8-9 March 2013 Snowstorm

 

A powerful coastal storm tracked across the center of the nation and exited the middle Atlantic coast on Thursday March 7th. Another upper level low was tracking across southern Canada and across the Great Lakes. These two systems acted in tandem as the upper low approached western New York overnight Thursday and eventually across Pennsylvania Friday morning. This allowed moisture from the coastal storm to become entrained from the coastal storm back into our region to produce the snow we observed.

The synoptic setup was unusual for our area, with the second upper low being the key player in terms of its interaction with the coastal system and moisture entrainment. Most of the forecast staff could not recall seeing a similar situation in at least the last 10 or 20 years, with an upper low rapidly diving southeastward from Canada while a larger closed low was positioned near or just off the coast. It seemed plausible that in the absence of the Canadian upper low, we may not have had a single flake of snow fall in the Albany CWA if the coastal low was the sole player. This made for an unprecedented situation from a local forecasting perspective.

Numerically, the NAM was the most consistent with the highest QPF totals with the global models roughly half as much. The NERFC guidance continued to support the lighter amounts (ie Non NAM members) with terrain enhancement adding an additional approximately one tenth to one quarter of an inch. The HPC WWD graphics on Thursday afternoon had 4-6" of snowfall for the duration of the event for all of eastern NY, and 6-8" for western New England using a blend of the latest SREF mean, ECMWF and GFS QPF. These totals looked a little high in the valley locations due to the SREF mean running high, and less QPF expected north and west of the Capital Region. Also, how quickly the snowfall would accumulate in the valley locations were an issue (evaporative/dynamic cooling of the column).

Leading up to and even during the event, QPF continued to be a challenge. As mentioned, the NAM was consistent with the highest totals with the global models suggesting much less totals. The NERFC values were also in agreement with the global models with some enhancements into the terrain which resulted in the initial geographic headlines for the terrain.

The 18Z NAM THU showed much lower totals in the Capital Region/upper reaches of the Hudson River Valley. Actually, a shift in the NAM guidance indicated the OKX forecast area, might get much higher snow amounts. The NAM forecasted widespread 1-2+ inches of QPF for our area 1-2 days prior to THU. It indicated high QPF, and in the post analysis it should have been considered more in terms of blends/higher confidence guidance. Also, the NERFC QPF was used, but slight adjustments were made in the valleys prior to 00Z which lowered forecasted liquid equivalent and snowfall totals some.

The HWO was updated during the day Tuesday to suggest a moderate to heavy snowfall event was possible Thursday into Friday. We issued Winter Weather Advisories early Thursday morning for the Catskills, Berkshires, Southern Green Mountains, and Northwest Connecticut for 3-6 inches. This advisory was then expanded early Thursday afternoon to include the Taconics with 3-8 inches continuing in the forecast. The advisory was next updated Thursday evening to expand across all of Ulster and Dutchess counties. During the early morning hours on Friday and the subsequent morning ESTF updates prior to the 630 am, the WSW was reissued for the same locations with 3-8 inches in the forecast.

As snowfall reports expanding in a denser coverage started to arrive toward sunrise, the Winter Weather Advisory was quickly expanded to include most of the Hudson River Valley (including the Capital Region) and a Winter Storm Warning was issued for Eastern Columbia and Eastern Dutchess County. In the end, more snow occurred than forecast.

We measured locally 3 inches with the synoptic observation and 3.1 inches measured at the airport early Friday morning (06Z) with snow continuing to fall. This event featured a wide spread in guidance solutions as late as Thursday with anything from a dusting to 2 feet. While there were a few 3 inch reports early in the midnight shift, there were many lesser reports, radar indicated mainly 15-20 dbz with some areas of 25-30 dbz snow to the south and east. This supported 0.2-0.3 inches per hour most areas. As the night progressed we had few additional reports and later radar reflectivities were less than 20dbz. There were a scattering of reports toward daybreak with widespread reports of 4-5 of snow already on the ground, and the event will end within a few hours. At this time all returns were under 20 dbz, snow had ended in northern Herkimer and Hamilton County.

Guidance suggested using slightly lower QPF amounts, and to low snow to liquid ratios focusing on snow to liquid ratios of 8-10:1. These were too low. The snowfall rates increased before daybreak and through daybreak in a manner that was not well-resolved in short term guidance or in radar trends. Radar was overestimating the snowfall most of the night and snowfall rates based on reflectivities around the Capital District prior to about 09z-10z were 1/10" to 1/4" per hour with higher reflectivities just to the east around the higher terrain. After about 09Z-10Z higher reflectivities (and snowfall rates 1/2" per hour and more) shifted west to the Capital District and continued much of the morning.

The radar trends captured the chaos from bandlets and mini-bands in the radar with this storm. This made snowfall forecasting extremely challenging forecasting the snow rates. This is potentially why the snow tallies were highly variable, and the Capital Region was in one of the sweet spots for the snow accumulations. Additionally, a check of the frost depth for March 7-8 showed it was still 10-11 inches. Snow falling on frozen ground will usually have no problem accumulating.

Ensemble data

Above:  GEFS forecasted 500 hPa heights and anomalies valid 00Z 8 March (left) and initialized 500 hPa heights at 00Z 8 March (right).  The northern stream system tracking out of the eastern Great Lakes was stronger than forecasted.

Above:  SREF forecasted 500 hPa heights and anomalies valid 00Z 8 March (left) and 21Z 7 March 500 hPa heights valid at 00Z 8 March (right).  The northern stream system tracking out of the eastern Great Lakes was stronger than forecasted.

Above:  GEFS forecasted mean sea level pressure and anomalies valid 00Z 8 March (left) and GEFS mean sea level pressure initialized and valid at 00Z 8 March (right).  The surface low was a little further offshore than originally forecasted.

Above:  SREF forecasted mean sea level pressure and anomalies valid 00Z 8 March (left) and 21Z 7 March mean sea level pressure valid at 00Z 8 March (right).  The surface low was a little further offshore than originally forecasted.

Above:  GEFS initialized 850 hPa temperatures valid 00Z 8 March (left) and SREF 21Z 7 March 850 hPa temperatures valid at 00Z 8 March (right).  The air mass over the northeastern U.S. was cold enough to support snow and the tightest thermal gradient was offshore southeastern New England.

 

Above:  GEFS probability for 0.50 inch liquid equivalent precipitation in 24 hours initialized 12Z 5 March and valid 00Z 8 March (left) and initialized 12Z 7 March and valid 18Z 8 March (right).  The higher probabilities extended further west as the onset of event neared.

 

Above:  SREF probability for 0.50 inch liquid equivalent precipitation in 24 hours initialized 21Z 5 March and valid 00Z 8 March (left) and 12 hour probability initialized 21Z 7 March and valid 18Z 8 March (right).  The higher probabilities were relatively consistent from run to run.

 

 

Above:  GEFS plumes diagrams for Albany, NY initialized 12Z 5 March (left) and 12Z 7 March (right).  Notice the clustering at a half inch or less.

Above:  SREF plumes diagrams for Albany, NY initialized 21Z 5 March (left) and 21Z 7 March (right).  Notice the clustering closer to a half inch with a few outliers with higher  and lower amounts.  Also there is a hint of mixed precipitation at the end of the event.

 

Above:  GEFS plumes diagrams for Bradley, CT initialized 12Z 5 March (left) and 12Z 7 March (right).  Notice the clustering around three quarters of an inch.

 

Above:  SREF plumes diagrams for Bradley, CT initialized 21Z 5 March (left) and 21Z 7 March (right).  Notice the clustering around three quarters of an inch but the clustering dropped a little in the later runs.

 

Above:  GEFS plumes diagrams for Monticello, NY initialized 12Z 5 March (left) and 12Z 7 March (right).  Notice the clustering below a half inch and some indication of a mix at the end of the precipitation.

Above:  SREF plumes diagrams for Monticello, NY initialized 21Z 5 March (left) and 21Z 7 March (right).  Notice the clustering near three quarters of an inch early that dropped to less than a half inch later.

Above:  GEFS 250 hPa winds and anomalies initialized and valid 00Z 8 March (left) and SREF (right).  Note the U wind anomalies are between -2 and -3 SD suggesting a relatively slow moving upper system at least partially cut off from the upper steering flow.

Forecast data

  

Above:  Loop of GEFS 850 hPa winds and anomalies initialized at 00Z 5 March (left) and 00Z 8 March (right), both valid 00Z 9 March.  Note the brief period of the nose of the maximum anomalies oriented into eastern NY suggesting a period of enhanced convergence in eastern NY.  The Axis of maximum anomalies shifted south into southern New England and Long Island afterward.

 

Above:  Loop of SREF 850 hPa winds and anomalies initialized at 00Z 7 March (left) and 00Z 8 March (right), both valid 00Z 9 March.  Note the brief period of the nose of the maximum anomalies oriented into eastern NY suggesting a period of enhanced convergence in eastern NY.  The Axis of maximum anomalies shifted south into southern New England and Long Island afterward.

 

Above:  Loop of GFS 850 hPa winds initialized at 12Z 7 March (left) and NAM (right) valid 00Z 9 March.  Note the brief period of the nose of the maximum anomalies oriented into eastern NY suggesting a period of enhanced convergence in eastern NY.  The Axis of maximum anomalies shifted south into southern New England and Long Island afterward.

 

Above:  Loop of the Weather Prediction Center probabilistic forecast snow totals.

 

 

Above:  Loop of the Hydrometeorological Prediction Center probabilistic forecast snow totals for >1 inch (left) and >2 inches (right).

 

 

Above:  Loop of the Hydrometeorological Prediction Center probabilistic forecast snow totals for >4 inches (left) and >6 inches (right).

Above:  Loop of the Hydrometeorological Prediction Center probabilistic forecast snow totals for >8 inches (left) and >12 inches (center) and >18 inches (right).

Above:  Loops of visible satellite imagery.  Note the extension of the enhanced cloudiness from New England into eastern NY illustrating the enhanced snowfall into eastern NY.

 

Above:  Loop of Infrared satellite imagery.  Note the extension of the enhanced cloudiness from New England into eastern NY illustrating the enhanced snowfall into eastern NY.

Above:  Loop of Water Vapor satellite imagery.  Note the extension of the enhanced cloudiness from New England into eastern NY illustrating the enhanced snowfall into eastern NY.

Above:  Loop of KENX radar reflectivity.  Note the enhanced reflectivities that build west into eastern NY.

Above:  Regional radar mosaic loop courtesy of The College of Dupage. 

Above:  Loop of soundings from Albany, NY.  Note the column remains below freezing the entire time but not in the dendritic growth zone.

 

Above:  Plots of 500 hPa heights showing the merging of the upper impulse from the Great Lakes and the offshore impulse.

Above:  Plots of mean sea level pressure showing the occluded surface low offshore.

Snowfall reports

PostMortems/CSTARPostMortems/2013/Mar_7_8_2013/PNS.txt

PostMortems/CSTARPostMortems/2013/Mar_7_8_2013/PNS_BGM.txt

PostMortems/CSTARPostMortems/2013/Mar_7_8_2013/PNS_BOX.txt

PostMortems/CSTARPostMortems/2013/Mar_7_8_2013/PNS_OKX.txt

 

NESIS rank

The storm was ranked a category 2 on the NESIS scale.