Snow Early to Arrive, But Main Event Still To Come

Another day, another poorly-forecast lowland snow event.

Our low-pressure system was stronger and faster than modeled, and as a result, it generated more and heavier precipitation than expected. Yesterday’s runs only had a light rain/snow mix at the coast at this morning, but the heavier-than-expected precipitation helped lower snow levels down to the surface there, and greater-than-expected lift allowed snow to develop over the interior lowlands as well. A particularly heavy band went through SW Washington, dropping approximately an inch of snow at most spots. A trained NWS spotter 5 miles NNW of Grand Mound, WA measured 1 inch of snow at 821 am with light snow continuing to fall (click here for the official report), and webcams along the coast show that similar amounts fell all the way down to sea-level. Westport got particularly hard-hit, here’s a webcam from Weather Underground user BeachToyBox in Westport.

Unfortunately, snow is not sticking at my elevation (200′) in NE Portland. Snowfall rates have let up a bit, but we are still seeing moderate snow. Here’s a picture taken around 9 AM when a heavy snow shower passed through. And in case you didn’t know, these photos always understate how much snow is falling.

Radar/Satellite/Surface Observations

No blog is complete without taking a look at the current (between 10:30 and 11) satellite, radar, and surface observations over the region. The radar shows light-moderate snow over SW  Washington and NW Oregon.

Much of the precipitation over the lowlands is due to a process known as isentropic lift, which is where air is lifted upward along an isentrope (a line of constant entropy). Isentropic lift occurs when less dense air moves into a region of more dense air and is lifted upwards as a result. We had cold and dry Canadian air enter the region in the wake of the low pressure system that brought Sunday’s snow, so the relatively warm and moist air coming off the Pacific with this storm is being lifted by this preexisting cold, dry, and dense airmass. When air is forced upwards, it cools, condenses into clouds, and if the lifting is robust/air is moist enough, forms precipitation.

The satellite shows a disjointed low pressure system moving SSE. It took me a long time to find the exact center of this low and I’m still not quite sure I got it right. Notice the stratiform clouds ahead of the low and the “popcorn” cumulus behind it… this is due to isentropic left and cold air filling in behind the low, respectively.

Finally, surface observations. As you can see, most places in the Portland metro area are just above freezing, and that, combined with higher sun angles to more efficiently warm surfaces, is preventing snow from sticking in most places. Additionally, relative humidities are high, so we won’t see wet-bulb cooling to lower temperatures below freezing. However, we should still see snow throughout the day because the temperatures just above the surface are very cold (for example, the 925 mb temperatures (approximately 2,500 feet up) are -5 degrees C; 23 degrees Fahrenheit.

To summarize, we have incoming precipitation due to isentropic lift with a weak, hard-to-track surface low some ~250 miles WSW of Astoria. Surface temperatures are above freezing in most locations, preventing snow from sticking to the ground.

The Forecast:

Here’s where things get tricky. This earlier-than-expected precipitation is but a precursor to the feature I mentioned in last night’s post: an area of frontogenesis created by moisture spinning northward from this low interacting with a shortwave trough dropping down from British Columbia. Here’s an image from this morning’s 12Z NAM.

These situations are tricky because the area of heaviest snow is highly localized and can be quite intense. The last time we had a situation with pronounced frontogenesis over the Portland metro area, we ended up with a surprise foot of snow that brought the city to a standstill for a week while Salem saw rain and Olympia northward was completely dry.

Timing:

Frontogenesis is expected to strengthen this afternoon (1-3 pm) over SW Washington as these two troughs interact. This frontogenesis will initially manifest itself as an intensifying blob of precipitation, organizing into a more pronounced, southward-moving band through the Willamette Valley in the evening as the upper-level trough near British Columbia picks up steam and heads southward. Snowfall is currently expected to be heaviest for the PDX metro area around 5-9 pm, Salem an hour or two later, and so on. The band should pass through Eugene by midnight or very early Wednesday morning.

Ryan Underhill posted these images from the latest 18Z NAM on the PDX WX Analysis Facebook group. Notice how the glob of snow develops into a band as it sinks southward. The models have been a complete mess today and I’m not looking into one particular solution, but this sequence provides a good visualization of how this storm will play out.

Snowfall amounts

The strength of the frontogenesis will be the deciding factor in how intense the snowfall is, but this morning’s National Weather Service forecast discussion mentioned the potential for snowfall rates as high as 1-2 inches per hour with the heaviest bands. This is definitely supported by the NAM above!

Here’s the updated forecast from the National Weather Service :

Unlike the snow we’ve seen so far today, the snow associated with this band is expected to stick on grassy surfaces and perhaps even roadways during the periods of most intense precipitation. This is because some snow melts while falling when temperatures are above freezing, and the more snow you have, the more melting occurs. Since melting takes heat energy, the temperature cools. This explains why ‘snowflakes’ are often very large when temperatures are above-freezing; the flakes begin to melt and become sticky in the process, forming large globs of snow. The potential for snow to stick on roadways will increase dramatically after sunset, so timing will play a huge role in how much this storm will affect the late-evening commute.

To summarize:

• Snowfall rates will increase this afternoon/evening as frontogenesis occurs over NW Washington/SW Oregon.
• This frontogenesis will initially manifest itself as a blob of increased snowfall rates, and it will transition into a more defined band as the evening goes on, traveling south all the while
• There is a ton of uncertainty in amounts and timing with this system due to temperatures being on the fringe, model disagreement, and the highly sensitive nature of this setup.
• Amounts for the Portland metro area should be between 2-5 inches and the heaviest snowfall should occur from 5-9 pm as this band passes through.
• Snowfall rates could be quite intense (1-2 inches per hour) with the band, and these high rates should help lower freezing levels. This should help snow begin to stick, especially if our intense snow occurs after sunset.

Update:

Since it’s 1:53 pm and I’m just now finishing this blog (analyzing a storm takes time, yo!), let’s just take a quick look at the radar. The Portland radar stopped reporting around 1pm, so here’s the most recent radar composite over the Pacific Northwest.

You can see the intensifying blob over SW Washington and some heavier, more defined showers along the coast. Expect precipitation to keep intensifying, and we should see a clear increase in the intensity of snowfall in the Portland metro area beginning 4-5 pm.

Pat yourself on the back for making it this far, and do some snow dances! Oh, and there’s a slight chance of snow Thursday morning, but let’s make it through this storm first.

Have a nice day,
Charlie