Warm, Wet, and Windy Weather Next Week

A major pattern change is in store for us next week. After dealing with very cold temperatures and heaps of snow for those lucky folks in Portland, we will see temperatures in the 50s, snow levels above 7000 feet, and very heavy rain as an atmospheric river plows through the Pacific Northwest. There is still a significant amount of model disagreement at this time, but minor flooding is possible over regions of Washington and especially Oregon next week.

Let’s start by looking at a really cool shot taken yesterday from the Moderate-resolution imaging spectroradiometer (MODIS) aboard NASA’s polar-orbiting AQUA Satellite. As you can see, all of Eastern Washington and Oregon is covered in snow, and there’s a large swath of white from Centralia to Salem thanks to the unexpected and epic snowstorm that impacted NW OR/SW WA Tuesday night and Wednesday morning. There are clouds masquerading as additional snow in the Southern Willamette Valley, but if you go further south, there’s also a ton of snow in S OR/N CA thanks to another unexpected and epic snowstorm from January 2-3rd that gave Medford its heaviest one-day snow total since 1919 and dropped 16 in 26 hours on Klamath Falls.

Fresh snow on the ground helps decrease temperatures because snow has a high albedo, meaning it is very effective at reflecting radiation back to space. This morning, Portland International Airport (KPDX) recorded a temperature of 11, their coldest temperature since December 1998 (also a low of 11), and the Hillsboro-Portland Airport (KHIO) bottomed out at 3 degrees, their coldest temperature since observations began. You have to go back to December 8, 1972, when a station in Hillsboro hit -2 degrees, to beat today’s low of 3. Thank you Chris Callais for these statistics.

But while fresh snow makes things cooler, snow eventually melts when warmer air moves into the region. And melting snow may play a big role in flooding next week, both in urban areas and on the many rivers flowing off the mountains.

Right now, rivers are running very low in Washington, but are near normal in Oregon. Whenever you are looking at the potential for a flood event in the future, it’s always smart to first look at river levels. If they are already low, it will take more rain to cause flooding than if they were high.

Anyway, let’s get a look a the current weather over the region.

Right now, we have a ridge off our coast that is directing the storm track to the north and increasing temperatures aloft. When you combine this with the extremely cold temperatures at the surface, you get a strong inversion and a very stable atmosphere. The radiosonde (weather balloon) launch from Salem this morning shows that temperatures were colder at the surface than they were between approximately 1,500 and 9,000 feet.

This ridge will continue to move eastward over us tonight, and tomorrow night/Sunday morning, a very weak system will dip to our south, bringing us nothing more than a few high clouds. Sunday and Monday morning will also feature a few clouds as systems tracking to the north drag very weak fronts through our region. No precipitation is expected with these features. The inversion shown in the above picture will hold through the weekend and into Monday morning, meaning that those in Portland can expect to see chilly nights with temperatures dipping into the teens in the city and single digits in outlying areas. Daytime highs will struggle to reach the freezing mark near Portland today and tomorrow, and should remain in the mid 30s Sunday and Monday. Areas like Seattle, away from the cold easterly influence of the Columbia River Gorge and with no snow on the ground, should see highs in the upper 30s to low 40s and lows in the upper 20s to low 30s over the weekend. Because of the persistent inversion, expect air quality to decrease and check for possible burn bans around your area.

The big change comes Monday afternoon, when the first of several warm and wet systems comes through the area. This system will be more of a “lead blocker” paving the way for other, stronger systems to affect the area in the middle of next week.

If you take a look at the upper-level chart below, you can see a stark difference to the pattern we’ve been stuck in for January and even much of last December. For most of those months, we had a large ridge in the Eastern Pacific and a large trough over us, giving us cold and dry weather. Next week, we’ll enter a pattern with a large trough in the Eastern Pacific helping direct strong, zonal flow into the Pacific Northwest and giving us warm, wet, and windy weather.

On Tuesday, a stronger system arrives into the area, bringing high winds to the coast and heavy rain throughout the region. This will usher in an “atmospheric river,” a narrow but deep plume of moisture that transports moisture from the tropics to the midlatitudes.

Another system arrives Wednesday, giving a reinforcing shot of rain and prolonging the period of gusty winds along the coast.

When it’s all said and done, the 48-hour rain totals from Monday-Wednesday afternoon are extremely impressive, with the potential for anywhere from 2-5 inches of rain in the lowlands and 5-10 inches in the mountains. With snow levels at 7-9000 feet, everything would be falling as rain in the mountains (besides the volcanic peaks), and this raises flooding concerns.

After Wednesday evening, we transition to moist northwesterly flow with snow in the mountains and chilly rain here in the lowlands. Good for rebuilding our base after all the heavy rain!

Thanks for reading,
Charlie