9 December 2009 First big snow of the season

 

This was a complicated significant "mixed" winter event involving a strong primary low moving through the central and eastern Great Lakes region with an impressive secondary low forming near ERN PA and NJ. The secondary wave deepened and intensified to 990-995 hPa, as it moved over Long Island in the late morning through the early pm. The track and intensity of the secondary low was pivotal for the higher snowfall amounts and for snow being the main ptype.

 

The NAM apparently did a better job with the thermodynamic profiles for the ptype in the event. The GFS was warmer and indicated a faster transition to liquid rainfall.

 

The 12Z KALB sounding was helpful for upgrading a larger portion of the CWA to a warning with the very cold thermal profiles for an all snow ptype through the rush hour. Also, ground truth coupled with radar returns of 25-35 dBZ's confirmed snow rates of 1-2+ inches per hour between 6 am-10 am. The 12Z ALB SCD of 4.2 inches and numerous NWS employee reports of 3-5+ inches helped the forecaster staff increase the advisories to warnings.

 

There was very limited mixed precipitation with the event, with only a narrow ribbon of IP/FZRA. There was a quick transition to rain in the mid Hudson River Valley in the mid to late morning. The precipitation shield diminished before a transition to -RA/-FZRA in most other locations.

 

The MSAS/LAPS data was important for the near term updates showing the isallobaric tendency of the deepening low.

 

BufKit profiles were good for the wind event east of the Capital Region. It was mainly an advisory event with 925 hPa E/SE winds around 40-45 kts. However, KDDH did have one gust to 59 mph. KAQW and KDDH mainly had advisory gusts in the 46-57 mph range. Downsloping limited snow accumulations in extreme western Bennington, Rensselaer, Columbia, Berkshires and portions of Washington counties. We received a notice from Steve LaPointe forwarding to us a report concerning 100+ mph winds in Rensselaer County. This report was received from LaPointe on 12/14/09. A viewer reported a 105.6 mph wind gust recorded on his Oregon Scientific Model 12A as well as a downed radio tower. Oregon Scientific instrument was mounted on a pole about 20 feet high.

 

18Z NAM showed 30AGL winds of 60 knots in the Berkshires. I believe 12Z NAM showed about 50 knots of easterly flow in downsloping areas of the Greens/Berks/Taconics.

 

 

Above: Heights and vorticity at 500 hPa from the NAM (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right) initialized at 0000UTC 7 December and valid at 1200 UTC 9 December (left), 1800 UTC 9 December (center) and 0000 UTC 10 December (right). Note the general consensus for the upper system to track through the Great Lakes, which implies warm advection and a potential for mixed precipitation. The negative tilt of the system and channeled vorticity tracking through southern NY and western and northern New England implies enhanced upper dynamics.

 

 

Above: Heights and vorticity at 500 hPa from the NAM (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right) initialized at 1200UTC 8 December and valid at 1200 UTC 9 December (left), 1800 UTC 9 December (center) and 0000 UTC 10 December (right). Note the general consensus for the upper system to track through the Great Lakes, which implies warm advection and a potential for mixed precipitation. Similar to the 0000 UTC 7 December guidance, the negative tilt of the system and channeled vorticity tracking through southern NY and western and northern New England implies enhanced upper dynamics.

 

 

Above: MSLP from the NAM (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right) initialized at 0000UTC 7 December and valid at 1200 UTC 9 December (left) and 0000 UTC 10 December (right). Note the slight differences in the resolving of a secondary surface low pressure wave tracking through southern NY and interior New England, with the primary surface low in the Great Lakes. This implies a surface low track that can support mixed precipitation over the interior northeastern U.S.

 

 

Above: MSLP from the NAM (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right) initialized at 1200UTC 8 December and valid at 1200 UTC 9 December (left) and 0000 UTC 10 December (right). Note the slight differences in the resolving of a secondary surface low pressure wave tracking through southern NY and interior New England, with the primary surface low in the Great Lakes. Similar to the 0000 UTC 7 December guidance, this implies a surface low track that can support mixed precipitation over the interior northeastern U.S.

 

 

Above: Temperatures at 850 hPa and 925 hPa from the NAM (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right) initialized at 0000UTC 7 December and valid at 1200 UTC 9 December (left) and 0000 UTC 10 December (right). Note that temperatures warm to near or just above freezing through the period over much of the interior northeastern U.S. implying the potential for a mix and changeover to rain.

 

 

Above: Temperatures at 850 hPa and 925 hPa from the NAM (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right) initialized at 1200UTC 8 December and valid at 1200 UTC 9 December (left) and 0000 UTC 10 December (right). Similar to the 0000 UTC 7 December guidance, note that temperatures warm to near or just above freezing through the period over much of the interior northeastern U.S. implying the potential for a mix and changeover to rain.

 

 

Above: Liquid equivalent precipitation prediction from the NAM (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right) initialized at 0000UTC 7 December (left) and initialized at 1200 UTC 8 December (right) valid at 0000 UTC 10 December. Note that much of the northeastern U.S. had an inch or more of liquid QPF.

 

 

Above: Precipitation type from the SREF initialized at 0300 UTC 7 December for rain (upper left), snow (upper right), freezing rain (lower left) and sleet (lower right), valid 1200 UTC 9 December (left), 1500 UTC 9 December (center) and 1800 UTC 9 December (right). Note the suggestion of snow changing to sleet and freezing rain and rain through the period.

 

 

Above: Precipitation type from the SREF initialized at 1500 UTC 8 December for rain (upper left), snow (upper right), freezing rain (lower left) and sleet (lower right), valid 1200 UTC 9 December (left), 1500 UTC 9 December (center) and 1800 UTC 9 December (right). Similar to the 0300 UTC 7 December SREF, note the suggestion of snow changing to sleet and freezing rain and rain through the period.

 

 

Above: GFSEnsemble MSLP and MSLP members (upper left), probability of 0.05 liquid equivalent in 6 hours (upper right), mean 850 hPa temperature (lower left) and mean liquid equivalent precipitation (lower right). Probability for 0.05 liquid equivalent and mean liquid equivalent precipitation are upper right and lower right, respectively in the far right 4-panel. The left two 4-panels were initialized 0000 UTC 7 December and the right 4-panel was initialized 1200 UTC 8 December. Note the categorical chance for precipitation, and the 0.60 or more liquid equivalent precipitation over much of the northeastern U.S.

 

 

Above: GFS 925 hPa and 850 hPa temperatures and heights and anomalies (color shaded), initialized 1200 UTC 8 December and valid 1500 UTC 9 December.

 

 

Above: NAM 925 hPa and 850 hPa temperatures and heights and anomalies (color shaded), initialized 1200 UTC 8 December and valid 1500 UTC 9 December.

 

 

Above: Wind anomalies at 850 hPa (left), heights and temperatures at 850 hPa (left center), MSLP and PWAT (center right), and 24 hour probability of 1.00 liquid equivalent precipitation from the GFSEnsemble initialized at 1200 UTC 8 December. Note the strong south to southeast winds at 850 hPa and the temperature and moisture advection. The chances for 1.00 liquid equivalent precipitation were categorical over much of the region.

 

 

Above: Wind anomalies at 850 hPa (left), heights and temperatures at 850 hPa (left center), MSLP and PWAT (center right), and 24 hour probability of 1.00 liquid equivalent precipitation from the SREF initialized at 0300 UTC 8 December. Note the strong south to southeast winds at 850 hPa and the temperature and moisture advection. The chances for 1.00 liquid equivalent precipitation were categorical over much of the region.

 

 

Above: (left) Ageostrophic wind and isotachs at 925 hPa from the 1200 UTC 8 December NAM (upper left), GFS (upper right), ECMWF (lower left) and GGEM (lower right).

(Center) MSAS surface temperatures and winds at 1500 UTC 9 December.

(right) MSLP and MSLP change at 1500 UTC 9 December.

 

Note the strong ageostrophic winds from the north, implying continued isallobaric wind flow and cold advection at low levels. The surface wind barbs and temperatures show the continued surface cold advection as well. The strongest surface pressure falls were over Long Island, implying that the secondary surface low could become the primary surface low, keeping the cold air in our region, and potentially resulting in no changeover to rain.