textproduct: Portland

SYNOPSIS

Today will be the last day of abnormally warm temperatures. Conditions remain dry while onshore flow increases. A significant weather pattern change is on track beginning Thursday as a frontal system swings through, bringing cooler temperatures and widespread chances for rain beginning Thursday night and through Saturday. There is also a 10-25% chance for thunderstorms Friday afternoon and Saturday afternoon. Conditions trend drier and slightly warmer Sunday through early next week, but we'll maintain slight chances for light rain showers across the Coast Range and Cascades.

DISCUSSION...Wednesday afternoon through Tuesday

There has been very little change in the latest forecast package. Satellite imagery early Wednesday afternoon shows marine stratus has mostly dissipated along the coast, just clinging on right along some beaches. Otherwise mostly clear skies with scattered high clouds are being observed across the rest of NW OR and SW WA, except for cumulus building along the Cascade crest. The cumulus is due to a weak upper-level trough passing over the PacNW today. This trough not expected to bring any shower activity west of the Cascade crest-line given the lingering dry airmass over our area.

The upper-level ridge that brought the heat the last couple days has shifted east of OR and WA as the aforementioned trough has moved inland. However, the warm and dry airmass is lingering through today, allowing one last day of very warm and dry conditions, though afternoon high temperatures will be a few degrees cooler than the last couple days (in the upper 80s to low 90s across interior valleys) as the trough is producing increased onshore flow. There remains widespread Moderate HeatRisk across the Willamette Valley, SW Washington lowlands, and Columbia River Gorge. This level of heat will affect those who are sensitive to heat, especially those without adequate cooling or hydration. Make sure to stay hydrated, wear light- colored and loose-fitted clothing, wear sun protection, taking frequent breaks from the heat, and limit strenuous outdoor activities during the hottest part of the day.

A large-scale pattern change to cooler and wetter conditions remains on track for Thursday through the weekend as ensemble and deterministic guidance is in excellent agreement of an upper-level low pressure system progressing into the Pacific Northwest from the Gulf of Alaska. Temperatures will cool to near normal with mid 70s across the interior lowlands. Most ensemble members have rainfall via a cold- frontal boundary reaching the coast by late Thursday afternoon/evening and then spreading into the I-5 corridor by late Thursday night into Friday morning. We'll remain in this cooler and wet pattern through at least Saturday, but at least total rain amounts at this time appear beneficial and generally non-impactful. Looking at the latest NBM, chances for 48-hour rain amounts exceeding 1 inch from 5 PM Thursday to 5 PM Saturday are around 40-70% for the coast and Coast Range, 10-30% along and east of the I-5 corridor, and greater than 80% across the Cascades due to westerly flow bringing orographic enhancement. Will note that the western-most parts of the Willamette Valley (west of I-5 including McMinnville and Corvallis) only have a 5-10% chance of exceeding 1 inch during this timeframe due to potential rainshadowing from the Coast Range.

By Friday afternoon, the upper-level trough moves further south and eventually centers itself right over-top the Pacific Northwest as a closed low on Saturday. Temperatures will further cool aloft allowing for increasing atmospheric instability during the daytime hours and facilitating CAPE values of 50-100 J/kg Friday afternoon and 100-200 J/kg late Saturday morning into the afternoon. This instability plus sufficient lift from the low will support a 10-20% chance for strong showers and thunderstorms across NW Oregon and SW Washington Friday afternoon, and a 15-25% chance for thunderstorms Saturday afternoon. For now, any convection appears rather weak and unorganized (our typical post-frontal pop-up storms). LREF guidance suggests effective shear remains below 35 kt; therefore, chances these thunderstorms become severe are unlikely (<5%). Otherwise, any strong shower or passing thunderstorms may produce brief heavy rain, gusty and erratic winds, and a few lightning strikes. Also can't rule out a cold air funnel cloud or two given the time of year, especially on Saturday.

The latter half of the weekend into early next week, the majority of ensemble members are showing the broader upper- level low feature shifting eastward and lingering along the interior mountain west, returning relatively drier conditions and a warm-up of a few degrees to our area. This will put high temperatures just below the seasonal normals with low 70s for the interior lowlands and upper 50s to low 60s for the coast). Will still maintain precipitation chances across the Coast Range and Cascades as westerly flow continues to support orographic enhancement of precipitation, though any additional rain amounts appear very light. Still, we'll have to keep an eye on subtle shortwave features rotating into the region north to south on the back side of the upper- level low which may expand the coverage of showers temporarily - low confidence on the placement/timing of these smaller shortwave features. Westerly winds throughout the week will be breeziest along the central Columbia River Gorge and across Upper Hood River Valley, with chances for isolated wind gusts exceeding 45 mph around 10-35% from Friday through early next week, with the highest chances on Sunday and Monday. -03/10/99

AVIATION

Scattered high clouds as relatively dry upper level trough moves across the region. Predominately VFR conditions continue except at the coast where shallow marine stratus persists with IFR to LIFR CIGs near KONP. Stratus has scattered out at KAST as of 19z. Lower chances for conditions improving to VFR along the central Oregon coast, based on a persistent pattern the past few days, although guidance does show around an 80% chance for VFR at KONP between 20-00z this afternoon. There is high confidence that marine stratus fills back in along the entire coast this evening after 01-03z Thu. Diurnally driven NW winds continue with gusts up to 20 kt expected later this afternoon.

KPDX AND APPROACHES...VFR expected with scattered high clouds through the TAF period. Northwest winds expected to increase to around 8-12 kt with gusts up to 20 kt by 23z this afternoon. /DH

MARINE

High pressure will maintain northwest winds across the coastal waters through tonight. Winds are generally expected to remain under 20 kt. Weakening high pressure will give way to a frontal system approaching the area on Thursday. Winds are expected to back west to southwesterly Thursday afternoon before the cold front pushes across the coastal waters Thursday night into Friday. Southwest winds are expected to be strongest across the inner coastal waters north of Cape Foulweather, but there is only around a 20-30% chance of wind gusts exceeding 21 kt at any given hour through Friday morning. West to northwest winds will persist behind the front later Friday and into the weekend as weak low pressure lingers near Vancouver Island. High pressure gradually builds across the waters late weekend. Seas around 3 to 5 ft through Friday build to around 6 to 9 ft this weekend as a west-northwesterly swell moves through the waters. /DH

PQR WATCHES/WARNINGS/ADVISORIES

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