textproduct: Portland

SYNOPSIS

High pressure will maintain dry weather across NW Oregon and SW Washington through tonight. On Sunday, a weak front will move over the area bringing another round of light rain. A pair of quick moving frontal systems associated with subtropical moisture will bring additional rounds of heavier rainfall and river rises, though probabilities for exceeding minor flood stage at the majority of rivers remains around 20-25% or less. Breezy to gusty southerly winds are also expected with the strongest winds likely Tuesday night. Onshore flow continues with showers on Wednesday. Lowering snow levels will allow for snow to fall at pass level in the Cascades. Precipitation continues through late next week, with potential for another strong system on Thursday.

DISCUSSION...Tonight through Friday

Water vapor satellite imagery this afternoon reveals an upper level ridge situated over the Pacific Northwest, with the main ridge axis shifting east of the Cascades. Visible satellite imagery shows widespread stratus through much of the Willamette Valley, but showing indications of scattering out, especially along the fringes with skies becoming mostly sunny this afternoon. There is a chance some areas in the central valley may remain socked in throughout the day. With the surface inversion in place, also seeing warmer temperatures into the mid-60s at higher elevations in the Coast Range and foothills of the Cascades. Good day for a hike!

Increasing clouds by Sunday as a weak front approaches the coast early Sunday morning. The upper shortwave trough quickly lifts north into British Columbia, removing much of the synoptic support for this system, and allowing for the front to decay and dissipate as it pushes inland. Rainfall amounts are not expected to be overly impressive on Sunday, with most likely amounts ranging from 0.1-0.4 inch along the coast and Coast Range, while amounts will likely range from a trace up to a quarter inch inland, highest amounts in southwest WA and lowest to the south.

More active weather looks to return by Monday, and continue through much of next week. A broad upper level trough will persist over the Pacific as strong low pressure systems spin in the Gulf of Alaska through late next week. A strong jet stream, associated with subtropical moisture at times is expected to be directed toward the Pacific NW during this time. The first system next week will be associated with IVT values peaking around 750-1000 kg/m/s, which would place it in the strong atmospheric river category. But, what makes this different from last week's AR event is both a shorter duration and a more southwest orientated flow. Ensembles are suggesting the initial surge of moisture Sunday night into Monday morning likely remains north over Washington, before the front quickly pushes through NW Oregon, followed by more showery conditions by Monday afternoon. Most likely rainfall totals in coordination with the WPC through Monday evening has decreased somewhat: around 0.25-0.75 inch within the interior lowlands, and from 0.75 up to 2 inches along the coast and Coast Range, and potentially up to 2.5 inches in 24 hours within the Cascades. The highest probabilities (50-70%) for exceeding 2 inches of rainfall are in the higher terrain of SW Washington and the northern Oregon Coast Range, but chances of exceeding 4 inches is less than 10%. The main concern with this pattern remains on rivers that are running high from last week's rainfall.

Another concern with this first system on Monday will be potential impacts from winds. South to southwesterly winds will likely increase ahead of the system late Sunday night into Monday morning. Wind gusts are most likely to peak along the coast around 40-50 mph and inland around 25-35 mph, though there is around a 10-20% chance of wind gusts exceeding 40 mph through the Willamette Valley. The combination of saturated soils and the strongest wind gusts could produce isolated downed trees and power outages.

The rain is still expected to ease, with scattered showers likely through Tuesday morning. But, the next frontal system arrives later Tuesday, bringing another round of rain and wind. Again, this frontal system appears to move fairly quickly across the area Tuesday night. While IVT values are modeled to be slightly lower, potentially peaking around 500-750 kg/m/s (moderate AR category), total rainfall amounts are likely to be similar to Monday's system. Models and their ensembles are indicating this system may be more dynamically driven as a stronger shortwave trough and associated vort max slide across the region. This also signals to potential for stronger, and perhaps more impactful winds across the area Tuesday night. Probabilities for wind gusts exceeding 40 mph have increased to around 50-60% through the southern and central Willamette Valley. Snow levels are expected to drop significantly by Wednesday morning with the passage of the upper trough. This will allow for snow to begin accumulating at pass level on Wednesday. The weather setup on Wednesday will feature onshore flow with zonal flow aloft, likely maintaining showers enhanced by orographic effects. There remains quite a large spread of uncertainty for snowfall amounts through Wednesday night, though probabilities for exceeding 6 inches is around 50%, and 10-30% for exceeding 12 inches in 24 hours. (The NBM's 10th and 90th percentiles at Santiam Pass are currently sitting at 1 inch to 20 inches!) So, if you plan on traveling across any of the Cascades passes next week, keep an eye on updated forecasts.

As the synoptic pattern progresses into late next week, uncertainty also remains fairly high. The GEFS and ECMWF ensembles, which were quite different just 24-hour ago, have come closer together, but the individual members remain wildly separated. It is likely (>90-95% chance) the rain will continue Thursday and Friday, but rainfall amounts are quite uncertain. That said, a stronger atmospheric river appears likely to set up somewhere along the west coast late next week, with the highest probabilities farther south near the Oregon- California border. But if that were to shift north, we could see much higher rainfall amounts impacting our forecast area.

AVIATION

Latest satellite imagery continues to highlight valley fog and low stratus, with largely IFR cigs but unrestricted vis at central and southern Willamette Valley terminals. To the north in the northern Willamette Valley, Portland-area, and inland along the Lower Columbia/Cowlitz Valleys, skies have mostly cleared. Increasing diurnal mixing will continue to improve conditions with prevailing VFR favored across the region by 21-24z Sat, however there remains a 30-40% chance that MVFR or worse stratus will persist generally from KSLE to the south. Along the coast, light offshore winds have maintained clear skies with VFR expected to continue through much of the period, while winds increase out of the south with gusts to 20 kt.

Continued clear skies ahead of a frontal system approaching the region early Sunday morning may allow for additional radiation fog to develop tonight, particularly if low clouds and rain showers along the front are delayed. More likely, MVFR cigs with light rain will arrive from the northwest by 09-15z Sun for most.

KPDX AND APPROACHES...Remaining low cloud cover continues to erode as diurnal mixing increases, with VFR conditions favored through at least 09-12z Sun. Largely southeast winds at around 5 kt expected through the period. Light rain showers and MVFR cigs along a front may arrive by 12z Sun. -36

MARINE

A ridge of high pressure shifting eastward away from the region through tonight will see tranquil conditions today give way to rain showers and increasing southerly winds on Sunday as a weak front tracks over the waters. Small Craft Advisories remain in effect from tonight through Sunday evening for wind gusts of 20-25 kt while seas rise to 7-10 ft.

A stronger system looks to affect the coastal waters on Monday. Ahead of a strong cold front, a tight pressure gradient will favor widespread southerly gales from late Sunday evening through Monday afternoon while a building west-southwesterly swell will see seas rise above 15 ft by Monday morning. While there is high confidence in gales, and a Gale Warning is now in effect from Sunday night through Monday afternoon, there remains a 25-45% chance in at least isolated storm-force gusts around 50 kt Monday morning across the inner waters, and 5-25% chance across the outer waters. Meanwhile, seas most likely build to 16-18 ft by Monday afternoon. Chances for seas above 15 ft are high, 65-95%, beyond 10 NM and north of Cascade Head, and less so to the south and east. Chances seas reach 18 ft or greater are only 15-35% beyond 30 NM north of Cape Falcon, and less than 10% elsewhere.

The weather pattern then looks to remain very active through the middle and latter portions of next week. Seas are very likely to remain elevated above 10 ft (greater than 90% confidence), and possibly (50% chance or higher) above 15 ft on Wednesday and Thursday as another potent weather system moves onshore. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 10 PM this evening to 1 AM PST Monday for PZZ210-253-273.

Gale Warning from 1 AM to 4 PM PST Monday for PZZ210-253-273.

Small Craft Advisory from 7 PM this evening to 10 PM PST Sunday for PZZ251-252-271-272.

Gale Warning from 10 PM Sunday to 4 PM PST Monday for PZZ251-252- 271-272.