textproduct: Portland

SYNOPSIS

Showery conditions with lowering snow levels. A variety of snow related hazards in effect for the Coast Range and the Cascades for today. 15% chance of low elevation accumulating snow in the Portland-Vancouver Metro with increasing chances as you move north. Atmospheric River potential is strengthening on Wednesday through Friday. Potentially impactful rainfall and snow in the south Washington Cascades, though confidence remains moderate at this time.

LONG TERM...Thursday through Monday

In the short term discussion we introduced the Atmospheric River (AR) and some of it's impact potential for Wednesday. However, that is not the end of the story. A great place to start is to look at Climatology and our "Extreme Forecast Index". This tool allows us to see how the current set up deviates from what is considered climatologically normal. For Thursday, the anomaly is quite strong for the Integrated water vapor transport and QPF. This is our first sign that we could be looking at a high impact scenario. As we then look at some of the synoptic features, there are a few that really stand out. Within the jet stream we sit just south of the main jet streak which, at 500 mb, is nearly 122 kt (that's very strong). This feature can be associated with significant lift and more active weather. We also see a band of warm air streaming in which has a much higher capacity for moisture than a colder airmass. These features remain nearly stationary through Saturday morning; meaning we will see nearly 72 hours of continuous rain.

Probabilistically, there again is a spread, but even the mean (the most likely solution) shows highly impactful rainfall totals. The dry solution (10th percentile) brings around 2.7 inches to Tillamook 3.5 inches to the north Oregon Coast Range, 2.15 inches to Portland, and 3 inches to the north Oregon Cascades. The wet solution (the 90th percentile) on the other hand produces 7.4 inches in Tillamook, over 8 inches in the north Oregon Coast Range, 5.15 inches in Portland and over 8.5 inches within the Cascades. The 90th percentile solution describes a scenario where essentially we get caught right in the bullseye of the AR where the dry scenario insinuates that the AR moved to the north.

With these types of rain amounts, concern for hydrological impacts rises further. Confidence remains low though as again, it will come down to exactly where this system sets up. However, we do have high confidence that somewhere will see those higher end amounts...it just comes down to where. Another factor to consider is that we would most likely not see any river rises until Friday or Saturday due to the water having to move through the smaller watersheds. At this point, the coastal and Coast Range rivers have the highest probability (around 30%) of reaching at least action stage, but there is still ample room for conditions to shift. With this in mind though, prepare for a very wet later half of the week.

Rain eases as we transition into Saturday night and Sunday as a weak area of high pressure forms, and troughing occurs inland over Idaho. This ridging will allow for a break in precipitation, but, if there is enough cold air that filters into the Columbia Basin, we could see some gustier winds. Not to rain on any parades for Monday, but yet another AR approaches the region Sunday night into Monday. At this time the bulk of the rain appears to move to the north and most long-range models agree on that pattern shift. However, we have seen just how fast conditions change and it easily could move southward. If this AR does move south, we could be looking at potentially another impactful rain even. -27

AVIATION

Widespread VFR conditions with showers encompassing much of the area. Rain is heaviest north of KCVO in Oregon but even with the heaviest showers only seeing temporary reductions in CIGs/VIS. Winds will continue to shift to the south which will bring gusty conditions to the coast after 18Z and through some of the airports within the Willamette Valley. There continues to be a threat for snow within the Coast Range and the Cascades with around a 15-20% chance for low-elevation snow at airports within the greater Portland-Vancouver Metro below 500 ft. For areas like KKLS those chances for snow are higher - around 30%. Any snow that does fall will be quite wet and has a high probability of melting quickly.

In the post frontal environment there will be a trend towards high end MVFR CIGs but this will generally be in the north as the southern portions of the forecast area have experienced very few impacts.

KPDX AND APPROACHES...VFR flying conditions likely with low chances for MVFR CIG/VIS to 20-50% any given hour. Scattered showers increasing again after 18Z Tue. Winds turn southerly and will amplify in the afternoon after 20Z Tue with the passing precipitation. Will see rain persist through the remainder of the forecast. -27

MARINE

The marine environment will be quickly evolving over the next 72 hours. Seas and winds are being driven by a low pressure system in the Gulf of Alaska off of the Alaska Panhandle. There is a persistent northwesterly swell with combined seas around 10-13 ft at 13 seconds. Small Craft Advisory remains in effect, but there will be periods of hazardous steep seas. Through the day winds will begin to slowly increase as winds become southerly. The pressure gradient will begin to tighten late tonight into Wednesday. As this occurs there will be a trend towards much higher wind speeds and more widespread hazardous seas.

On Wednesday winds continue to show Gale Force speeds but are generally concentrated on the central and northern waters as of right now. Given that the direction is not truly due south, the southcentral waters will be less impacted. However, where gales are not present, hazardous seas will be. Recent models suggest that the chance for gales may be slightly delayed, however, there remains at least a 70% chance for gale force winds as early as late Wednesday morning. Because there remains some level of uncertainty, have issued a gale watch for the Columbia River Bar. A fresh southerly swell will combine with the southerly wind wave and we will see seas build to 14-17 ft by Wednesday night and early Thursday morning. A hazardous Seas Watch has been issued for PZZ253/273.

The pattern remains active late this week and into this weekend, but confidence in the occurrence of hazardous conditions is low at this time. Will note that a strengthening atmospheric river is forecast for Wednesday through at least Friday with strong winds aloft. Some of these winds could mix down to the surface and see isolated high wind gusts. -27

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 5 PM PDT this afternoon for ORZ106. Winter Weather Advisory until 5 PM PDT Wednesday for ORZ126- 127. WA...Winter Weather Advisory until 5 PM PDT this afternoon for WAZ211. Winter Storm Warning from 5 PM this afternoon to 5 PM PDT Wednesday for WAZ211. PZ...Small Craft Advisory from 8 AM this morning to 5 PM PDT Wednesday for PZZ210. Gale Watch from Wednesday afternoon through late Wednesday night for PZZ210. Small Craft Advisory until 8 AM PDT Wednesday for PZZ251>253- 271>273. Gale Watch from Wednesday morning through late Wednesday night for PZZ251-252-271-272. Hazardous Seas Watch from Wednesday morning through late Wednesday night for PZZ253-273.