textproduct: Portland

SYNOPSIS

High pressure building over the area will keep conditions mostly rain free with patchy drizzle in the north. Rainfall accumulation will be minimal. On Sunday, a cooler front will move over the area bringing another round of rain. Rain will start off lighter but then intensify on Monday. Another strong Atmospheric River will move over the region Monday bringing heavy rain and increased chances for flooding. Rain will persist with another strong system arriving on Tuesday into Wednesday.

SHORT TERM...Now through Sunday Night

High pressure over the area is giving a brief reprieve from the heavy rain that has plagued the region the last several days. Fog across the Willamette Valley this morning has begun to dissipate, with weather observations showing visibilities improving above one quarter of a mile. Therefore, the Dense Fog Advisory across the Willamette Valley has been cancelled. Patchy fog may continue but should gradually dissipate by early Friday afternoon.

Tonight into Saturday remains dry. If conditions remain overcast today with minimal cloud breaks, then fog will have difficulty forming overnight. Because of the moist lower atmosphere, stratus is favored and thus, a lower probability of fog. As it stands, high resolution guidance suggests around a 30% chance of dense fog redevelopment (or maintenance) into Saturday morning. Guidance suggests fog and low stratus should dissipate by early Saturday afternoon as the upper level ridge moves overhead, making way for some brief 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 inch 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. Inland sites have a 10% chance of seeing rain as high as 0.5 inch. Will note that this system is taking a northerly track which is favoring a similar rain location pattern with the central and southern Willamette Valley seeing the least amount of rain. -27/10

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 forecast shows around 1.0-1.5 inches of rain along the coast and Coast Range with 1.5-2.5 inches along the Cascades. Now if the frontal system shifts northward precipitation totals will be much less ranging from 0.5-1.0 inch along the coast with up to 1.25 inch in the Cascades. If the system makes a direct hit though, we could see precipitation totals around 1.5-2.5 inch along the coast and up to 4 inches along the Cascades. 48 hour rainfall totals ending Wednesday at 4 AM for 2 inches or more of rain is around 50-75% along the coast, and greater than80% along the Cascades. The Willamette Valley around 5-15% chance, and the valleys of southwest Washington around 40%.

Another concern with this first system on Monday and Tuesday are winds. Gusts greater than 40 mph are likely with greater than a 60% chance of those wind speeds along the coast, near 70% along the Coast Range but much lower inland at less than a 15% chance. 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. 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 50-60% through Highway 26, Santiam Pass, and Willamette Pass. 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. -27/10

AVIATION

An upper level ridge of high pressure continues to shift over the region today with relatively light westerly flow aloft. With lingering surface moisture and calmer conditions, low stratus has developed at the coast and across inland valleys. IFR to LIFR conditions are expected to persist through this morning at most terminals, gradually improving this afternoon. There is at least a 50% chance that CIGs at most terminals 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, KEUG and KONP which are likely to see conditions improve to VFR by 20-22z this afternoon. Another round of IFR to LIFR fog and/or stratus is favored tonight into Saturday morning.

KPDX AND APPROACHES...A broken layer of IFR to MVFR stratus is expected to persist through this morning, becoming predominately MVFR by 20z this afternoon. That said, there is a 30-40% chance that conditions improve to VFR today if clouds are able to scatter out more. Stratus is likely to redevelop overnight with chances for IFR fog and/or stratus increasing after 08z Saturday. Winds expected to remain light less than 5 kt. /DH

MARINE

The marine environment is going to start of settled but quickly ramp up midweek with an incoming series of frontal systems. High pressure will keep winds settled with gusts up to 15 kt, and seas below 10 ft.

On Monday a robust frontal system will move in causing winds to quickly ramp up. Will see a southerly wind wave compound with a strong westerly swell. There is around a 50% chance of gale force winds on Monday with models increasing this probability with each run it appears. Seas at this point will rise quickly from 8 ft at 10 seconds to 14 ft at 12 seconds. These steep and hazardous seas will continue through Tuesday. The next system moves in on Wednesday which will bring in an amplified fresh swell. Winds will not necessarily be a problem, but rather strong westerly flow in combination with westerly swells will cause seas to build considerably up to 18 ft. there is a 10% chance of 20 ft seas in the outer waters. -27

HYDROLOGY

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. Confidence in the presence of flooding is low at this point, but something to keep an eye on. For specific flood forecasts for area rivers, please visit www.water.noaa.gov. -27

PQR WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory until 10 AM PST this morning for ORZ109-110- 114>118. WA...None. PZ...None.