textproduct: Portland

SYNOPSIS

The next seven days are highlighted by below average temperatures for this time of year with periodic chances for rain showers and a good deal of cloud cover. Most of the showery activity expected from Sunday onward will be focused over the mountains, with the bulk of rain falling in the lowlands today through Saturday. Note there is also a 15-25% chance of thunderstorms Saturday afternoon, however these would be isolated and short-lived with minimal impacts aside from brief heavy rain causing ponding of water in low-lying urban areas and the potential for a few flashes of lightning.

DISCUSSION...Friday through Thursday night

Most locations across northwest Oregon and southwest Washington have observed anywhere from 0.2-0.4 inches of rain over the past 12-24 hours with the passage of a frontal boundary, with some areas observing slightly below or above that range. Radar observations from early Friday morning showed scattered post-frontal rain showers ongoing across the area, while surface weather observations depicted an occluded front over the coastal waters beginning to move inland. This secondary frontal boundary is weaker than the first front that moved through the area yesterday into last night. That said, this front will help maintain light rain showers over the area today into tonight with the most frequent showers expected over the mountains. Given the showery nature of precipitation today and tonight, additional rain amounts will vary significantly from location to location, with some spots picking up only a few hundredths of an inch and others picking up over 0.25 inches. The lowest rain amounts are favored in the Willamette Valley to the west of the I-5 corridor where rain shadowing on the leeward side of the Coast Range is expected; these areas have only a 1-5% chance for over 0.25 inches of rain between 5 AM PDT Friday and 5 AM PDT Saturday. Meanwhile, the coast, coastal mountains, Cascades, and Cascade foothills have a 65-95% chance. With the excess cloud cover and showers in place today, expect high temperatures mainly in the 60s, expect 50s in the Cascades.

Forecast rain amounts increase for all locations from the coast to the Cascades on Saturday with temperatures remaining cool. The reason rain amounts increase on Saturday is because a closed upper level low currently centered off the B.C. coast will begin moving directly over the Pacific Northwest, bringing increasing moisture, instability, and lift. Forecast REFS ensemble mean soundings depict steepening mid-level lapse rates Saturday afternoon with surface-based CAPE values increasing to 200-300 J/kg and equilibrium levels increasing close to 20,000 feet. This suggests some heavier showers and isolated short-lived thunderstorms will be possible, and NBM thunder probabilities of 15-25% back this up. Given the weak environmental wind shear profile in place, the main hazard with stronger showers or thunderstorms would be brief heavy rain and ponding of water in low-lying urban areas. It is also worth mentioning that isolated non-mesocyclonic cold air funnel clouds are occasionally observed in this type of weather pattern, however the rotation with these typically fails to lower to the ground and thus damage does not occur. If rotation with one of these were to lower to the ground, it would be considered a weak landspout, and damage with these is extremely isolated if any occurs at all. Note probabilities for 24-hour rain amounts over 0.25 inches are higher on Saturday compared to Friday, peaking between 70-95% along and east of the I-5 corridor to the Cascade crest, 30-50% west of the I-5 corridor within the Willamette Valley, and 50-80% for the coast and coastal mountains. Locations that observe repeated heavy showers could see over 0.5-1.0 inches of rain, especially in the Cascade foothills.

Rain amounts trend much lighter on Sunday as the upper level low shifts eastward into Idaho. However, at least some light showers will linger over the area with moist onshore flow on the backside of the low, especially over higher terrain. Sunday will also be another cool day with a good deal of cloud cover and highs in the 60s across the lowlands. Cooler than average temperatures and chances for rain showers are in the forecast Monday through Wednesday as well given little change to the overall weather pattern. Note most of the showery activity next week should be confined to higher terrain. -23

AVIATION

Cool and moist onshore flow will maintain cloud cover and scattered rain showers across northwest Oregon and southwest Washington through 12Z Saturday, resulting in a mixed bag of flight conditions. For coastal locations, expect predominately IFR/LIFR conditions through 18Z-20Z Friday with conditions improving towards mainly MVFR thereafter. For inland locations expect a mixture of MVFR/IFR conditions, improving to high-end MVFR to low-end VFR around 20-23Z Friday. Expect occasional drops in visibility with passing showers, down to 2-3 SM at times.

KPDX AND APPROACHES...Expect ceilings to range between 800-1500 feet through 19-20Z Friday before lifting to 2000-3500 feet thereafter. Off-and-on rain showers will persist through 12Z Saturday, resulting in brief drops in visibility from time to time. Surface winds are expected to be out of the south to southwest, sustained between 5-10 kt with gusts up to 15 kt. -23

MARINE

An occluded front over the coastal waters was beginning to push inland early Friday morning, bringing an abrupt shift from southerly winds to westerly winds over the waters. By sunrise on Friday, westerly winds are expected over both the inner and outer waters with gusts up to 15-20 kt. Buoy observations from early Friday morning reported steep wind-driven seas around 5 to 7 ft and 5 to 6 seconds, steepest over the inner waters where a Small Craft Advisory remains in effect through 5 AM PDT Friday. However, seas are expected to ease quickly after 5 AM as wind speeds decrease behind the frontal passage. Buoy 089 backs this expectation up, reported seas of only 4 ft at 17 seconds. Therefore, an extension of the aforementioned Small Craft Advisory is not expected.

Winds and seas will increase again Saturday into Sunday as a pair of fronts move southward over the coastal waters and persistent northwesterly winds allow for the development of a fresh northwest swell upwards of 7-9 ft at 9-10 seconds, highest over the outer waters beyond 10 NM offshore. Given seas will become steeper and winds will gusts up to 20-25 kt over the outer waters, a Small Craft Advisory has been issued for the outer waters from Saturday morning through Sunday morning.

Although high pressure builds over the waters on Monday, northwest winds will persist and will maintain a fresh northwest swell with significant wave heights peaking near at least 8-10 ft. There is a 30-40% chance significant wave heights will peak over 10 ft beyond 30 NM offshore on Monday, mainly to the north of Cape Foulweather. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 5 AM PDT early this morning for PZZ210-251-252.

Small Craft Advisory from 5 AM Saturday to 11 AM PDT Sunday for PZZ271>273.