8:09 PM

On December 18th to the 19th, 2009 an area of low pressure tracked from the Gulf of Mexico to the Mid Atlantic coast producing heavy snowfall throughout the Mid Atlantic.  Record breaking snowfall was observed in both Philadelphia, Pennsylvania and Upton, New York.  The heaviest snowfall was observed from southern and central New Jersey through central and eastern Long Island with lesser amounts to the north and west.

Model Guidance Performance:

Leading up to this storm, the model guidance performed relatively poorly for the most part.  In the medium range, there was a strong suggestion by the majority of the model guidance that the low pressure system would remain well to the south and east of the northern Mid Atlantic.  In fact, there was a strong suggestion by the GFS that the low pressure system would track through the Florida peninsula and out into the Bahamas at one point.  The ECMWF did not perform much better with a strong suggestion that the storm would remain far enough out into the Atlantic to produce only a light 4 inches or less over southern New Jersey.  This forecast solution was seen up until two days before the winter storm.

Within 48 hours of the actual storm, models still had a difficult time with the actual forecast.  The GFS remained well south and east of the actual storm track up until 12 hours before the event.  The ECMWF/UKMET/Canadian solution performed much better in this time period with a better handle on the storm track and potential precipitation.  Once again in what has been a theme since late Fall, somehow the NAM lead the way in terms of forecasting the precipitation gradient, storm track, and intensity.  Despite the handicap of resolution issues on the NAM compared to the other model guidance in the 48 hour period, the NAM overall performed very well.

The model guidance had a difficult time with three important factors in this storm.

The first was the handling of the 500 MB evolution over southeastern Canada.  The support for the storm track remaining well south and east of the northern Mid Atlantic was due to the idea of a strong upper low over Quebec.  The overall theme of the guidance was to suggest the a disturbance dropping south from the Hudson Bay would remain weak while a dominant upper low will remain over Quebec.  Meanwhile, the 50/50 upper low over the Canadian Maritimes weakened was well.  The position of these features would lead to a suppressed storm track.  The reality however was the upper level disturbance dropping south from the Hudson Bay was much stronger and the disturbance over Quebec was much weaker.  As a result, the long wave trough was in a better position to support a strong winter storm.

The second feature that was handled poorly was the strength of the Sub Tropical Disturbance.  Up until 36 to 48 hours of the storm impact, the strength of the Sub Tropical Disturbance was under forecasted in strength.  As a result, the forecast of the potential intensification of the surface low off the Mid Atlantic was not handled well.  The implications of a strong Sub Tropical Disturbance would mean that the overall phase would be stronger and the potential for the 500 MB features could close off for at least a short time.

Finally, the magnitude of the dry air was also under forecasted, leading to exaggerated QPF totals over northern Pennsylvania, northwestern New Jersey, and the Hudson Valley.  The dry air lead to a longer than expected duration of virga over these locations.

Storm Impacts:

The blizzard of  2009 started to impact the northern Mid Atlantic early on Saturday morning over southern New Jersey with light snow.  The snow quickly expanded north and west into the Philadelphia metropolitan area by day break with a steady light to moderate snow.  Meanwhile, surface temperatures had fallen to the mid 10′s to lower 20′s over the northern interior and mid 20′s to lower 30′s along the coast.  Dew points however were a more influential factor with a range of the negative single digits over the Hudson Valley to the lower 10′s over much of the coastal plain.

As the low pressure started to intensify off the North Carolina coast, moisture continued to push north throughout the northern Mid Atlantic in the morning hours.  The very dry air over much of eastern Pennsylvania, just northwest of Philadelphia, the northern half of New Jersey, and much of the New York City metropolitan area lead to a prolonged period of virga up through the afternoon hours.  However, a much different story was unfolding over the New Jersey coast.  A strengthening thermal gradient at the mid and lower levels of the atmosphere supported strong lifting over central and southern New Jersey and back through the immediate Philadelphia metropolitan area.  As a result, steady moderate to at times heavy snowfall resulted.

By the evening hours, the low pressure system moved to 50 miles off the Virginia coast.  Very heavy snow and strong wind gusts were experienced along much of the New Jersey coast and over the Philadelphia metropolitan area.  Light snow finally broke through the dry air and was slowly starting to accumulate over locations north and west of Philadelphia and central New Jersey.  As the low pressure system continued to deepen to 990 MB, mesoscale banding started to form over Monmouth and Ocean Counties of New Jersey.  Snowfall rates started to increase to an inch per hour and visibility crashed to below a mile at times.

On Saturday night, the surface low moved into the southern New Jersey coastal waters and stalled briefly for roughly 3 hours from 7 PM to 10 PM.  As the low pressure system deepened to 980 MB east southeast of Atlantic City, very strong mesoscale banding developed from central New Jersey through much of Long Island.  Snowfall rates increased to 1 to 3 inches per hour within these bands.  Additional weaker bands formed over much of New Jersey and along the western shores of the Delaware River.

Light snow continued over northeastern Pennsylvania and the Hudson Valley.  These locations missed on the heaviest snowfall on Saturday night as the best dynamics remained focused to the south and east.  The strong lifting along the coast limited the potential for snowfall in these locations.

The intensity of the snowfall was best illustrated over northeastern New Jersey and the five boroughs of New York City as much of the snowfall up until Saturday evening accumulated to around an inch.  Six hours later after the snow bands, 10 to 15 inches accumulated over much of this region.  That is a 1.5″ to 2.33″ of snow per hour.

The low pressure system exited on Sunday morning with lingering light to moderate snow from eastern New Jersey through Long Island and Connecticut.  All snowfall ended by mid Sunday afternoon.

This winter storm produced record breaking snowfall for Philadelphia at 23.2 inches and in Upton, New York at 26.3 inches.  East Patchogue, New York in Suffolk County had the most snowfall of all the northern Mid Atlantic with 27.5 inches.  The following are official snowfall totals from the National Weather Service.

CONNECTICUT

 

…FAIRFIELD COUNTY…

NORWALK               12.5   735 AM 12/20   SKYWARN SPOTTER

DARIEN                11.5  1200 PM 12/20   CT DOT

NEW CANAAN            10.3  1200 PM 12/20   CT DOT

BRIDGEPORT             9.0   700 AM 12/20   SKYWARN SPOTTER

SHELTON                8.8  1056 AM 12/20   SKYWARN SPOTTER

DANBURY                7.0  1200 PM 12/20   CT DOT

 

…MIDDLESEX COUNTY…

EAST HADDAM           23.5   830 AM 12/20   SKYWARN SPOTTER

CLINTON               20.0   900 AM 12/20   HAM RADIO

OLD SAYBROOK          20.0  1200 PM 12/20   CT DOT

WESTBROOK             19.0  1030 AM 12/20   SKYWARN SPOTTER

CLINTON               16.0   830 AM 12/20   PUBLIC

MIDDLETOWN            12.0   830 AM 12/20   SKYWARN SPOTTER

 

…NEW HAVEN COUNTY…

MADISON               12.2  1027 AM 12/20   SKYWARN SPOTTER

NORTH BRANFORD        12.0   830 AM 12/20   PUBLIC

NEW HAVEN             11.0  1200 PM 12/20   CT DOT

BETHANY               10.1  1030 AM 12/20   SKYWARN SPOTTER

MILFORD               10.0  1200 PM 12/20   CT DOT

NORTH HAVEN            8.5   830 AM 12/20   SKYWARN SPOTTER

BRANFORD               8.0  1006 AM 12/20   SKYWARN SPOTTER

BEACON FALLS           7.5   600 AM 12/20   CT DOT

NAUGATUCK              7.4  1250 PM 12/20   SKYWARN SPOTTER

WATERBURY              6.3  1200 PM 12/20   CT DOT

MERIDEN                6.0   140 PM 12/20   SKYWARN SPOTTER

 

…NEW LONDON COUNTY…

BALTIC                26.0  1100 PM 12/20   PUBLIC

NORWICH               20.0  1200 PM 12/20   CT DOT

OLD LYME              20.0   940 AM 12/20   HAM RADIO

MONTVILLE             19.0   900 AM 12/20   SKYWARN SPOTTER

GALES FERRY           18.5  1045 AM 12/20   SKYWARN SPOTTER

GROTON                18.0  1200 PM 12/20   CT DOT

OAKDALE               17.4  1100 AM 12/20   SKYWARN SPOTTER

LISBON                15.2  1200 PM 12/20   SKYWARN SPOTTER

NORTH FRANKLIN        15.0   850 AM 12/20   SKYWARN SPOTTER

 

NEW JERSEY

 

…BERGEN COUNTY…

PARAMUS                9.5   515 AM 12/20   SKYWARN SPOTTER

SADDLE BROOK           9.0  1000 AM 12/20   SKYWARN SPOTTER

PALISADES PARK         8.8  1000 PM 12/20   SKYWARN SPOTTER

BERGENFIELD            8.2   900 AM 12/20   SKYWARN SPOTTER

RUTHERFORD             8.0   515 AM 12/20   SKYWARN SPOTTER

RAMSEY                 7.8  1000 PM 12/20   SKYWARN SPOTTER

GARFIELD               7.5  1100 AM 12/20   SKYWARN SPOTTER

MAHWAH                 7.2   830 AM 12/20   SKYWARN SPOTTER

RIVERVALE              7.0   700 AM 12/20   SKYWARN SPOTTER

TENAFLY                6.8   700 AM 12/20   SKYWARN SPOTTER

FORT LEE               6.2  1000 AM 12/20   SKYWARN SPOTTER

RIDGEWOOD              6.2  1000 AM 12/20   SKYWARN SPOTTER

FAIR LAWN              6.0  1002 AM 12/20   SKYWARN SPOTTER

GLEN ROCK              6.0  1000 AM 12/20   SKYWARN SPOTTER

NORTHVALE              5.5  1100 AM 12/20   SKYWARN SPOTTER

DUMONT                 4.0   815 AM 12/20   SKYWARN SPOTTER

 

…ESSEX COUNTY…

NEWARK                11.2  1100 AM 12/20   FAA CONTRACT OBSERVER

MILLBURN               8.3   715 AM 12/20   SKYWARN SPOTTER

NEWARK                 8.0  1100 AM 12/20   PUBLIC

 

…HUDSON COUNTY…

JERSEY CITY           10.0   515 AM 12/20   SKYWARN SPOTTER

HARRISON               9.5   515 AM 12/20   SKYWARN SPOTTER

KEARNY                 7.0   800 AM 12/20   PUBLIC

 

…PASSAIC COUNTY…

CLIFTON               11.0   515 AM 12/20   SKYWARN SPOTTER

RINGWOOD              10.0   515 AM 12/20   SKYWARN SPOTTER

 

…UNION COUNTY…

ELIZABETH             11.0   515 AM 12/20   SKYWARN SPOTTER

CLARK                 10.0   515 AM 12/20   SKYWARN SPOTTER

ROSELLE PARK          10.0  1200 PM 12/20   PUBLIC

WESTFIELD             10.0   200 AM 12/20   PUBLIC

SPRINGFIELD            8.0  1215 PM 12/20   SKYWARN SPOTTER

PLAINFIELD             6.8  1201 PM 12/20   SKYWARN SPOTTER

 

NEW YORK

 

…BRONX COUNTY…

BRONX                  9.5   900 AM 12/20   SKYWARN SPOTTER

THROGS NECK            9.5   800 AM 12/20   SKYWARN SPOTTER

EAST TREMONT           9.2   830 AM 12/20   SKYWARN SPOTTER

CITY ISLAND            9.1   800 AM 12/20   SKYWARN SPOTTER

RIVERDALE              8.6   215 PM 12/20   PUBLIC

 

…KINGS COUNTY…

BROOKLYN MARINE PARK  14.0   700 AM 12/20   PUBLIC

SHEEPSHEAD BAY        13.2   430 AM 12/20   SKYWARN SPOTTER

BOROUGH PARK          12.5  1000 AM 12/20   SKYWARN SPOTTER

BROOKLYN              11.8   700 AM 12/20   SKYWARN SPOTTER

 

…NASSAU COUNTY…

WANTAGH               18.0  1100 AM 12/20   SKYWARN SPOTTER

OLD BETHPAGE          17.9  1022 AM 12/20   SKYWARN SPOTTER

MERRICK               17.6  1020 AM 12/20   SKYWARN SPOTTER

PLAINVIEW             16.8   800 AM 12/20   SKYWARN SPOTTER

BELLMORE              16.6  1100 AM 12/20   PUBLIC

ROCKVILLE CENTRE      15.1  1200 PM 12/20   PUBLIC

MINEOLA               15.0   800 AM 12/20   SKYWARN SPOTTER

WOODBURY              15.0  1200 PM 12/20   PUBLIC

WOODMERE              15.0  1100 AM 12/20   PUBLIC

BETHPAGE              14.6   800 AM 12/20   SKYWARN SPOTTER

MASSAPEQUA            14.2  1153 AM 12/20   PUBLIC

OCEANSIDE             14.2   700 AM 12/20   COOP

SEAFORD               14.0   959 AM 12/20   SKYWARN SPOTTER

FLORAL PARK           13.5  1000 AM 12/20   SKYWARN SPOTTER

LIDO BEACH            13.5   900 AM 12/20   SKYWARN SPOTTER

MALVERNE              13.1  1100 AM 12/20   SKYWARN SPOTTER

LYNBROOK              13.0  1100 AM 12/20   SKYWARN SPOTTER

LONG BEACH            12.5   330 AM 12/20   SKYWARN SPOTTER

MILL NECK             12.0   213 PM 12/20   PUBLIC

 

…NEW YORK COUNTY…

NEW YORK              11.4  1100 PM 12/20   SKYWARN SPOTTER – UPPER

NYC/CENTRAL PARK      10.9   700 AM 12/20   CENTRAL PARK ZOO

MANHATTAN              9.5   210 AM 12/20   SKYWARN SPOTTER

HARLEM                 9.0   800 AM 12/20   SKYWARN SPOTTER

 

…ORANGE COUNTY…

TUXEDO PARK            6.0   545 AM 12/20   SKYWARN SPOTTER

WESTTOWN               3.8   830 AM 12/20   SKYWARN SPOTTER

WARWICK                3.7   811 AM 12/20   SKYWARN SPOTTER

GOSHEN                 2.5   800 AM 12/20   SKYWARN SPOTTER

NEW WINDSOR            2.5   700 AM 12/20   SKYWARN SPOTTER

 

…PUTNAM COUNTY…

PUTNAM VALLEY          4.5   635 AM 12/20   SKYWARN SPOTTER

 

…QUEENS COUNTY…

NYC/JFK ARPT          14.2   700 AM 12/20   FAA CONTRACT OBSERVER

FRESH MEADOWS         12.5   900 AM 12/20   PUBLIC

BAYSIDE               11.5   900 AM 12/20   PUBLIC

WHITESTONE            11.1  1130 AM 12/20   PUBLIC

NYC/LA GUARDIA         8.8   700 AM 12/20   FAA CONTRACT OBSERVER

 

…ROCKLAND COUNTY…

MONSEY                 5.5   715 AM 12/20   SKYWARN SPOTTER

NEW CITY               5.0   940 AM 12/20   SKYWARN SPOTTER

STONY POINT            4.0   927 AM 12/20   SKYWARN SPOTTER

PIERMONT               3.9  1000 AM 12/20   SKYWARN SPOTTER

 

…SUFFOLK COUNTY…

EAST PATCHOGUE        27.5  1200 PM 12/20   PUBLIC

MASTIC                26.5   938 AM 12/20   PUBLIC

UPTON                 26.3   156 PM 12/20   NWS OFFICE

HOLTSVILLE            25.0   900 AM 12/20   SKYWARN SPOTTER

MOUNT SINAI           25.0  1115 PM 12/20   NWS EMPLOYEE

BRIDGEHAMPTON         24.0   800 AM 12/20   SKYWARN SPOTTER

EAST SETAUKET         24.0   705 AM 12/20   SKYWARN SPOTTER

NORTH BABYLON         24.0   258 PM 12/20   SKYWARN SPOTTER

NORTH PATCHOGUE       24.0  1000 AM 12/20   PUBLIC

PATCHOGUE             24.0   335 PM 12/20   NWS EMPLOYEE

ISLIP                 23.9   100 PM 12/20   FAA CONTRACT OBSERVER

STONY BROOK           23.7   940 AM 12/20   SKYWARN SPOTTER

MIDDLE ISLAND         23.5  1100 AM 12/20   NWS EMPLOYEE

SOUTHAMPTON           23.0   900 AM 12/20   NWS EMPLOYEE

CENTEREACH            21.0  1000 AM 12/20   NWS EMPLOYEE

MANORVILLE            21.0   130 PM 12/20   PUBLIC

RIVERHEAD             21.0   930 AM 12/20   SKYWARN SPOTTER

RONKONKOMA            20.5   949 AM 12/20   NWS EMPLOYEE

CUTCHOGUE             20.0  1147 AM 12/20   NWS EMPLOYEE

SAG HARBOR            20.0   730 AM 12/20   SKYWARN SPOTTER

SAYVILLE              19.8   700 AM 12/20   NWS EMPLOYEE

LINDENHURST           19.5   915 AM 12/20   SKYWARN SPOTTER

BAITING HOLLOW        18.9  1205 PM 12/20   NWS EMPLOYEE

EAST NORTHPORT        18.5  1000 PM 12/20   PUBLIC

HOLBROOK              18.5   310 AM 12/20   PUBLIC

COMMACK               18.2   500 AM 12/20   SKYWARN SPOTTER

DEER PARK             18.0  1000 AM 12/20   PUBLIC

MEDFORD               18.0   330 AM 12/20   SKYWARN SPOTTER

ROCKY POINT           18.0   157 PM 12/20   NWS EMPLOYEE

BAY SHORE             17.0  1000 AM 12/20   NWS EMPLOYEE

SMITHTOWN             16.4   620 AM 12/20   SKYWARN SPOTTER

WEST ISLIP            16.0   230 PM 12/20   SKYWARN SPOTTER

HUNTINGTON            15.2  1115 AM 12/20   SKYWARN SPOTTER

PORT JEFFERSON        15.0   900 AM 12/20   SKYWARN SPOTTER

MELVILLE              14.0   100 PM 12/20   PUBLIC

 

…WESTCHESTER COUNTY…

PORT CHESTER          12.5   733 AM 12/20   SKYWARN SPOTTER

YONKERS               12.2  1015 AM 12/20   SKYWARN SPOTTER

LARCHMONT             12.0   700 AM 12/20   SKYWARN SPOTTER

SCARSDALE             12.0   730 AM 12/20   SKYWARN SPOTTER

EASTCHESTER           11.5  1000 AM 12/20   SKYWARN SPOTTER

YONKERS               11.0   725 AM 12/20   SKYWARN SPOTTER

NEW ROCHELLE          10.0   900 AM 12/20   PUBLIC

MOUNT VERNON           9.5  1030 AM 12/20   PUBLIC

HASTINGS-ON-HUDSON     9.0   600 AM 12/20   SKYWARN SPOTTER

ARMONK                 8.5   643 AM 12/20   SKYWARN SPOTTER

WHITE PLAINS           7.0  1200 PM 12/20   SKYWARN SPOTTER

HARRISON               6.4   800 AM 12/20   SKYWARN SPOTTER

BRONXVILLE             5.5  1100 AM 12/20   PUBLIC

CHAPPAQUA              5.5  1040 AM 12/20   SKYWARN SPOTTER

 

***********************PEAK WIND GUST***********************

 

LOCATION            PEAK WIND    TIME/DATE    COMMENTS

GUST       OF

(MPH)   MEASUREMENT

 

 

CONNECTICUT

 

…NEW LONDON COUNTY…

GROTON/NEW LONDON       41   216 AM 12/20   ASOS

 

NEW JERSEY

 

…ESSEX COUNTY…

NEWARK                  44  1108 PM 12/19   ASOS

 

NEW YORK

 

…QUEENS COUNTY…

NYC/JFK ARPT            43  1016 PM 12/19   ASOS

 

…SUFFOLK COUNTY…

FARMINGDALE             53  1037 PM 12/19   ASOS

MONTAUK                 48   158 AM 12/20   ASOS

SHIRLEY                 44  1136 PM 12/19   ASOS

ISLIP                   40  1254 PM 12/20   ASOS

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