textproduct: Portland

SYNOPSIS

Wet conditions continue through Thanksgiving as another frontal system impacts the region bringing additional widespread rain and breezy winds. Conditions gradually trend drier Friday into Saturday with showers largely pinned to higher terrain features. Snow levels drop to the passes by Saturday but limited accumulation is anticipated. A cooling trend impacts the region Sunday into early next week with additional chances for precipitation arriving on Tuesday.

DISCUSSION...Now through Tuesday night

Current radar and satellite observations show our current swath of rainfall slowly shifting northward late this afternoon with the south Willamette valley beginning to see a break from the rainfall. This northward trend in the precipitation continues into the evening/overnight hours, and confidence is high we'll experience a region wide lull in the rainfall until the next frontal system arrives from the west Thursday morning. Both ensemble and deterministic guidance continue to indicate that the center of this low pressure system likely makes landfall somewhere just north of Astoria, OR along the Washington coastline Thursday evening. Rain amounts with this system are expected to somewhat similar to the previous front with 0.2-0.45 inch for the central and southern Willamette Valley, 0.4-0.75 for the northern Willamette Valley into the SW Washington lowlands, 0.5-1.5 inches along the coast and Coast Range, and Oregon Cascades south of Clackamas County, and 0.8-2 inches for the north Oregon and SW Washington Cascades. Snow levels will remain just above pass level with this second system as well.

Winds are also expected to increase as the front and low pressure center move inland. Expect winds along the coast to begin increasing Thursday morning, then inland areas Thursday afternoon as the aforementioned low finally onshore tightening pressure gradients near the surface. There are some discrepancies between the various sets of guidance as to just how breezy winds will get with the NBM markedly lower than high resolution ensembles like the HREF and REFS. Given that winds aloft are expected to be elevated (~40-50 knots at 850 mb) with neutral to slightly unstable conditions present in the boundary layer to facilitate vertical mixing Thursday afternoon, nudged wind/wind gust values towards the high resolution ensemble means. With this in mind expect peak gusts of 35-45 mph along the coast and Cascades crests, and up to 25-38 mph for inland valleys (highest Salem to Portland). However, it's worth highlighting there is a 15-25% chance for peak gusts to exceed 45 mph in the north Willamette Valley into the Portland/Vancouver Thursday late afternoon and evening which would cause additional impacts. We'll need to watch the progression of this low pressure closely. High confidence winds decrease Thursday night into Friday.

The weather on Friday into Saturday looks pretty benign other than shower chances continuing. Ensemble guidance is in good agreement of a strong upper ridge beginning to build in the eastern Pacific through the weekend. A good number of ensemble members and their deterministic counterparts continue to suggest a weak shortwave will move along the flow aloft over the PacNW on Saturday, which is stretching out the chances for precipitation. However, there still remains decent uncertainty in the exact location of the shortwave, whether it will move over western WA and OR, or more to the east in eastern WA, OR, and into Idaho. This will affect exactly how much cooler air will funnel into NW Oregon and SW Washington. If the wave is more to the west, more colder air will move into NW Oregon and SW Washington. If the wave is more to the east, the coldest air will remain east of the Cascades. Latest NBM guidance indicates cooling Sunday morning with Monday morning being the coldest. Sunday morning has a 20-50% chance of low temperatures falling below 32 degrees with probabilities increasing to 40-80% for Monday morning. The highest probabilities are in typical colder spots outside of urban areas, especially in the Willamette Valley between Corvallis and Eugene. One thing to note is probabilities of sub-freezing temperatures around the Portland metro area are only around 10-25% for each day. When looking at the probability of temperatures falling to 25 degrees (Cold Weather Advisory criteria), most lowland locations are below 10%, though the cold pocked in the central/south Willamette Valley mentioned earlier has around a 15-25% chance.

By Tuesday, ensembles are in agreement that a slightly stronger shortwave and associated frontal system will slide south from western Canada into the PacNW, breaking down the high pressure over the region and returning precipitation chances over the area. Quite a bit of uncertainty exists in the strength of this system and the impacts associated with it, though cold weather could stick around in the wake of it. -99/03

AVIATION

A warm front lifting north will bring a brief break in the rain overnight. But, weak ridging aloft and clearing skies are allowing for fog and low stratus to develop across much of the Willamette Valley this evening. As of 04z Thursday, LIFR conditions have developed from KEUG to KUAO and will likely persist through 10-12z. Expect similar conditions to develop after 06z across northern portions of the Willamette Valley as the higher clouds and light drizzle push farther north. An approaching cold front will push another round of rain onshore around 09z Thu, spreading inland early Thu morning. Expect conditions to trend toward predominately MVFR through Thursday evening. Southerly winds are also expected to increase as the low pressure system moves closer to the south Washington coast on Thursday, with wind gusts up to 35 kt along the coast and up to 25 kt inland by late Thu afternoon.

KPDX AND APPROACHES...As of 04z Thu, light rain ending as the warm front lifts north. Clearing conditions will likely be brief as low stratus and possibly fog develop. But, breezy east winds near KTTD could potentially maintain VFR conditions overnight. Guidance suggests around a 40% chance of MVFR CIGs at the PDX terminal through 10-11z, before the probabilities increase toward 80% with the return of rain by around 11-12z. Southeast winds around 8-10 kt expected to increase through Thursday, with gusts up to 25 kt later Thu afternoon and evening. /DH

MARINE

A warm front will continue to lift north overnight while southerly winds increase across the coastal waters tonight through Thursday. A 996 mb low pressure system currently located about 450 miles west of the mouth of the Columbia River will move toward the south Washington coast through Thursday evening. An initial cold front is expected to move across the coastal waters Thursday morning, with isolated wind gusts up to 35 kt. More widespread gales are now expected behind the front, with south to southwesterly winds increasing through Thu afternoon, with wind gusts up to 40 kt likely. Seas will also increase, becoming steep and hazardous through Thu evening, peaking around 15 to 18 ft at 11-13 seconds late Thu afternoon through Thu evening. There remains around a 10% chance that significant wave heights reach 20 ft. Gale warnings are in place for all waters later Thu morning into Thu afternoon, and transition into hazardous seas to cover the large fresh swell that will move into the waters later Thursday. Conditions ease late Thursday night into Friday as the low weakens, with seas expected to fall below 10 ft by Friday afternoon. Hazardous conditions are not expected through the weekend as weak high pressure builds over the waters. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 1 AM to 11 AM PST Thursday for PZZ210- 251>253.

Gale Warning from 11 AM to 6 PM PST Thursday for PZZ210-251>253.

Hazardous Seas Warning from 6 PM Thursday to 2 AM PST Friday for PZZ210-251>253-271>273.

Small Craft Advisory until 7 AM PST Thursday for PZZ271>273.

Gale Warning from 7 AM to 6 PM PST Thursday for PZZ271>273.