from the south is pushing the AR northward, which is resulting in the bulk of the precipitation riding the northern edge of the ridge and forcing the majority of the perception towards the Olympic Peninsula and Vancouver Island.

The center of this upper level ridge will move inland around southern California, with the ridge axis pushing into our CWA today. This will bring generally dry weather and decreasing cloud cover over our forecast area, with the further south you go, the more sunny/blue 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.

As the ridge gradually shifts eastward on Tuesday, the ridge itself will deform slightly 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 continue to be directed towards the Olympic Peninsula and Vancouver Island, the southern periphery of the AR will bring precipitation back into the forecast for the majority of the CWA. The highest chances (60-80%) for rain will be across southwest Washington and along the coast from Tillamook northward. From Wednesday to Friday, the upper ridge remains settled over the southern portion of the Great Basin with minimal weakening 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 25-40% 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%. However, there are a few model solutions that suggest that the high pressure could weaken significantly or trek further southward and either of those two scenarios would result in the AR dropping further southward. If this were to occur, then we could receive even more rain. In this case, the wettest scenarios for 24 hour rain each day from Wednesday through Saturday (10% chance of occurring) would be around 1.10-1.75 inches along the coast, Coast Range and the Cascades, with 0.30-0.80 inches along the I-5 corridor. The highest totals would be observed for areas north of Salem, OR.

Despite the renewed precipitation chances this week, spring-like temperatures in the 60s and potentially low 70s will continue across interior valleys through Friday, with the warmest daytime highs expected south of Salem, OR. Chances for Eugene exceeding 75 degrees F are around 20-40% from Wednesday to the end of the week. Given the warmer, temperatures we could see 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 (70-80%) suggests the ridge will begin to break down and result in zonal flow across the Pacific NW. This pattern change will bring cooler temperatures, with daytime highs in the upper 40s to upper 50s across the forecast area. /42-10

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 will continue to develop along the north Oregon coast, with a 50-70% chance of MVFR cigs spreading south to the central Oregon coast starting around 12Z-14Z Monday. Additionally, there's a 10-20% chance of IFR conditions developing for areas along the coast north of KTMK after 12Z-14Z Monday. Another round of light showers will move through the region, mainly north of KSLE, through 18Z Monday. Little to no impacts to flight conditions are expected as inland terminals may only observe virga or a trace of rain. There is a 10-20% chance that MVFR conditions could develop for inland terminals with the showers between 12Z-18Z Monday. 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 10-20% chance of MVFR conditions to develop from 14Z-18Z Monday as another round of virga/light showers move over the area. Generally light and variable winds. /42

MARINE

Southerly winds are expected across all waters and will to increase today with gusts up to 25 kt, the strongest gusts are expected across the northern coastal waters. Seas will also build today in response to the increasing wind waves and southwesterly swell, this will result in seas building towards 8 to 11 ft with the highest seas in the northern and central water zones. Breezy southerly winds continue through at least mid-week as a quasi- stationary front remains offshore. Winds are expected to increase on Thursday with around a 30% chance of gusts up to 25 kt across the northern and central waters as the front pushes inland towards the end of the week. As the front moves in, expect winds to become northerly, which will persist through the weekend. /42

PQR WATCHES/WARNINGS/ADVISORIES

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

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