textproduct: Portland

SYNOPSIS

Broad upper level troughing over the Gulf of Alaska will maintain dry southwest flow aloft over northwest OR and southwest WA through Thursday morning, bringing mainly dry weather and slightly cooler than normal high temperatures in the mid 60s to lower 70s. Troughing over the Gulf of Alaska deepens early Thursday and begins progressing eastward towards the Pacific Northwest. Confidence is now high this trough will move inland late in the week, bringing widespread rain to the entire area late Thursday into Friday. Weak high pressure builds in behind the trough on Saturday, bringing dry weather. Chances for showers increase again Sunday into Monday, but only up to 10-30%.

LONG TERM...Thursday through Monday night

Weather conditions begin to change considerably Thursday into Friday as a broad upper level trough over the Gulf of Alaska deepens and progresses eastward over the Pacific Northwest. Now that model ensemble guidance is in excellent agreement this trough will move inland without significant deamplification, confidence is high for widespread rain over all of northwest OR and southwest WA late Thursday into Friday (80-90% chance). The exact timing rain will begin remains uncertain, but most guidance suggests rain will begin sometime between Noon-11 PM Thursday with rain beginning closer to the start of that range at the coast/Coast Range and closer to the end of that range for the Willamette Valley, Portland metro, Cascades/foothills, Columbia River Gorge, and Cowlitz Valley. Rain is expected to end fairly quickly from west to east Friday evening into Friday night as weak high pressure builds in behind the trough.

Forecast rain totals with this system have increased to 0.3-1.0 inches, with the highest totals in the mountains and the lowest totals in the Willamette Valley, Cowlitz Valley, and Portland metro. Chances for rain totals over 1 inch range between 15-35% in the Willamette Valley, Cowlitz Valley, and Portland metro to 35-55% for the coast, Coast Range, and western slopes of the Cascades. It appears the only way locations in the Willamette Valley, Cowlitz Valley, or Portland metro could see rain amounts that high is if heavier showers and thunderstorms hit a given area on Friday. Confidence in the thunderstorm potential on Friday remains low, however probabilities for thunder have increased slightly to 20-25%. LREF guidance currently suggests limited instability with SBCAPE values most likely peaking around 50-150 J/kg with a 10% chance for values as high as 250-300 J/kg. The latter would only occur if there are sufficient breaks of sun Friday morning into the afternoon to allow for enough surface heating to steepen lapse rates. If breaks of sun do occur, then the potential for heavier showers and low-topped thunderstorms would increase Friday afternoon. Given what appears to be a decent wind shear profile in place at that time (0-3 km shear values around 20-30 kt), any stronger shower or thunderstorm would have the potential to produce gusty outflow winds. Even away from convection, environmental surface winds look to be rather breezy with wind gusts most likely peaking within the 20-30 mph range. In fact, the NBM is showing a 5-15% chance for max wind gusts as high as 40 mph or stronger on Friday. While wind gusts of this magnitude are unlikely to occur, it is worth mentioning as these wind speeds would be enough to result in minor tree damage and isolated power outages given leaves are still on trees.

Those hoping for dry weather come Saturday are in luck, as models remain in good agreement for high pressure behind the trough. As such, chances for rain on Saturday are less than 10%. Chances for showers increase slightly Sunday into Monday, but only up to 10-30%. Temperatures will likely stay cooler than average over the weekend with highs in the 60s. It is also worth mentioning overnight lows will trend chillier, especially in the Upper Hood River Valley where morning lows may get cold enough to support frost Saturday morning and Sunday morning; this is when the NBM is showing a 30-50% chance for morning lows of 37 degrees or colder in the Parkdale area and 10-15% chance near Odell. -23

AVIATION...Through 12Z Wednesday

At 1145Z Tuesday, widespread stratus remained in place from the coast to the Cascades, resulting in a mix of high-end IFR to low-end MVFR ceilings along the coast and MVFR ceilings inland. Between 13-16Z Tuesday, chances for IFR ceilings below 1000 ft increase to 10-25% for inland TAF sites, except 40-50% at KEUG. Expect low stratus to scatter out and lift to VFR thresholds sometime between 19-23Z Tuesday, with ~21Z being the most likely outcome. Winds will stay under 5-10 kt throughout the TAF period.

KPDX AND APPROACHES...MVFR ceilings around 2500 ft will continue through at least Tuesday morning before low clouds scatter out and lift to VFR thresholds sometime between 19-23Z Tuesday, with ~21Z being the most likely outcome. While unlikely to occur, there is a 10-15% chance for broken IFR ceilings near 1000 ft or lower between 13-16Z Tuesday. Winds will range anywhere from calm to light northwest around 3-5 kt throughout the TAF period. -23

MARINE

High pressure will maintain predominately northerly winds across the waters through at least Wednesday night. An exception would be Tuesday afternoon to Wednesday morning, when winds may turn more westerly to southwesterly for the waters north of Cape Falcon with the passage of a weak front. Winds become northeasterly over the waters for a brief period of time Thursday morning and early afternoon ahead of an approaching frontal system. This frontal system will move over the coastal waters late Thursday into Friday, bringing increasing northwesterly winds and rain. This front will be relatively stronger than the frontal passage Tuesday afternoon into Wednesday morning, and chances for small craft advisory level wind gusts of 21 kt or stronger have increased to 60-80% with this system. The highest probabilities are over the northern waters to the north of the Columbia River Bar. In fact, there is even a 15-25% chance for max wind gusts of 34 kt or stronger, as some guidance suggests a stronger area of low pressure is possible over the coastal waters with this system. However, the most likely outcome is for a relatively weaker low with peak gusts somewhere in the 20-25 kt range.

Seas will most likely range between 6 and 9 ft through Friday, but are not expected to be particularly steep at any point in time due to dominant wave periods of 10 to 13 seconds. However, there is a 15-20% chance seas will peak at 10 ft or higher late Thursday night into Friday. This would only occur if the aforementioned area of low pressure comes in stronger than what most guidance suggests. -23

PQR WATCHES/WARNINGS/ADVISORIES

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