WunderBlog Archive » Category 6™

Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.

All Eyes on East Coast as Big Snowmaker Looms for Tuesday

By: Bob Henson and Jeff Masters 5:30 PM GMT on March 13, 2017

A classic late-season nor’easter has all the ingredients to produce what could be near-record-heavy March snow and dangerously strong winds in coastal cities from Washington, D.C., to Boston, and onward into southeast Canada. But if the devil were ever in the details, it’s right now. The dividing line between rain and snow in this storm, dubbed Stella by The Weather Channel, will be close enough to the Interstate 95 corridor to keep forecasters sweating. Right now it appears that the meteorological bounty will be mainly in the form of snow for the big East Coast cities. However, just a minor shift in track could bring rain or sleet into the metropolitan areas, at least for a brief period.

There’s no doubt that a big storm is in the cards. Upper-level energy from two sources—a large trough swinging across the Midwest and an upper-level wave at the base of this trough over the Southern Plains—will be joining forces along the East Coast by Tuesday. The interplay between these two factors is just one of the elements of uncertainty on the table.

What’s clear is that a surface low off the central Gulf Coast on Monday morning will begin to strengthen near the Outer Banks Monday night, then rapidly intensify as it moves northeast along and/or just off the East Coast through the day on Tuesday. As shown in Figure 1 below, the 12Z Monday run of the GFS model deepens the low 23 millibars in 24 hours, from 2:00 am EDT Tuesday to 2:00 am Wednesday, as it moves from near Cape Hatteras, NC (1000 mb), to the central coast of Maine (977 mb). This would bring the low very close to the official definition of a meteorological “bomb”—a midlatitude low that deepens at least 24 millibars in 24 hours.

Figure 1. The surface low associated with Winter Storm Stella will deepen very rapidly as it progresses from offshore Delaware to Maine from 2:00 am EDT Tuesday, March 14, to 2:00 am EDT Wednesday. Colors depict the altitude, in tens of meters, of the 500-millibar surface, corresponding to the upper-level low digging sharply into the East Coast and supporting the strengthening of the surface low. Image credit: tropicaltidbits.com.

Snow lovers will be delighted to hear that Monday morning’s models are leaning toward a storm track just far enough offshore to keep the big cities mainly in the snow camp, especially north of Washington. This had been thrown into some doubt on Sunday night, when the major models began inching the system just far enough toward the coast to raise the odds of rain or sleet nudging into the urban corridor, especially Washington and Boston. There is still room for the models to swing a bit eastward or westward before the storm arrives, though.

Regardless of precipitation type, this storm’s intensity and rapid development will make it a formidable wind producer. Where it snows heavily, downed trees and power lines could jeopardize electricity and transportation for an extended period. Tuesday morning will be an especially tough point: we can expect at least some major airports to be closed, and highway travel will be extremely unpleasant where it’s not impossible. Blizzard warnings are in effects from northern New Jersey across the New York metro area and western/central Long Island into southern Connecticut. Winds gusts of up to 55 mph are expected on Tuesday, especially near the coast.

Figure 2. Snowfall totals projected by The Weather Company/IBM's Deep Thunder model (formerly known as RPM) for the period from 8:00 am EDT Monday, March 13, through 7:00 pm EDT Tuesday, March 14. "It does look like eastern PA through northern NJ and into central upstate NY will be the 'jackpot' zone," said Michael Ventrice (TWC). Image credit: The Weather Company, An IBM Business.

Outlooks for some major cities

Washington: The forecast remains especially tricky for D.C., not only because the rain/snow transition line will angle from southeast to northeast very close to the metro area, but also because the storm’s heaviest precipitation rates will kick in from D.C. northeastward. This leaves Washington on the edge of two transition zones sitting at right angles. It’s quite possible that a slice of warmer air will wrap toward the D.C. area ahead of the fast-strengthening surface low, bringing a period of rain and/or sleet near or into the city. On the other hand, the storm’s heaviest precipitation will hit the District in the early morning hours Tuesday, a time frame that will maximize the odds of accumulating snow (which would be tougher to accomplish at midday this late in the season).

The probabilistic NWS guidance posted at 8 am EDT Monday morning gave the District as little as 3” or as much as 13” of snow, with best odds of around 5”. Capital Weather Gang was slightly more pessimistic in its outlook issued just before noon EDT Monday, giving the northwest part of the District best odds of 3” - 7” but deeming 1” - 4” most likely in the southeast District. “We have bumped down snowfall totals in the immediate metro area for the second time in the last 12 hours as the likely storm track has shifted closer to the coast, which will draw in more mild air,” wrote CWG’s Jason Samenow.

New York: The New York City area is just far enough northwest of the expected storm track to give it the best odds among the I-95 cities of avoiding a changeover to rain during the storm. Some of the very heaviest snow could be within 100 miles of the city, especially across northern New Jersey into central Connecticut and perhaps extending into the metro area as well. The probabilistic snow outlook for New York, issued by the city’s National Weather Service office at 6:00 am EDT Monday morning, pegged the most likely snowfall amount for New York City at a very impressive 20”, with a reasonable chance of as little as 11” or as much as 23”.

Boston: Snow totals are more likely to be higher in Boston than in D.C., but again there’s the chance here that warm air wrapping around the nearby surface low will work its way near or into the city. “This will cut into snowfall totals,” tweeted meteorologist Michael Ventrice at The Weather Company (@MJVentrice), pointing to output from TWC’s Deep Thunder model. “If this changeover happens, it would be just for 1-2 hours, then flip back to snow as the low pulls away,” he added. The 11:30 am EDT briefing from NWS/Boston did not include the type of probabilistic guidance noted above for Washington and New York, but it showed an expected range of 12” - 18” for Boston.

Figure 3. The rapidly strengthening surface low associated with Winter Storm Stella will be located just east of the Delmarva Peninsula at 8:00 am EDT Tuesday, March 14, 2017, according to the 12Z Monday run of the GFS model. Precipitation amounts shown are for the six-hour period ending at 8:00 am EDT Tuesday. The rain-snow dividing line evident near Washington, D.C., could end up further northwest or southeast, and the transition zone will move northeastward over time. Image credit: www.tropicaltidbits.com.

What might be Stella’s place in history?
“Nail biter!” said NOAA’s Paul Kocin when I asked him for his take on the storm just after noon on Monday. Kocin is coauthor with NWS director Louis Uccellini of the definitive reference book “Northeast Snowstorms.” “As you can tell, this is a nightmare to forecast, with the rain/sleet/snow transition region right near the big cities,” he added. Northwest of the urban corridor, it’s not so tough, Kocin said: “Easy for Allentown, Poughkeepsie, Hartford, etc.”

There is certainly the potential for Stella to go down in the record books among the bigger March storms in Northeast history. The two greatest March storms on record for the region as a whole are the Great Blizzard of 1888 and the 1993 Storm of the Century, both of which we discussed in our Friday post. Below are the top three March snowstorms for the three cities discussed above.

12.0”: March 27-28, 1891
11.5”: March 28-29, 1942
10.7”: March 7-8, 1941

New York
21”: March 12-14, 1888
18.1”: March 7-8, 1941
14.5”: March 3-4, 1960, and March 1-2, 1914

25.4”: March 31–April 1, 1997
19.8”: March 3-5, 1960
15.5”: March 3-5, 1981

A modest coastal flooding threat by nor’easter standards
The exceptionally strong winds of Stella will drive a peak storm surge of 1 - 3 feet on Tuesday along much of the coast of Delaware, New Jersey, New York, Connecticut, and Massachusetts, including Cape Cod and the islands. The main concern for coastal flooding will come Tuesday during the late morning/early afternoon high tide cycle, when the strong winds of the storm will coincide with high waters from a higher-than-usual high tide, due to the full moon that occurred on Sunday. The highest storm tides are expected in western Long Island Sound, where the peak storm surge is expected about two hours after high tide early Tuesday afternoon. Wave heights on the ocean waters off the coast are forecast to range from 12 to 18 feet, with breaking waves of 6 to 10 feet at the shore, especially along the Atlantic-facing Delaware and Jersey beaches.

The waves and storm surge will cause significant coastal erosion. However, only minor to moderate coastal flooding is expected, due to the relatively rapid motion of the storm across the area. Slow-moving nor’easters are a much bigger coastal flooding threat, since the wind has more time to pile large amounts of water up against the coast. According to Monday morning run of the experimental NOAA Extratropical Storm Surge Model, these are the peak storm surge (height of the water above normal) and storm tide (height of the water above the high tide mark) expected on Tuesday along the coast:

Boston, MA: 2.6’ storm surge, 0.7’ storm tide (peak storm surge occurs at low tide, so a relatively low peak storm tide occurs)
Bridgeport, CT: 3.0’ storm surge, 1.5’ storm tide
King’s Point, NY (east side of NYC): 3.5’ storm surge, 2.5’ storm tide
The Battery, NY: 2.3’ storm surge, 1.4’ storm tide
Sandy Hook, NJ: 2.6’ storm surge, 1.6’ storm tide
Atlantic City, NJ: 2.1’ storm surge, 1.6’ storm tide
Lewes, DE: 2.1’ storm surge, 1.3’ storm tide

In advance of Stella, a dramatic Great Lakes low
A compact area of low pressure called a mesovortex developed over southern Lake Michigan on Sunday, March 12, associated with the strong upper-level trough that will help intensify Winter Storm Stella on Tuesday. Below is a remarkable loop of preliminary, non-operational imagery from the new GOES-16 satellite (animation courtesy NWS/Marquette). See the CIMSS Satellite Blog for more on this feature.

We'll be back with a new post on Tuesday.

Bob Henson and Jeff Masters

Winter Weather

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.