textproduct: Portland

SYNOPSIS

Light widespread rain today transitions to isolated showers in the terrain and far northern Oregon/SW Washington on Saturday. Mild temperatures near to slightly above seasonal normals over the weekend. Colder air moves in from the north early next week, bringing low chances for low elevation rain/snow mix and light to moderate Cascade snow. Then, a non- impactful atmospheric river takes aim at northern Oregon and Washington Wednesday into Thursday, pushing out the cold air and bringing the return of widespread rain.

DISCUSSION...Friday afternoon through Thursday

Radar imagery Friday afternoon shows a band of light rain moving through much of NW OR and SW WA as a weak front approaches the coast. Although high pressure is set up over the eastern Pacific, the Pacific NW remains on the eastern periphery of the flow, which is allowing moisture to continue streaming into the area from the north into the weekend. Once again daytime temperatures will be below normal for early March, peaking right around 50 degrees for the interior lowlands, low 50s along the coast, and low 40s to upper 30s over the Cascades. As high pressure over the eastern Pacific spreads east into the Pacific NW tonight into Saturday, the front will push to the north, also pushing precipitation chances north. Mostly dry weather is expected for Saturday, though can't rule out a few stray showers along the coast, Cascades, and inland locations north of Highway 20, especially in SW WA. Temperatures will also warm around 8-10 degrees on Saturday, peaking in the upper 50s to low 50s for the interior lowlands, just slightly higher than normal (which is in the mid 50s). A low pressure system moving south from Alaska along the northern Canadian coast will cause the upper level flow to become zonal over the PacNW on Sunday. This will allow a weak front to push south through the Pacific NW on Sunday along with another round of widespread light precipitation. Temperatures will lower around 5 degrees, peaking in the mid 50s for the interior lowlands.

A bit of a pattern change is expected early next week due to cold air intrusion into the region, leading to the potential for low elevation snow. Ensemble guidance remains in good agreement that cold air from Canada will funnel into the PacNW behind the front, especially as the upper flow continues deepening slightly Monday into Tuesday. About 75% of the GEFS, EPS, and GEPS members indicate 850 mb temperatures falling to -4 to -6 degrees C or colder by early Monday morning at PDX with nearly 80% of the members falling to -6 to -8 degrees C or colder Monday night into early Tuesday. Additionally, precipitation chances continue Monday into Tuesday as moisture continues streaming over the eastern Pacific ridge, and a weak shortwave along the flow pulls that moisture south into the region. Daytime temperatures each day will be well above freezing in the lowlands, so the main time period for any lowland snow potential is in the late overnight to morning hours when temperatures are the coldest.

Snow levels will likely fall to around 1500-2500 ft by Monday morning as the cold air pushes into the region. For elevations lower than this, on Monday morning, there's only a 10-25% chance of low temperatures falling to freezing for elevations below 500 ft, though there is a 25-50% chance for elevations above 500 ft. However, precipitation chances are expected to be showery and light, leading to low probability (less than 10% chance) for any accumulating snow. Coldest temperatures are expected late Monday night into Tuesday morning with a 50-70% chance of temperatures falling below freezing for elevations below 500 ft except for the Portland metro area (not including Vancouver), where probabilities are closer to 10-25%. Above 500 ft, probabilities increase to 70-90% chance. However, there are also lower chances for precipitation with light accumulations during this time, except for in the Cowlitz Valley. This is leading to only a 5-20% chance of 0.1 inch of snow for the Portland/Vancouver metro area and south, except for up to 30% chance for elevations above 500 ft. Chances increase to 30-45% for the Cowlitz Valley. Probability of 1 inch of snow is now less than 5% for locations below 1000 ft, except for 10-20% in the Cowlitz Valley. The other thing to take into account is road temperatures will likely be on the warm side since overnight temperatures will be in the 40s leading up to early next week. This will make it difficult for snow to accumulate on roadways as it will likely melt on contact. Ultimately, the most likely scenario is no accumulating snow below 500-1000 ft, though some snowflakes or slushy rain may be observed.

Higher chances for snow are expected for elevations above 1000-1500 ft Monday into Tuesday, including the Coast Range, Willapa Hills, Cascade foothills, and Cascades. However, limited precipitation amounts lead to limited snow forecast over the majority of these areas. At this point, accumulating snow looks to be mainly for locations north of Highway 20, with a 40-60% chance of snow reaching 6 inches in 48 hours ending at 5 AM on Wednesday. The far northern Oregon Cascades and SW Washington Cascades are forecast to receive more precipitation. These locations have an 80-95% chance of at least 6 inches of snow and a 60-80% chance of at least 12 inches of snow in the same 48 hour timeframe.

On Wednesday, moisture associated with a non-impactful atmospheric river will begin pushing into the region, pushing out the colder air mass as widespread precipitation returns to the region. Quite a bit of uncertainty remains in the timing of both the steady precipitation initiation as well as when the colder airmass begins warming. This is leading to a 20-40% chance for morning low temperatures below freezing for lowland locations outside of the Portland metro area (which has a less than 10% chance). This is once again leading to the potential for a rain/snow mix for some locations below 1000 ft, but similar to Monday, there is low chance for accumulating snow at the lower elevations. As the warmer air associated with the atmospheric river moves inland, snow levels are expected to rise above 4500-5000 ft by Wednesday evening. Additional moderate to heavy snow is possible in the SW Washington Cascades Thursday into Friday, but not much else south of this.

In terms of other impacts, rain amounts will be steady, but flooding is not expected with this event. Current 48 hour rain totals ending 5 AM Friday are 0.5-1.5 inches for the interior lowlands and 0.75-2.5 inches for the coast, Coast Range, and Cascades. Higher rain amounts in general are expected to be north of Highway 20 as current guidance suggests the atmospheric river will aim towards far northern Oregon and Washington. Additionally, winds could get a little breezy at times, though potential for impactful winds has gone down. There's now only a 5-20% chance for wing gusts of at least 35 mph for inland valleys and 20-35% chance for wind gusts of at least 40 mph for coastal communities. Next week is shaping up to be active, so keep an eye on the forecast! -03

AVIATION

Moist, north to northwest flow aloft continues today as a weak front pushes across the region. This will maintain light, steady rainfall at local terminals while conditions are expected to gradually deteriorate through the day and into tonight. Expect conditions to continue to trend toward predominately MVFR this afternoon as the front continues to push inland and CIGs lower. There is around a 20-40% chance of IFR CIGs after 22-00z at interior lowland terminals due to the potential for both lowering CIGs and reduced VISBY. Higher chances (50-60%) for IFR along the coast after 00-03z Saturday. Winds generally remain southerly around 5-10 kt.

KPDX AND APPROACHES...As of 21z, MVFR conditions are in place with CIGs around 1500 ft and light rain reducing vsbys slightly. Expect these conditions to persist near the lower end MVFR, with around a 20-30% chance of IFR conditions after 22z this afternoon. Southerly winds around 4-7 kt expected to persist through the period. /DH

MARINE

A weak cold front will push across the coastal waters today, bringing southwest to westerly winds with gusts up to 15 kt. A persistent northwesterly swell will maintain seas at around 6 to 8 ft at 10-11 seconds through Saturday. Another weak front will drop across the coastal waters on Sunday, with northwest winds behind the front likely producing gusts up to 20 kt, though there is about a 30-50% chance that winds reach Small Craft Advisory criteria, highest across the northern waters. The northwest swell is also expected to increase slightly, building seas to around 8 to 10 ft into Monday. A progressive weather pattern continues, with the potential for a stronger system arriving to the coastal waters by the middle of next week. At this time, there is around a 40-50% chance for Gale force wind gusts on Wednesday. /DH

PQR WATCHES/WARNINGS/ADVISORIES

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