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. The best chances for widespread rain showers are this afternoon through Saturday evening with a 15-25% chance of thunderstorms Saturday afternoon. Most of the showery activity expected from Sunday onward will be focused over the mountains.

DISCUSSION...Friday afternoon through Thursday night

Visible satellite imagery from early Friday afternoon show widespread stratus has dissipated for much of the area east of the coast with cumulus clouds associated with developing showers taking precedence. A stationary front that has been situated off of the west coast since late last night is finally beginning its push eastward. The forcing from this front along with daytime heating with the gaps in the clouds is allowing for more convective showers to form across the region. Model soundings indicate these showers will be pretty shallow, though instability could improve to around 100 J/kg this afternoon, allowing some showers to become stronger and produce periods of heavier rain and possibly a lightning strike or two. 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. Expect high temperatures mainly in the 60s today, except 50s in the Cascades and along the coast.

Forecast rain amounts increase for all locations from the coast to the Cascades on Saturday with temperatures remaining cool. The surface front will pass through the region tonight, and a closed upper level low currently centered off the B.C. coast will begin moving directly over the Pacific Northwest Saturday, 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. General longwave troughing over the western US continues Monday through Wednesday, keeping temperatures cooler than average temperatures with shower chances continuing, mainly confined to the mountains. -23/03

AVIATION

Cool/moist onshore flow, cloud cover, and scattered rain showers will continue across the airspace through at least 18Z Sat, bringing a mixed bag of flight conditions.

For coastal terminals, conditions will vary by location, with current VFR/MVFR conditions at KAST expected to improve to persistent VFR by 06-12Z Sat, and current IFR/LIFR conditions at KONP expected to improve to MVFR/IFR conditions by 06-12Z Sat. For inland terminals, most terminals have improved to VFR as of 20Z Fri, so expect widespread VFR for inland terminals by 00Z Sat. Passing showers may cause occasional drops to 2-3 SM visibility at times during the TAF period. Conditions may deteriorate again after 12Z Sat, with chances of MVFR CIGs increasing to 20-30% at inland terminals and IFR/LIFR chances increasing to around 20% at coastal terminals. Westerly to southwesterly winds at or below 10 kt through the TAF period. May see afternoon gusts up to 20 kt along the coast, notably at KAST.

KPDX AND APPROACHES...VFR conditions through at least 9-12Z Sat. There is a 20-30% chance of MVFR CIGs after 09Z Sat. Off and on showers through at least 18Z Sat may cause brief drops in visibility from time to time. Southerly winds will shift more southwesterly this afternoon/evening, with sustained winds between 5-10 kt. Gusts up to 18 kt possible until 02Z Sat. ~12

MARINE

Winds and seas will increase 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 night.

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 8-11 ft. There is a 20-40% chance significant wave heights will peak over 11 ft beyond 30 NM offshore on Monday, mainly to the north of Cape Foulweather. ~12/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 5 AM Saturday to 11 PM PDT Sunday for PZZ271>273.