March 11-15, 2010 Historic Nor’Easter (Click on images for larger displays)

 

This was a storm of historical proportions, especially in terms of wind and rain. There was some snow, sleet and freezing rain in higher elevations of the Catskills, Helderbergs, Berkshires, NW CT, Southern VT and the Adirondacks. There were many forecast challenges associated with this storm, especially the degree of impact of the winter precipitation, magnitude of the flooding, and magnitude of the winds.

 

All sources of guidance and ensembles pointed to a major storm 5 or more days prior to onset, but as usual the fine details of precipitation types, potential flood impacts and wind impacts could not be determined until about 12-24 hours prior to onset. Part of the problem was determining how much cold air would be able to anchor itself in the region to produce winter precipitation. The ECMWF was most accurate in depicting sub freezing temperatures at the surface over higher elevations, 5 days prior to onset. Although temperatures through the rest of the boundary layer were less consistent from guidance sources. By 1-3 days prior to the event, the GFS and GFSEnsembles suggested colder surface temperatures in areas of higher terrain. By 1-2 days prior to the event, even the NAM12 was focusing on colder temperatures around areas of higher terrain. Another aspect of precipitation type that was exceptionally difficult to resolve, was the elevation dependence of snow vs. sleet vs. freezing rain and rain. The levels at which each precipitation type were occurring were not only different from the Catskills to western New England, but fluctuated throughout the day with the diurnal changes in temperatures. MADIS surface observations and temperatures in HYDROVIEW were extremely helpful in displaying temperatures and distiguishing the locations and elevations receiving various types of precipitation. Furthermore, RTMA analysis was beneficial to reveal the freezing temperatures across the terrain.

 

The pure model snowfall forecasts for snowfall in higher elevation were not consistent, with some guidance predicting feet of snow, and others <7 inches. Very short-fuse Winter Storm Warnings and Winter Weather Advisories were issues due to the high levels of uncertainty. It was decided that due to the multiple precipitation types in short distances and elevations, all precipitation types would be highlighted in the Warnings and Advisories, and some distinguishing of elevations would be provided in the statements. In fact, since some parts of the Berkshires reported up to 0.25" of ice, and parts of the Helderbergs reported accumulating sleet and snow, that further reinforced the idea to include all precipitation types in the WSW statements. We initially began the warning idea of 1500' elevation. However, by Saturday afternoon, several reports of sleet and ice were received at elevations around 1000'. We quickly made the change to lower the freezing levels which proved to be more correct.

 

The Flood Watch was issued far in advance of the storm, due to the consistent signal in guidance for multiple inches of liquid equivalent precipitation, especially in the Catskills and western New England. The highly anomalous boundary layer easterly and southeasterly winds, far exceeding -5 SD, suggested heavy precipitation favoring the Catskills and western New England. Within 2 days of the onset of the storm, the NAM12 consistently from run-to-run showed a dramatic display of just how much the precipitation would favor higher terrain, and valleys would experience remarkable downslope and much less precipitation (nicely depicted by the NAM12). Once it was deemed that the Catskills would experience winter precipitation, the flood watch updates had to be carefully worded to still support the potential for flooding.

 

Wind forecasts were another challenge. The anomalous winds were expected to reach Wind Advisory criteria, especially in areas of higher terrain, but there was the possibility for some localized damage from stronger winds. Again, the winds exceeded -5 SD, translating to 70+ knot winds, and the potential to mix the full force of the wind to the surface was in question. Wind Advisories were issued for much of the forecast area. Some of the observed winds outside the forecast area, from New York City and White Plains through Long Island and southern CT, were alarmingly strong, but it was decided the strongest winds were along the coast, away from the inland friction zone. Winds at Bennington, North Adams and Rutland gusted to Wind Advisory levels during the storm. However, as the storm was winding down, we received reports of numerous trees down from the Poughkeepsie area through the Hudson Valley just east of the Capital District, through the Berkshires, NW CT and southern VT. Additional contributions to the fallen trees were a recently thawed moist ground and dormant time of year for tree growth.

 

 

 

Above:  Heights and Vorticity forecasts at 500 hPa from 00Z March 9, and 12Z March 12 from multiple sources of guidance.  Although there was broad agreement on the evolution and track of the slow-moving upper level system, there were important differences when looking at the specific tracks of the upper features in all sources of guidance.  Subtle differences in track and timing could result in significant differences in impacts throughout the northeastern U.S.

 

 

 

Above:  MSLP forecasts from 00Z March 9, and 12Z March 12 from multiple sources of guidance. Although there was broad agreement on the evolution and track of the slow-moving surface low pressure center, there were important differences when looking at the specific tracks of the surface low pressure centers in all sources of guidance.  Subtle differences in track and timing could result in significant differences in impacts throughout the northeastern U.S.

 

 

 

Above:  Ageostrophic winds at 925 hPa from 00Z March 9, and a forecast loop from 12Z March 12 from multiple sources of guidance.  Note the strong north to northwest ageostrophic flow suggesting colder and dryer air continuing to advect into the region during the storm.

 

  

 

Above:  Precipitation and snowfall forecasts from multiple sources of guidance.  Notice the heaviest precipitation was forecasted to favor areas of higher terrain, especially the Catskills and western New England.  The mesoscale model forecasts captured the dramatic differences in predicted precipitation from the higher terrain to the valleys.  Significant snowfall was being predicted in the highest elevations.

 

 

 

Above:  Temperature forecasts at 850 hPa, 925 hPa and a loop of forecasted surface temperatures from multiple sources of guidance.  Notice the 925 hPa and 850 hPa temperatures were suggesting changing precipitation types in different areas at different times, but still favoring snow, sleet and/or freezing rain at higher elevations.  The different sources of guidance were in noticeable disagreement as to the evolution of the 850 hPa and 925 hPa temperatures, creating high uncertainty in precipitation type forecasts. Surface temperature forecasts from different sources of guidance were also in considerable disagreement, but generally showed the coldest temperatures in areas of highest terrain.

 

 

 

Above:  Wind barbs and U wind anomalies at 850 hPa, valid 00Z 14 March from the GEFS and GFS.  Notice the consistency from run-to-run, and as the event neared, the predicted anomalies increased due to better resolution of the system.  Also notice the remarkable areal extent of anomalies exceeding -4 SD, illustrating some of the most widespread extreme wind anomalies ever observed in a cool season nor’easter.

 

 

 

Above:  Wind barbs and U wind anomalies at 850 hPa, valid 00Z 14 March from the SREF and NAM.  Notice the consistency from run-to-run, and as the event neared, the predicted anomalies increased due to better resolution of the system.  Also notice the remarkable areal extent of anomalies exceeding -4 SD, illustrating some of the most widespread extreme wind anomalies ever observed in a cool season nor’easter.

 

 

 

 

Above:  Wind barbs and U wind anomalies at 250 hPa, valid 00Z 14 March from the GEFS and GFS.  Notice the remarkable areal extent of anomalies exceeding -3 SD.  In some cases the wind anomalies exceeded -4 SD, which represents some of the most extreme 250 hPa U wind anomalies ever observed, illustrating how cut off the storm was from the steering flow.

 

 

 

 

Above:  Wind barbs and U wind anomalies at 250 hPa, valid 00Z 14 March from the SREF and NAM.  Notice the remarkable areal extent of anomalies exceeding -3 SD.  In some cases the wind anomalies exceeded -4 SD, which represents some of the most extreme 250 hPa U wind anomalies ever observed, illustrating how cut off the storm was from the steering flow.

 

 

  

Above:  PWAT and anomalies from the GEFS and SREF showing anomalies of 2-3 SD above normal.  The moisture being entrained into the storm was well above normal.

 

Above:  MSLP and anomalies from the GEFS.  Note the surface high over southeastern Canada of nearly 2 SD above normal, and the surface low pressure over North Carolina and Virginia at around 3 SD below normal.  The strong pressure gradient resulted in the strong low-level winds and the strong surface high helped anchor colder air in interior New England and interior NY.

 

  

 

Above:  GEFS probabilities for 2.00”, 3.00” and 4.00” of liquid equivalent precipitation.  Notice the very large area of high probabilities for >2.00”, and some of the largest areas of probabilities for 3.00” and 4.00” ever observed in a cool season nor’easter.  Compare the probabilities with the locations of maxima in the observed precipitation at the bottom of this page.

 

 

 

Above:  SREF probabilities for 2.00”, 3.00” and 4.00” of liquid equivalent precipitation.  Notice the very large area of high probabilities for >2.00”, and some of the largest areas of probabilities for 3.00” and 4.00” ever observed in a cool season nor’easter.  Also note the remarkable agreement between the GEFS and SREF with regard to the 3.00” and 4.00” probabilities.  Compare the probabilities with the locations of maxima in the observed precipitation at the bottom of this page.

 

 

Loops of GEFS plumes from Albany, NY and Islip, NY.  Notice the consistent clustering from run-to-run at values above 1.00”, and maximum values of 3-5”.  Also note that as the storm got closer, there were many members that suggested winter precipitation at Albany, NY.

 

 

Above:  Loops of SREF plumes from Albany, NY and Islip, NY.  Notice the consistent clustering from run-to-run at values above 1.00”, and maximum values of 3-5”.  Also note that the spread was consistently greater than in the GEFS,  but was still pointing to a big precipitation event.

 

 

 

 

 

Above:  Successive loops of water vapor satellite imagery.  Notice the large area that the storm covered, and the multiple smaller impulses embedded within the larger circulation.  Also notice the barotropic circulations that developed over the Dakotas on the western periphery of the large circulation.

 

 

Above:  Visible satellite imagery.  Notice the large area that the storm covered, and the multiple smaller impulses embedded within the larger circulation.  Also note the areas of convection and lightning embedded within the large circulation.

 

 

Above:  Radar loop over the course of 2 days, showing the enhanced precipitation in higher elevations, and the break-up of the precipitation in valley areas.

 

 

 

 

Above:  Radar loop from KDIX showing strong convection with high winds and hail locally just under 1” in diameter.  The 12Z sounding from KOKX showed remarkable instability above 900 hPa that supported the development and maintenance of the convection.

 

 

 

Above:  Observed stage 4 precipitation over the 3+ day storm.  Notice the widespread 3”+ across NJ, southern NY and southern/eastern New England.  Also note the enhanced precipitation in higher terrain, similar to what was predicted in the mesoscale NAM12.  Finally, notice where the heaviest precipitation fell, in eastern Massachusetts and New Hampshire, where the GEFS and SREF probabilities for 3.00” and 4.00” were highest.