textproduct: Portland

SYNOPSIS

High pressure will maintain dry weather across northwest Oregon and southwest Washington through Saturday. Potential for fog development continues across the Willamette Valley tonight into Saturday morning. On Sunday, a weak front will move over the area bringing another round of light rain. Monday to Tuesday, a quicker moving atmospheric river will bring another round of heavier rain and increased chances for river flooding. Rain persists throughout the entire week with another stronger system arriving mid to late next week.

SHORT TERM...Today through Sunday night

Satellite imagery as of early Friday afternoon shows lingering low stratus across the central Willamette Valley, Tualatin Valley, and southwest Washington. Some of the low stratus is trying to burn off, but there is low confidence it completely goes away considering the lower sun angle and limited daytime heating we have. However, some lucky folks across the Portland/Vancouver Metro and southern Willamette Valley cleared out late this morning, leaving behind some brief afternoon sunshine. Conditions remain dry as high pressure from an upper level ridge builds over the Pacific Northwest.

Areas that clear this afternoon will be more susceptible to fog development tonight into early Saturday afternoon. Efficient radiational cooling, calm winds, and a moist surface will support potential fog development across the Willamette Valley this evening through mid-morning Saturday. Based on high resolution guidance, there is a 50-80% chance for visibilities falling below 1/2 mile across the Valley, with the highest chances in the southern Willamette Valley (our typically foggiest spot). Will note that any locations that don't clear out this afternoon and maintain low stratus will likely maintain the stratus deck through tonight rather than develop fog. An inversion aloft from the high pressure will keep the clouds low. Guidance suggests most of the fog and low stratus will dissipate by early Saturday afternoon as the upper level ridge moves over head, making way for some brief afternoon sunshine.

Saturday night into Sunday, the next frontal system will move into the Pacific Northwest, returning clouds and chances for rain. Snow levels briefly fall to 6000-7000 feet, so it's still looking like rain will be falling at pass-level with this system. Rainfall totals are not expected to be nearly as robust as we saw earlier this week. 24 rainfall totals will be highest along the coast with the most likely scenario favoring around 0.25-0.75 inches on Sunday. There is a 10% chance of rainfall totals closer to 0.75-1.25 inches. If these rain amounts are received, hydrological concerns are heightened for coastal rivers. For inland sites, locations north of Salem have a 10% chance of seeing rain as high as 0.5 inch while locations from Salem southward to Lane County have a 10% chance of seeing up to 0.30 inch. Will note that this system is taking a northerly track which is keeping the central and southern Willamette Valley and central Oregon Cascades drier than other locations more north. -10/27

LONG TERM...Monday through Thursday

While this weekend looks to be drier, it's not time to put away your rain boots as yet another strong series of persistent atmospheric rivers are in store. Confidence is increasing that the first stronger frontal system will arrive Monday into Tuesday. The upper level synoptic pattern is showing the frontal system taking a northerly track with a broad ridge still present aloft over much of the west. This means that we may be on the southern edge of the main rain band on Monday. But don't let that sway you - we still have the potential to be very wet with impactful rainfall totals. The coast and southwest Washington will be the most susceptible to the heaviest rain. Current rain forecast from 4 AM Monday to 4 AM Wednesday shows around 1-2 inches along the coast, 1.5-3.5 inches of rain across the Coast Range and Willapa Hills, and 2.5-4.5 inches across the Cascades. Now if the frontal system shifts northward precipitation totals will be much less ranging from 0.75-2 inches along the coast with up to 1-2.5 inches across the Cascades (10% chance for this drier scenario). If the system makes a direct hit though, we could see rain totals around 2.5-5 inches along the coast and 3.5-5.5 inches across the Cascades (10% chance for this wettest scenario) along the Cascades. 48 hour rainfall totals ending Wednesday at 4 AM for 2 inches or more of rain is 20-30% along the coast, around 50-70% across the Coast Range and Willapa Hills, and 70-80% across the Cascades. The Willamette Valley is around 10-20% chance, and the valleys of southwest Washington around 30-40%.

Another concern with this first system on Monday and Tuesday are winds. Winds gusts greater than 40 mph are likely coast at 80%+ chance, near 60-70% across the Coast Range, and 20-30% for interior lowland valleys chance (breeziest on Tuesday). The southwesterly flow though is a more gusty wind prone scenario for coastal areas and the interior valleys so it is something to watch for. The combination of yet another wet system with gusty winds will increase probability for downed trees.

While the Monday and Tuesday system look to be the most robust at this time, we aren't out of the water so to speak as there will be wave after wave of additional precipitation Wednesday through the end of the week. At this point confidence is incredibly low as it will greatly depend on the overall flow. There appears to be around a 50-60% chance of IVT values greater than 500 kg/m*s. This value is a fair representation for the potential of moisture transport. From an impact perspective, it will be a very wet week with each day after Monday offering the potential for impactful rainfall.

There may be some positive signs next week for snow and winter sport lovers. Snow levels drop behind the Monday-Tuesday system, potentially bringing snow back down to pass-level. NBM chances for a foot of snow or greater in 48 hours from 4 AM Tuesday to 4 AM Thursday are 40-50% through Highway 26 and Santiam Pass, and 20-30% through Willamette Pass. These chances have fallen since yesterday. Will note that if the system early next week shifts northward, then we'll end up on the warm side of that system and snow levels will rise significantly. -10/27

AVIATION

An upper level ridge of high pressure continues to shift over the region today with relatively light westerly flow aloft. Low stratus continues to persist across much of the central Willamette Valley and into the Tualatin Valley. IFR conditions are expected to persist at most inland terminals. There is at least a 50% chance that CIGs remain MVFR or lower through 04z, before conditions are expected to deteriorate again tonight. The main exceptions are near the fringes of the stratus deck, including KTTD-KDPX and KEUG, which are currently VFR with scattered clouds. Another round of IFR to LIFR fog and/or stratus is favored tonight into Saturday morning.

KPDX AND APPROACHES...The stratus has scattered out enough this afternoon, with predominately VFR conditions expected to persist through at least 04z Saturday. Stratus is likely to redevelop overnight with chances for IFR conditions increasing after 08z Saturday. Similar to today, the stratus will again likely break up by Saturday afternoon, trending toward VFR after 20z Sat. Winds expected to remain light less than 5 kt. /DH

MARINE

High pressure over the coastal waters will maintain relatively calm conditions through Saturday morning. Winds remain 10 kt or less while seas hover around 4 to 6 ft. Winds become southerly by Saturday afternoon as a decaying front approaches the coastal waters. Southerly winds increase Saturday night into Sunday with wind gusts likely (70-90% chance) exceeding Small Craft Advisory thresholds (22+ kt). Expect southerly winds to remain elevated through Sunday. Seas build slightly to around 7 to 8 ft through Sunday evening due to the increased wind chop.

A much stronger front moves into the waters late Sunday night into Monday, bringing increasing winds and hazardous seas. There remains high confidence for southerly gale force wind gusts on Monday. Probabilities for wind gusts exceeding 48 kt (storm force) remain low, around 20%, except near 30-40% across the inner waters if a low level coastal jet were to set up. Steep and hazardous seas are expected to build through Monday afternoon and evening, likely peaking in the mid to upper teens. There is a 10% chance of significant wave heights exceeding 20 ft. Seas are expected to remain elevated through the middle of next week as an active weather pattern continues. /DH

HYDROLOGY

Minimal changes to the flood headlines. There continues to be two rivers currently at and forecast to be at minor flood stage through at least the 13th of December. The Pudding River at Aurora and the Cowlitz River at Kelso will remain elevated for at least the next 24 hours, and then will slowly fall below flood stage. However, confidence in the easing of the rivers is low due to the incoming rain and how they have responded. The Cowlitz for example may fall just below minor flood but it's proximity to that stage will prompt the warning to remain until further notice. The Pudding, in a similar way, is forecast below but on the upper end of action stage. Therefore there is a chance that this river specifically may have the warning extended.

Looking forward to the next week, there continues to be the threat for flooding due to swollen rivers, saturated soils, and the potential for more robust rain. It won't take much to get our rivers back into action or flood stage. There still remains uncertainty with exactly which rivers will flood and by how much, but something to keep an eye on. For specific flood forecasts for area rivers, please visit www.water.noaa.gov. -10/27

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 10 PM Saturday to 10 AM PST Sunday for PZZ210-253-273. Small Craft Advisory from 7 PM Saturday to 10 AM PST Sunday for PZZ251-252-271-272.