textproduct: Portland

SYNOPSIS

Our latest dry streak comes to an end today as a frontal system ushers in the return of widespread rain and breezy to locally gusty south/southwest winds. After the cold front passes to our east by Tuesday night, lingering showers persist through Wednesday with at least some mountain snowfall above pass level before tapering off by Thursday. High confidence in dry weather with a warming trend late week into the weekend. Chances increasing for another weather system late weekend into early next week, though the exact details of this event remain uncertain.

DISCUSSION...Tuesday through Monday

The much advertised weather system slated to impact NW OR and SW WA today is approaching the PacNW early this morning. The closed surface low is located over the eastern Pacific west of the OR/WA border. As the low has moved northeast aiming towards British Columbia, a warm front associated with the low has been moving north through the waters just off the coast. As of 2 AM, rain ahead of the warm front has begun pushing inland along the far OR and SW WA coast, and the front will graze the coast around Clatsop County and north over the next few hours. As the low continues approaching British Columbia later this morning, the cold front associated with this surface low will stretch south from western Canada to offshore of northern California and approach the coast. Widespread rain will spread inland ahead of and along the front as it pushes inland late this morning into the afternoon hours. Expect periods of moderate to locally heavy rain at times, mainly in midday and afternoon hours. Rain will decrease fairly quickly this evening after the passage of the cold front. Latest guidance indicates that the interior lowlands could be a bit rain shadowed, so total rain amounts have decreased a touch here. 24 hour rain amounts from 5 AM Tuesday to 5 AM Wednesday are around 0.15-0.40 inch for the interior lowlands, 0.5-1 inch along the coast, 0.5-1.5 inch for the Coast Range and Willapa Hills, and 0.5-1.25 inch for the Cascades.

Breezy winds will accompany the cold frontal passage today. While rainfall doesn't look particularly impactful, we'll have to keep a close eye on these winds, which are expected to peak along the coast between 8 AM to 5 PM and inland between 11 AM to 6 PM. The strongest winds will be along the coast, Coast Range, and Willapa Hills where the latest NBM continues to indicate a 65-90% chance for maximum wind gusts to exceed 40 mph. In terms of maximum wind gusts exceeding 50 mph, south of Tillamook county there's only a 5-10% chance, but those chances increase to around 15-40% north of this, especially for wind prone beaches and headlands. Model soundings indicate winds near the top of the boundary layer around 850 mb will peak around 50-65 kts ahead of and along the front. A strong stable layer below this will mainly prevent vertical mixing of these strong winds to the surface, though convection right along the cold front could provide just enough mixing at times to allow for some of these stronger winds to reach the surface briefly somewhere along the front. Essentially, most coastal locations will experience max gusts of 40-45 mph, but can't rule out a few gusts above 50-55 mph, especially for the north OR and SW WA coast. Shifting inland, probabilities for maximum gusts to exceed 40 mph are mainly less than 10%, except for 15-30% in the central Willamette Valley south of the Portland metro area to the Salem area. The most likely scenario here is peak wind gusts of 25-35 mph with some isolated gusts closer to 40 mph. Winds will also fall quickly behind the frontal passage by this evening.

A secondary upper-level shortwave axis moves eastward behind the front Wednesday into early Thursday while the region sits in a convective post-frontal airmass. Thus, showers will continue on Wednesday, most frequent over the Cascades and coast/Coast Range, before slowly tapering off Wednesday night into early Thursday. While elevated initially on Tuesday, snow levels finally drop below the passes early Wednesday morning when the heaviest of stratiform precipitation has ended, leading to limited snow over the Cascades. The NBM probability of at least 6 inches of snow over the Cascades (both OR and SW WA) is less than 5-15%, except for the highest peaks. Snow levels lower to 1500-2000 ft Wednesday night into early Thursday morning, with a 10% chance of snow levels falling near valley floor. However, precipitation probabilities along with amounts are so low by this point that even if some wet snow does mix in, it would not accumulate or lead to any impacts, especially as road surfaces would be too warm.

By Thursday afternoon, the shortwave moves east of the area with upper level flow becoming more zonal behind it. Generally zonal flow with slight ridging continues Friday into Saturday, bringing dry conditions with clearing skies. Expect a warming trend with daytime temperatures in the mid to upper 50s on Thursday, the low to mid 60s for Friday, and upper 60s/near 70 on Saturday/Sunday for the interior lowlands. Over the weekend, the NBM indicates a 30-50% chance for high temperatures to meet or exceed 70 degrees across the Portland metro area and Willamette Valley, with slightly higher probabilities (50-65%) south of Salem near Eugene.

Both deterministic and ensemble guidance indicate a deep Pacific trough moving east through the Pacific and approaching the West Coast later on Sunday into Monday, bringing increasing chances of precipitation. However, this far out, there is quite a bit of uncertainty in regards the speed/amplitude of the incoming feature. Latest model runs indicate a better chance for rain to begin later Sunday into Monday as opposed to earlier on Sunday. This uncertainty has lead to a large NBM 25th-75th percentile temperature spread, showing highs from the upper 50s to the mid 70s are all in play for the interior lowlands Sunday afternoon. Should ensemble guidance trend even slower with the aforementioned trough, Sunday may end up being the warmest day of the upcoming late week/weekend time period. We'll have to see how forecast solutions converge in the coming days. -03/99

AVIATION

A frontal system approaching the airspace is producing rain along the north OR and southwest WA coast. As a cold front swings southeast, rain will become widespread along the coast and push inland by 14-16z Tue with periods of moderate rain between 18-23z Tue. VFR conditions transition to MVFR cigs and vis after 15-18z Tue along the coast and 18-21z Tue for inland terminals with the heaviest rain. Additionally, there are increasing chances for coastal terminals to experience a period of IFR conds from 18z Tue to 01z Wed. Conditions expected to improve to VFR for inland terminals after the frontal passage, generally after 01-03z Wed. There's a 30-60% chance MVFR conditions continue at coastal locations into Wednesday morning.

Light and variable winds will increase quickly as the front approaches, after 13-16z Tue along the coast and 17-20z Tue inland. Peak wind gusts will likely occur between 18-23z Tue with southerly wind gusts up to 35-40 kts along the coast and 25-30 kts inland. There is also the potential for LLWS at inland terminals beginning around 14z Tue until surface wind gusts increase as there will be south- southwesterly winds of around 40-50 kt at 2000 ft. While LLWS will mostly be speed shear, directional shear may be present initially, especially for terminals around the Portland metro area.

KPDX AND APPROACHES...Rain begins around 13-15z Tue with moderate rain at times between 17-23z Tue. VFR conditions deteriorate to MVFR between 17z Tue to 02z Wed. Light and variable winds will become southeasterly and increase early through the morning, with sustained winds around 10 kt by 15z Tue and gusting up to 20-25 kt by 18z Tue. Increasing south- southwesterly winds aloft (40-50kt around 2000 ft) will introduce LLWS beginning 14-15Z Tue. While this LLWS is expected to be mostly speed shear, the frontal passage may bring some directional shear. -03

MARINE

A strengthening surface low is moving northeast through the eastern Pacific towards British Columbia this morning. A warm front associated with the low is moving north through the waters, bringing southerly winds. The associated cold front will move through the waters later this morning through the afternoon, producing gale force wind gusts. Widespread gusts of 34 kt or greater are expected across all waters, with a 40-60% chance of peak gusts reaching 45 kt. A Gale Warning remains in effect from 5 AM to 5 PM today for all coastal waters, including the Columbia River Bar.

Seas are also expected to build today in response to the increasing wind waves followed by a westerly swell behind the frontal passage. Combined seas will likely build to at least 13 to 15 ft, peaking by late this afternoon into this evening. Chances for seas exceeding 18 ft remain low (5% or less). Southwest to westerly onshore winds with gusts up to 20-25 kts at times will continue behind the frontal passage through Wednesday, while seas are expected to gradually subside to around 9 to 12 ft by Wednesday morning. A Hazardous Seas Warning has been issued for zones PZZ271, 251, and 210 and a Small Craft Advisory for zones PZZ272, 273, 252, and 253 beginning at 5 PM tonight after the Gale Warning ends for a combination of elevated winds and seas. High pressure then rebuilds over the waters later Thursday into Friday returning northerly winds. -03/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Gale Warning until 5 PM PDT this afternoon for PZZ210-251>253.

Hazardous Seas Warning from 5 PM this afternoon to 11 PM PDT this evening for PZZ210-251-271.

Small Craft Advisory from 5 PM this afternoon to 11 PM PDT this evening for PZZ252-253-272-273.

Gale Warning until 5 PM PDT this afternoon for PZZ271>273.