Fourth Nor’easter in a Row Throws Shade on Second Day of Spring

A burst of wet, heavy snow swept its way up the U.S. East Coast on Wednesday, threatening to set major records for late-season snowfall. In a paradox of an equinox, springtime arrived on Tuesday to find winter storm watches plastered along the East Coast from Virginia to Massachusetts. By Wednesday morning, more than 75 million Americans were affected by winter storm warnings or winter weather advisories, and snowfall rates had reached 1”/hour as far north as Newark, New Jersey.

The last three weeks have seen four nor’easters—dubbed Riley, Quinn, Skylar, and Toby by the Weather Channel—dump heavy snow on various parts of the mid-Atlantic and Northeast. Such storms aren’t unheard of in late winter and even early spring, but four in a row of this caliber don’t occur every winter.

Several grace notes will make this latest nor’easter a bit easier to bear:

—The effect of late-March sun will help keep daytime temperatures near freezing, which could hasten melting even as the snow falls.

—Relatively warm ground temperatures may also cut down on snow depth, especially where snowfall rates are on the lighter side.

—This will not be a rapidly strengthening “bomb cyclone,” so on the whole, winds will be less fierce than in Toby’s three predecessors.

Figure 1. GOES-16 visible satellite image of Winter Storm Toby at 1348Z (9:48 am EDT) Wednesday, March 21, 2018. Image credit: NCAR/RAL.

Even though Toby won’t be a rapid intensifier, it will be a slow mover, which will prolong the effects of wind and heavy snow on trees and power lines. A few areas of snow continued to extend west of the Appalachians on Wednesday morning, including a persistent, narrow band of heavy snow in the Louisville, Kentucky, area that was pushing local amounts well above 4".

Top official snowfall amounts by state earlier in the day (4 am EDT Wednesday) included:

Pennsylvania:  13.0”, Greencastle
Maryland:  6.8”, near Westminster
Indiana:  5.0”, Curby
Kentucky:  5.0”, Goshen
Virginia:  6.0”, Covington City
West Virginia:  7.0”, Athens
Ohio:  3.5”, Arcanum
Delaware:  1.4”, near New Castle Country Airport

Because of the mitigating effects above, it’s quite possible that snow totals on the ground will fall well short of the amounts that end up being officially measured. See the weather.com article on Toby for frequent updates.

Coastal flooding: mostly minor

Toby's storm surge is expected to cause minor flooding (which is more disruptive than damaging) along much of the Mid-Atlantic and Northeast coast, from Duck, North Carolina to Portland, Maine. The NWS has issued a Coastal Flood Warning along the coasts of Delaware and southern New Jersey, where a storm surge of 2 - 2.5' capable of causing moderate flooding is expected. Moderate flooding is considered damaging but not destructive. The highest water levels are expected to occur during the high tide cycle late Wednesday night.

Two cities to watch for possible snow records

As detailed in a weather.com writeup by Brian Donegan, Wednesday could see several major snow records tumble.

New York City:  A foot of snow at Central Park, on the high end of expectations, would vanquish the post-equinox spring snow record of 11.8” from March 20-21, 1958. However, Toby is not expected to challenge the city’s all-time March record of 21” from the legendary storm of March 12-14, 1888, or the runner-up of 18.1” from March 7-8, 1941. Snow records go back to 1869 at Central Park.

Philadelphia:  The all-time March record of 12.0” from the Storm of the Century (March 13-14, 1993) could end up being a bridge too far, as several hours of rain fell in place of snow on Wednesday morning. An all-time spring record would be even tougher to accomplish, as Philadelphia scored an amazing 19.4” on April 3-4, 1915. Snow records date back to 1885 in Philly.

One thing that could help bring these records within reach is the more-frequent snow measuring intervals (at least every six hours) that are now in standard use by the National Weather Service. Decades ago, snowfall was often measured just once or twice a day by sticking a ruler into the snowpack. Since the 1990s, official snow observations have been carried out by measuring the depth on a snowboard that is cleared after each observation, normally every six hours. This standardized, more-frequent method can lead to higher snow totals for a given storm than would have been observed many decades ago, because it reduces the amount of compaction that occurs between observations. See this writeup by Matt Kelsch (University Corporation for Atmospheric Research) for more on the sometimes-“flaky” history of snow measurement.

Serious flood threat continues for coastal Southern California

A potent atmospheric river will send copious amounts of moisture slamming into the hillsides of coastal Southern California from Wednesday into Thursday. Mandatory evacuations are in place in and near parts of Santa Barbara County where wildfires raged late last year. Major fires can seal the soil, allowing rain to run off much more quickly and carry debris downstream. The hazard may continue to increase into Thursday morning, when the atmospheric river is expected to be almost perpendicular to the coastal hills of Ventura and Santa Barbara counties. Rainfall amounts are expected to range from 2 to 5 inches at lower elevations and 5 to 10 inches at higher terrain.

Flash flooding is possible in other parts of the SoCal coast, including the Los Angeles area, from this biggest storm of the 2017-18 winter thus far. We’ll have more on this major storm in our next post.

Jeff Masters contributed to this post.