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FXUS66 KPQR 040600 AAA AFDPQR

Area Forecast Discussion...Updated National Weather Service Portland OR 1000 PM PST Sun Dec 3 2023

Updated aviation...

SYNOPSIS
Yet another atmospheric river will bring periods of heavy rain to the area Monday night through at least Tuesday night. A frontal system immediately behind the aforementioned atmospheric river will bring more rain Wednesday through Thursday, albeit relatively lighter. As such, river flooding and urban flooding will remain a concern through at least Wednesday afternoon.

SHORT TERM
Sunday afternoon through Tuesday night...Radar, satellite, and surface weather observations from 1:30 PM PST Sunday showed rain showers occurring across northwest OR with very little precipitation occurring over southwest WA. The atmospheric river responsible for the heavy rain that occurred Saturday night/Sunday morning has shifted south of Salem and has decayed a bit. As such, most of the rainfall with this system has already fallen. How much rain has fallen from approximately 1 PM Saturday through 1 PM Sunday? Well, observations across the area should 24-hr rain amounts ranging between 0.5-1.75 inches across southwest WA, 1-4 inches over the north OR Coast Range, 2-6 inches over the central OR Coast Range, 0.65-1.25 inches over the Portland/Vancouver metro, 1-2 inches over the central/southern Willamette Valley, 2-4.5 inches over the Oregon Cascades, around 2 inches in the Columbia River Gorge, and 0.9-1.5 inches in the Upper Hood River Valley.

While the amount of rain that has fallen thus far hasn't been quite enough to cause widespread river flooding, there have still been some reports of minor urban flooding and small stream flooding. There was also a report of a rockslide around 7 miles north of Mapleton on Highway 36. In addition, sharp rises are currently occurring on the Siletz River near Siletz and on the Siuslaw River near Mapleton. Both rivers will likely approach minor flood stage later this afternoon.
Will need to monitor these river levels closely as even a slight uptick would warrant a Flood Warning.

Moving into Sunday night/Monday morning, models are showing a warm front lifting northward over northwest OR into southwest WA. This warm front will bring another round of rain with it, albeit light.
This means urban flooding won't be much of a concern Sunday night and Monday morning, however river levels will remain elevated on the Siletz and Siuslaw Rivers so locations near either of these rivers should be prepared for potential flooding.

However, urban flooding and river flooding concerns increase again Monday night through Tuesday night as yet another atmospheric river takes aim at southwest WA and northwest OR and brings more heavy rain to the area. A Flood Watch remains in effect for locations west of the Cascades through Wednesday afternoon to cover this threat. The GEFS/EPS continue to suggest this will be a strong atmospheric river with IVT values peaking around 850-900 kg/ms. That being said, the GFS/EURO are showing very weak synoptic scale and mesoscale forcing.
This means rain rates may not get as high as expected given the strength of the AR, which could result in total rain amounts that are less than expected. In addition, confidence is somewhat low regarding the exact duration of the heaviest rainfall with this system. Right now, most model guidance suggests a prolonged period of heavy rain for 36-48 hours. Despite the ongoing uncertainty, river levels will already be elevated going into this event. This means river flooding will remain a concern, even if observed rain amounts wind up a bit less than the current forecast. Forecast rain amounts and expected impacts for the system early next week are discussed below in the hydrology discussion. -TK


LONG TERM
Wednesday through Saturday...Post frontal showers will persist through the day as the remnants of the atmospheric river (AR)
continues to shift eastward. Will mainly be showery, but will still see additional accumulations. Overall the flow is fairly messy and unorganized. Will say that there are some bands of increased vorticity advection paired with the region having upper air support from the jet stream. Cannot rule out thunderstorms in the late afternoon and evening - especially along the coast.

Thursday a cold front will move in bringing yet another round of rain, but not necessarily as strong. This system is not an AR, but rather a true airmass shift. A cold air wrapped low aloft will move over Vancouver Island through the day. This cold front will also bring cooler temperatures in comparison to what we will experience early this week. 850 mb temperatures will be around -3 deg C. Snow expected over the mountains, but the track of the low will determine the target of the precipitation. Either way, with snow levels dropping, snow at the passes is likely. Confidence is low at this time for both the track and strength of this system.

On Friday through Saturday there is a very weak signal for a potential AR. However, the pattern is not necessarily "text book" in that it is going to be a warm airmass converging with a much cooler airmass from the north. Expect precipitation to continue through the week, but at this point, it is too early to nail down on any specifics. -Muessle

HYDROLOGY
Periods of rain will continue Sunday night through Wednesday, heaviest Monday night through Tuesday night as a strong atmospheric river aims at southwest Washington and northwest Oregon.
This is also when flooding concerns are highest. Additional rain amounts expected Sunday night through Tuesday night (48 hour rain totals): 3 to 5 inches along the south Washington coast, north Oregon coast, and central Oregon coast, 3 to 7.5 inches over the Coast Range and Willapa Hills (highest amounts in higher terrain), 1.5 to 3 inches across the interior lowlands of southwest Washington and northwest Oregon (highest from Salem northward), 3 to 7 inches across the south Washington and north Oregon Cascades, 1 to 2 inches for the Lane County Cascades, 2 to 4.5 inches for the Columbia River Gorge, and 1 to 2 inches for the Upper Hood River Valley.

Given the forecast rainfall amounts expected over the remainder of this weekend into the middle part of next week, river levels will continue to rise through Wednesday. Forecasts for most rivers indicate a smaller peak late Sunday night into Monday morning with another higher peak Tuesday afternoon into Wednesday morning.

Most coastal rivers show a 10-20% chance of reaching minor flood stage at some point within the next 10 days. This includes the Naselle River near Naselle, the Nehalem River near Foss, the Nestucca River near Beaver, the Grays River near Rosburg, and the Siletz River at Siletz. The Wilson River near Tillamook has the best chance of reaching minor flood stage as the probability is currently at 45%.
Probabilities for major flooding along coastal rivers have decreased to around 5% or less,

Additionally, many other rivers across northwest OR and southwest WA are showing anywhere from a 5-35% chance of reaching at least minor flood stage in the next 10 days, except for the mainstem Columbia and Willamette Rivers where the chance of reaching minor flood stage remains lower at 5% or less. While these probabilities have decreased, there is the potential for probabilities to increase again if QPF amounts increase with future model runs. To view current and forecast river stages for any river gage location across southwest WA and northwest OR, visit https://water.weather.gov/ahps2/index.php?wfo=pqr

Lastly, urban flooding is possible Monday night through Wednesday, especially in low-lying areas with poor drainage. If you have gutters and/or storm drains that still need to be cleared out, now is the time. Significant ponding of water and localized roadway flooding is expected on roads. Any motorists with travel plans should also allow extra time for their commute as hydroplaning will be a risk. Never drive through flooded roads. -TK

AVIATION
Observations from around the region show mainly MVFR conditions from KSLE southward and IFR to low end MVFR farther north into the Portland metro as rain has diminished in intensity but become more drizzly in nature. Will see rain redevelop through 12z as warm front lifts northward, with IFR/MVFR mix continuing to impact the terminals. Gusty south-southwest winds will also continue to increase 12-18z Mon and persist through much of the period. Easterly Gorge component to surface winds will also bring LLWS concerns to KPDX and KTTD as winds just off the surface remain out of the southwest. Also added a short period of LLWS to KEUG through about 09z when surface winds should become more southerly. Rain and MVFR persists through much of the period, although locations south and east of a roughly K4S2-K6S2 line may see a bit more improvement towards high end MVFR/VFR as heavier rains shift northward during the afternoon.

** The new Aviation Weather Center website is live. The new website can be found at aviationweather.gov **

KPDX AND APPROACHES...IFR conditions at the terminals this evening as light rain/drizzle lingers. Rain will re-intensify through 12z as warm front lifts northwards, with a mix of IFR/MVFR continuing overnight. Expect LLWS concerns after 12z as surface winds increase out of the ESE but winds off the surface remain out of the SW. LLWS may linger through much of the day, depending on how much surface winds can veer southerly after 18z Mon. Rain and MVFR expected to persist through the end of the period. /CB

MARINE
Brief break in gales through through the evening ahead of the next gales. However Hazardous Seas Warning remain in effect with seas the 13 to 16 ft range with a period around 13 seconds.

Gales develop late tonight across the waters. Gales may be prolonged as the tight pressure gradient remains over the waters into Monday night before the front move ashore Tuesday morning.
May even see a brief period of storm force winds Monday evening but latest models have backed off slightly on that idea.

Seas approaching 20 ft Monday into Monday night. GWES probabilistic guidance shows about a 50% chance for seas exceeding 20 ft Tuesday morning. There is another batch of swell arriving Tuesday afternoon. GWES probabilistic shows about 90% seas exceed 20 ft, but less than 10% for exceeding 25 ft.

Also with the elevated seas continuing through mid-week, will maintain the threat of sneaker waves along our coast. /JH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Flood Watch through Wednesday afternoon for Cascade Foothills in Lane County-Central Coast Range of Western Oregon-Central Oregon Coast-Central Willamette Valley-Coast Range of Northwest Oregon-Greater Portland Metro Area-Lower Columbia- North Oregon Coast-Northern Oregon Cascade Foothills-South Willamette Valley.

WA...Flood Watch through Wednesday afternoon for Greater Vancouver Area-I-5 Corridor in Cowlitz County-South Washington Cascade Foothills-South Washington Coast-Willapa Hills.

PZ...Hazardous Seas Warning until 4 AM PST Monday for Coastal waters from Cape Shoalwater WA to Florence OR out 60 NM.

Gale Warning from 4 AM Monday to 10 AM PST Tuesday for Coastal waters from Cape Shoalwater WA to Florence OR out 60 NM.

Small Craft Advisory until 10 AM PST Monday for Columbia River Bar.

Gale Warning from 10 AM Monday to 4 AM PST Tuesday for Columbia River Bar.

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