Wednesday, January 20, 2016

Some Thoughts on Snow - Past, Present, and Near Future

This has been a down year for snow lovers, unless you are in the Pacific Northwest, Sierra Nevada, or the Rockies. The eastern two-thirds of the country has been experiencing a very mild winter in terms of both temperature and snowfall.

This map shows the status of the Accumulated Winter Severity Index as of January 19. The index characterizes the severity of winter using temperature, snowfall, and snow depth data.

We are now probably in one of the more "snow active" periods of this winter so far. Storms continue to pound into the west coast, increasing the snowpack in the Cascades and Sierras. Cold Arctic air streaming across the warm and open waters of the Great Lakes flipped the lake-effect snow machine to "on", resulting in feet of new snow in the lee of the lakes from Michigan to New York.

72-hour accumulated snowfall ending 7:00 a.m. EST January 20.

A relatively weak upper wave crossing through the Midwest dropped from one to six inches of in a band from eastern Nebraska east-southeast into northern Kentucky on Tuesday night, and light snow occurred further southeast into Tennessee, northern Alabama, and Georgia today. Highlighting the winter weather this week is the chatter the past two to three days about the impending winter storm along the east coast.

As of today snow cover across the U.S. stands at 52.6 percent with an average depth of 5.2 inches. That's not bad considering the first part of the winter.

Snow depth and extent across the U.S. as of January 20, 2016

A year ago snow cover was half of this, sitting at 25 percent with an average depth of 2.5 inches. The largest and most welcome changes since last year are in the western U.S. In the western coastal region, which includes the Sierra Nevada mountains, snow extent is 17 times greater than last year (8.6 percent this year vs. 0.5 percent in 2015), and the average depth is 2.6 inches compared to a measly 0.1 inch last year. This is critically important for California's water supply and recovery from drought. Snow water equivalent, the amount of water in the snow, ranges from 95 percent of normal in the southern Sierra to 125 percent of normal in the northern Sierra. However, there is a ways to go before SWE reaches what's normal for April 1. The snow melt and runoff in the spring and summer is the water source during the dry season.

Status of the snow water equivalent in the California Sierra Nevada mountains.

In the Pacific Northwest, snow extent is 48.6 percent at this time, compared to 31.2 percent a year ago. Average snow depth is 25.4 inches compared to only 10.3 inches last year.

You can find more of this information for other parts of the country as well as maps at the National Operational Hydrologic Remote Sensing Center web site.

As I mentioned earlier, the buzz so far this week has been the impending winter storm in the eastern United States. The forecast models have been pretty consistent this week with the overall picture. There have been some run-to-run shifts in the track and position of the low, but generally the mid-Atlantic region has remained under the gun. The finer details have yet to be worked out, as the upper level disturbance that will spawn this storm just came ashore Tuesday night in the Pacific Northwest and today was over the central Rockies. This morning a Blizzard Watch was issued for the District of Columbia north through Baltimore for Friday through Saturday night.

If it plays out as forecast, this will be a paralyzing storm for the region. As much as 10 inches to two feet of snow may occur before it's all over, all whipped around by winds from 20 to 30 mph and gusts to 40 mph. Heavy snow could extend as far north as New York City. There is likely going to be a sharp cutoff in snow amounts at the northern edge of the system so a slight shift in track north or south could make a big difference in snow amounts. In addition, onshore winds will likely result in some coastal flooding. While most of the attention will be on the snow, a potentially damaging ice storm could be in store for the western half of North Carolina into southern Virginia. Further south along the Gulf coast sever weather is a possibility.

Impacts won't just be related to snow. For a good take on the likely impacts from this storm, check out Gary Szatkowski's blog, "Winter Weather Impacts Are About More Than Snowfall Totals".
Gary is the Meteorologist-in-Charge at the National Weather Service Office in Mt. Holly, NJ which covers the Philadelphia area.

Stay up to date with the last information, watches, and warnings at your local National Weather Service Office web site. The first round of snowfall forecasts were issued this afternoon and will be updated as more information comes in and details come into more focus.

Watches, warnings, and advisories in effect as of 8:00 p.m. EST January 20.

A message to CoCoRaHS observers - as part of your preparations for this storm try and take some time to review snowfall measurement and reporting procedures. It's been a long time since there has been a significant snow to measure in the eastern U.S., and the snow amounts and winds expected will present some challenges. The short videos (~2-4 minutes) on the CoCoRaHS YouTube channel are a great way to review this material. You can find them  under Training Animations.

One last thing - be safe. No observation is worth risking life and limb. If it is too icy or otherwise too hazardous for you to get out to your gauge, wait until the next day and submit a multi-day report.

We'll be looking forward to seeing all of your reports.

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