textproduct: Portland

SYNOPSIS

High pressure will maintain dry weather through Friday. Offshore flow will bring unusually warm temperatures Thursday and Friday along with gusty easterly winds in the Cascades, foothills, and western Columbia River Gorge. A weak Pacific cold front will bring chances for light rain Friday night into Saturday, before high pressure brings a quick return to dry weather Saturday night into Sunday. Widespread rain is likely (75-85% chance) on Monday with an approaching low pressure system.

SHORT TERM...Now through Friday

The short term forecast is highlighted by a warming trend and continued dry weather. An upper-level shortwave ridge is currently moving overhead, but will shift east late today with 500 mb flow become west-northwest. A weak upper-level impulse embedded within the west-northwest flow and a decaying surface cool front will bring a 10-15% chance of light rain to the Long Beach Peninsula and Willapa Hills Wednesday morning, resulting in trace amounts of rain if any at all. Otherwise, the rest of southwest WA and all of northwest OR should remain dry.

Temperatures will peak Thursday and Friday as a secondary upper-level ridge amplifies over the region and low-level offshore flow develops. Model guidance is in good agreement for widespread high temperatures ranging between 75-80 degrees on both days for inland valleys, and around 70 degrees at the coast. Although this is around 10-15 degrees above normal for mid-October, record high temperatures should be out of reach for most locations. Eugene is the only location that has a chance to tie or break the current record of 81 degrees in 2002, as this site has a 30% chance of reaching 81 degrees or warmer according to the 13Z/Oct. 13 iteration of the NBM.

In addition to the warmer temperatures, winds will become gusty out of the east in the Cascades, Cascade foothills, and western Columbia River Gorge. Corbett, Troutdale, Wood Village, Washougal, and Camas will also become quite breezy, especially late Thursday through Friday morning. This is in response to a strengthening surface pressure gradient; the NAM and GFS both suggest the pressure gradient between Troutdale and the Dalles will peak around -4 to -5 mb on Thursday. As such, the aforementioned areas should expect max wind gusts as high as 25-35 mph, except up to approximately 45 mph on wind-prone exposed ridgetops and in the western Gorge. Similar pressure gradients and wind speeds are anticipated on Friday. Beyond a few small downed tree branches where wind gusts are strongest, impacts are not expected. For the rest of northwest OR and southwest WA, winds should remain under 10-15 mph. Offshore winds should dissipate Friday night as low-level flow transitions back to an onshore flow regime ahead of an incoming Pacific cold front. This front will bring chances for light rain to the area Friday night into Saturday, which is discussed below in the long term discussion. -23

LONG TERM...Friday night through Monday night

A weak Pacific cold front is set to move inland Friday night into Saturday, which will bring cooler temperatures this upcoming weekend and chances for light rain. Rain chances begin at the coast Friday night (15-35%, except 40-50% from Seaside northward to Astoria and the Long Beach Peninsula). Rain chances then spread inland to the Cascades Saturday morning (20-40%, except 45-65% over the western Columbia River Gorge, south WA/north OR Cascades and adjacent foothills). Locations that do observe rain can expect light amounts ranging anywhere from a trace to 0.15 inches, resulting in no impacts.

Chances for rain decrease Saturday afternoon into Saturday night with the return of high pressure. Model ensemble guidance is in agreement for the continuation of high pressure through late Sunday afternoon, resulting in a dry day with seasonable temperatures in the 60s, except 50s at the coast and in the Cascades. However, a developing low pressure system offshore will approach the region Sunday night into Monday, bringing a 75-85% chance of rain to all of northwest OR and southwest WA from the coast to the Cascades between early Monday morning and early Tuesday morning. Rain could begin as early as Sunday evening, however the vast majority of ensemble guidance holds rain off until Sunday night or Monday morning. This system has the potential to bring a more meaningful amount of rain considering how dry conditions have been thus far this October, and the NBM currently shows a 45-65% chance for at least 0.25 inches of rain between 5 AM PDT Monday and 5 AM PDT Tuesday. Fortunately, this is not enough rain to cause flooding. Chances for impactful wind gusts over 40 mph are currently less than 10% with this system. -23

AVIATION...Through 18Z Wednesday

Mid to high level clouds will continue moving over northwest Oregon and southwest Washington Tuesday into Wednesday as a weak system moves into British Columbia. Given that this system is further north, precipitation is not expected across our area and will continue to have VFR conditions with SCT to BKN mid to high level clouds throughout the TAF period. An exception would be KAST, where there is a 20-40% chance for MVFR CIGs or lower at any given hour after 04-06Z Wed. Light northwesterly or variable winds around 5 kt or less today across all terminals.

KPDX AND APPROACHES...VFR with mid to high level clouds through the TAF period. Variable winds under 5 kt. -03

MARINE

Benign marine conditions expected this week. Southerly winds continue through Wednesday due to a weak front, then return northerly by the end of the week. Seas remain around 4-6 ft this week with periods of 13-14 seconds through Wednesday and 11-13 seconds late this week. A steeper northwesterly swell arrives this weekend, building seas to 7-9 ft by Sunday and 11-13 ft early next week. There is a 20-35% chance for seas exceeding 15 ft on Monday for the waters north of Cape Foulweather and beyond 10 NM, and a 10-20% chance elsewhere. Will also note that the long periods from the swells will result in a moderate sneaker wave threat this week. Caution should be used when in or near the water. -03/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.