textproduct: Portland

SYNOPSIS

Warm temperatures into Thursday. Shower and thunderstorm chances over the Lane County Cascades this evening. Shower and thunderstorm chances spread over the north Oregon and southern Washington Cascades Thursday afternoon and evening with the potential for some thunderstorms to become severe. Non- severe shower and thunderstorm chances west of the Cascades Thursday afternoon and evening. Cooler temperatures return Friday, followed by a drier weekend. Warming temperatures return early next week.

DISCUSSION...Wednesday afternoon through Tuesday

Weak high pressure is building over the eastern Pacific Ocean off of the PacNW today into Thursday as the PacNW sits between a large closed low pressure system centered over California and a deepening low pressure system south of Alaska. Temperatures today and Thursday will be much warmer than previous days this week with the interior lowlands warming to the mid 70s to low 80s each day. More importantly, this upper level pattern is producing shower and thunderstorm chances across the region today and Thursday as moisture wraps around the north side of the CA low into central Oregon today and spreads north into northern Oregon and Washington tomorrow. Today's chances remain limited to the Lane County Cascades with a 40-60% chance of showers and a 15-20% chance of thunderstorms.

Chances for showers and thunderstorms become much more widespread and increase significantly on Thursday. The trough over the northeastern Cascades deepens closer to the PacNW as a strong shortwaves moves along the CA closed low over Oregon, leading to strong convergence and lift. CAPE increases significantly by midday/afternoon to 500-1000 J/kg in the Lane County Cascades and 1200-1600 J/kg over the rest of the northern Oregon and southern Washington Cascades and Hood River County. This is a significant amount of convective potential for this area. Additionally, models show 0-6 km shear of 30-40 kts concurrently. Guidance does show CAPE decreasing from south to north through the afternoon hours, lingering above 1000 J/kg the longest in Hood River and Skamania Counties and east into the evening hours, and this area is where thunderstorms could be strongest.

SPC indicates a Marginal Risk (level 1 out of 5) of severe thunderstorms over the northern Oregon and southern Washington Cascades as well as a Slight Risk (level 2 out of 5) of severe thunderstorms over a portion of Hood River County. What this means is there is a 30-45% chance of thunderstorms where some thunderstorms may become severe with hail up to at least the size of a quarter (1 inch) and wind gusts up to at least 60 mph. Additionally, there's a 15-25% chance for non-severe thunderstorms west of the Cascades with best chances generally around and east of Interstate 5. These strong showers or thunderstorms could produce small hail and gusty winds less then 60 mph. Thunderstorms are expected to form along the Cascade crest in the afternoon and evening hours (except late morning in the Lane County Cascades) with best chances for severe thunderstorms between 4-10 PM PDT. Showers and thunderstorms that form along the Cascades could then move west into the foothills and interior lowlands with best chances of this also between 4-10 PM PDT.

Heavy rain and flooding potential is increasingly becoming a concern for both the Cascades and the interior lowlands. REFS, HREF, and model soundings indicate PWAT values of 1.2-1.4 inches over the interior lowlands, which is extremely high for late May. This is right around the climatological maximum of 1.24 inches for May 28 based on sounding analysis at Salem, OR. PWAT values do decrease somewhat as elevation increases in the Cascades, but values are still around 0.75-1 inch which is still very high for this time of year. HREF and REFS are indicating rain rates could reach 0.25-0.5 inch per hour over the Cascades and western foothills with a few outlying members suggesting rain rates up to 0.8-1 inch per hour possible. For the lowlands, this guidance is indicating rain rates up to 0.25-0.3 inch per hour are possible, mainly north of Salem. These rain rates could create flooding issues for urban areas, low lying areas, and any other flood prone areas as well as landslides if the higher rain rates materialize.

On Friday, the closed low over California finally begins moving east as the trough continues to deepen over the eastern Pacific. Shower chances continue on Friday morning, shifting to mainly over the Cascades by the afternoon. Temperatures also cool back into the mid 60s for the interior lowlands Friday and Saturday as the trough moves over the PacNW.

A significant warming trend is then expected Sunday into Tuesday. Interior lowland temperatures warm into the 70s on Sunday, low to mid 80s on Monday, and upper 80s to possibly around 90 degrees on Tuesday. NBM indicates a 20-50% chance of high temperatures exceeding 90 degrees on Monday for locations around and north of Salem and a 50-70% chance on Tuesday for locations around and north of Corvallis. This pushes these areas into Moderate Heat Risk with a 10-25% chance of Major Heat Risk on Tuesday. -03

AVIATION

Onshore flow continues near the surface as an upper level low spins over central California tonight through Thursday. At the coast, low stratus has already sloshed back along the coastline with high confidence in MVFR/IFR conditions persisting through at least a good chunk of Thursday morning before some partial clearing/slowly rising CIGs returns in the afternoon - best chances near KAST. Inland expect predominantly VFR conditions although low clouds seeping through the coast range gaps has the potential (30-60% chance) to bring a period of degraded CIGs to the central and southern Willamette Valley between 12-17z Thursday with only a 20-25% further north towards the Portland metro during this same time-period.

Otherwise, the focus shifts to thunderstorm development along the Cascade crests during the early to mid afternoon hours with moderate confidence this activity attempts to shift east to west towards the Willamette Valley approaching 22z Thur to 04z Friday. However, the latest model guidance suggests thunderstorms likely weaken as they move off the Cascades, in which case they may end up as just an organized band of showers by the time they are within the vicinity of the inland terminals. A period of breezy/erratic winds are still possible in this scenario however. Given the lack of confidence regarding convective mode and timing late in the TAF period, did not add a mention of thunder to any sites in the 06z TAF package.

KPDX AND APPROACHES...Expect VFR conditions to prevail with few high clouds tonight into Thursday morning. Chances (20-25%) for MVFR stratus return between 14-18z Thursday but VFR CIGs are still favored. After ~00z will have to be on the lookout for weakening thunderstorms along the Cascades attempting to work westward towards the terminal. For now it appears the main thunderstorm impact may be moreso for any the east approach into the airport - something to keep an eye on. -99

MARINE

High pressure offshore will maintain breezy north to northwesterly winds across the coastal waters through the remainder of the week. The strongest wind gusts are expected to peak later today through this evening with gusts up to 25-30 kt, highest across the outer waters south of Cape Foulweather. Otherwise, expect frequent gusts to around 20 kt. Seas around 9 to 10 ft this afternoon remain relatively steep and choppy, before building to around 11 to 12 ft later tonight into Thursday as a mid to long period northwest swell pushes through the waters. Wave heights then gradually subside back down to around 9 to 10 ft by Friday morning. Therefore, have extended the Small Craft Advisory for all coastal waters, including the Columbia River Bar, through Thursday night. High pressure is expected to strengthen this weekend across the coastal waters leading to a return of breezy northerly winds. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 5 AM PDT Friday for PZZ210-251>253- 271>273.