April 21, 2012: Powerful Storm Delivers Rain, Snow

Forecast Highlights:


- One of the strongest storms this season to affect the region tomorrow, Monday
- Up to 3-4 inches of rain, 50-55 mph wind gusts expected for NYC area
- Unseasonably late snowstorm for Pennsylvania, flakes may reach NW NJ
- Seasonable temperatures return this week


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Today's Observations:

As a slow moving cold front slowly approached the area, warm temperatures were observed today, peaking in the mid 70s to 80 degrees north and west of NYC, in the lower to mid 70s in NYC, and mid 60s to low 70s in coastal NYC, Long Island and southern CT. Cloud cover increased throughout the afternoon hours as the front approached, with thunderstorms, locally heavy, having affected the western parts of the area this evening. The weakening line of light-moderate rain is currently near north central NJ and slowly moving east, reaching NYC by 11:30 PM to 12 AM, but struggling to make it east of NYC.

As the cold front stalls east of NYC tonight, a low pressure currently near the southeastern US will move up the coast while intensifying and pulling in large amounts of moisture. By tomorrow night, this low pressure will intensify into one of the strongest storms of the season, with heavy rain and strong winds expected in the east side of the storm, including NYC, with an unseasonably late heavy wet snowstorm for western Pennsylvania, where as much as a foot of wet snow may fall. Near to below average temperatures will stick around through the rest of the week following the storm, with temperatures cooling down again towards next weekend.


Sunday - Monday: Strong Storm Brings 2-3+ Inches Rain, 50 MPH Gusts


Forecasting this storm over the last week was not very easy, as although models showed the possibility of a big storm early on, the idea of a large storm unfolding was questionable given the very dry pattern, with drought conditions present across the region, and the trend throughout the season for storms to remain relatively weak and progressive without much amplification. This storm marks a change in this pattern as a much more amplified set up is expected during this time frame with a strong, slow moving low pressure moving NNW through the region. At first, I questioned the big storm idea in my forecasts, and although the big storm idea was considered a reasonable possibility, as it became clear that this storm would end up amplified, it also was apparent that my original thinking of a more progressive storm following this winter's pattern would not end up verifying. With a weak, progressive storm out of the question, the region is about to observe one of the strongest storms of the entire season, with impacts ranging from heavy rain and flash flooding to strong winds and even heavy snow in some areas.

Storm Development: The low pressure that will become the strong coastal low is currently a 1002 mb low located off the western coast of Florida. As the southern stream and northern stream phase, the low pressure will intensify as it moves up the coast, deepening to about 996 mb on Sunday afternoon while near eastern North Carolina. At that time, there will be an area of heavy rain near the eastern Mid Atlantic extending up to southern NJ and Washington DC, with light to moderate rain covering the rest of the region except for northwestern areas. The low pressure will continue to move north from that point, with the rain expanding and spreading further north. With the low also intensifying, a tighter pressure gradient will develop with increasingly windy conditions developing near the New Jersey coast and spreading north.

The worst of the storm will take place on Sunday night across most of the region. The low pressure will slow down as it tracks through the eastern half of Pennsylvania, getting down to about 984-986 mb. Heavy rain will move up the north and eastern sides of the storm, spreading through the NYC area followed by New England. Windy conditions will also accompany the heavy rain, with gusts to 45-55 mph possible near coastal areas. Meanwhile, in the western side of the storm where cold air will be pulled in, the rain will begin to change over to heavy wet wind-driven snow near far western NY, the western half of Pennsylvania, and the higher elevations of West Virginia. As the low pressure slowly drifts north through the region on Monday, precipitation will weaken across the region, with some rain persisting in New England while moderate-heavy wet snow continues in the western parts of the region.


Forecast for NYC Area: Rain is expected to develop after at least 1-3 PM across the area. A band of locally heavy rain is likely to move through around the mid-late afternoon hours, with a possible short time frame with lighter rain around the evening. As the low pressure approaches, winds will steadily increase through the evening hours, with the peak of the storm expected to take place between at least 8 PM and 2 AM. During this time frame, heavy rain will fall, with rain rates up to 3/4 to 1 inch per hour possible. Strong winds are also expected, with gusts up to 40-50 mph in the immediate NYC area and 45-55 mph near coastal areas. The rain will weaken by 2-3 AM, with occasional showers and rising temperatures between 2 AM and sunrise.

Temperatures will peak in the first half of Monday in the upper 50s to lower 60s across the area with mainly cloudy skies for the rest of the day along with breezy winds, gusting up to 35-40 mph. Isolated showers will continue especially north/west of NYC, and it is not out of the question that snow flakes may mix with the showers on Monday night in NE PA and possibly in the higher elevations of NW Sussex county.


Regional Outlook: This will end up as the biggest storm across the region since early December, making it nearly 4 and a half months since the last significant storm in the region. Widespread moderate to heavy rain is expected for many places, with amounts near 2-3 inches, locally up to 4 inches, from the eastern Mid Atlantic up the Interstate 95 corridor and further west into Maine. For the NYC area, at least 2 to 3 inches of rain are expected as well, locally up to 4 inches. While the rain will help the region considering that New England is in a severe drought, too much rain at once will still cause some flooding, and especially with heavy rain expected in a short period of time, the dry ground may enhance any flash flooding.

Another notable part of the storm is the heavy wet snow associated with it. There have not been many snowstorms this winter, and if amounts end up near the latest forecasts, this may actually end up as the 2nd biggest snowstorm of the season; this would add to an already odd winter season, as both of the biggest snowstorms actually ended up outside of winter itself, in October and in April. Western Pennsylvania, New York and West Virginia may observe conditions similar to what interior parts of the NYC area saw during late October, with a heavy, wet wind-driven snow accumulating on trees full of leaves, which may result in widespread power outages and tree damage. Above, I posted a snow map for this storm; while it shows the general snow totals, accumulations will highly depend on elevation with this storm, and some places may see less than shown above while others see more.


Stay tuned for occasional storm updates tomorrow reflecting the updated forecasts as well as the observations on this storm.