2 March 2007 mixed precipitation storm

 

Pre-storm set-up:

 

The NAO, which had been neutral through the month of February, was just beginning to trend slightly positive, as the polar vortex near Hudson Bay Canada was also retreating north (Fig. 1a).  The PNA was trending from slight negative to neutral (Fig. 1b).  A high amplitude upper ridge developed over eastern Canada consistent with the neutral NAO trending slightly positive (Fig. 2).  This upper ridge would play an important role in maintaining strong upper level confluence in eastern Canada that supported a strong, cold surface high pressure center that would keep the low-level cold air in place over much of the interior northeastern U.S. during the 2 March storm. 

 

a)b)

 

Figure 1.  Current and predicted phases of the a) NAO and b) PNA.

 

 

Figure 2.  Plot of 500 Mb heights, height contours, wind barbs, and temperatures for 12Z 2 March 2007.  Note the upper ridge around Hudson Bay Canada.

 

This overall North American upper-level pattern supported a more northerly storm track across the continental 48 states, especially for strong southern-stream based systems tracking out of the southwestern U.S associated with widespread convection.  In fact, the 2 March storm originated in the eastern Pacific Ocean around 26 February, tracked into the southwestern U.S. on the southern periphery of a strong upper low, and tracked into the central plains on 28 February while phasing with northern stream upper energy (Figs. 3a-e).  By 1 March, the system was tracking into the Midwest and western Great Lakes, as it continued to cut off from the main westerlies, slowing the movement of the system through the Great Lakes (Figs. 3a-e).  Severe weather and deadly tornadoes occurred during this time period in the mid Mississippi valley. 

 

a)b)

c)d)

e)f)

 

Figure 3.  Water vapor satellite images with lightning overlay for a) 1800 UTC 27 February, b) 1800 UTC 28 February, c) 1800 UTC 1 March, d) 1800 UTC 2 March, and visible satellite images with lightning overlay for e) 1800 UTC 28 February, and e) 1800 UTC 1 March.

 

Late on 1 March and through 2 March, a strengthening upper-level impulse tracked around the southern periphery of the upper low, enhancing convection across the northern Gulf Coast States and Tennessee Valley.  Widespread severe weather and deadly tornadoes occurred in along the Gulf Coast States and Tennessee Valley, as the surface low pressure center developed in the Ohio Valley and mid Atlantic states, eventually strengthening and tracking through southern New York and New England the evening of 2 March (Figs. 4a-b).  A very anomalously long and broad 850 Mb jet with southerly V winds >5 SD above normal was forecasted to track from the southeastern U.S. through the northeastern U.S. with peak winds of near 90 kt (Figs. 5a-b). The “fist” of the 90 kt jet core (not “nose” since it was so wide and strong, a “nose” just didn’t do justice to the width and magnitude of the jet core) tracked through the mid-Atlantic and just south of Long Island (Figs. 6a-b), while 50 kt winds extended north into southern Canada. Due to the very slow movement of the upper low center into the Great Lakes, only slight low-level cooling and drying occurred behind the system that brought the mixed precipitation to the northeast U.S. and severe weather to the Gulf Coast and Tennessee Valley.  The deeper cooling and drying would have to wait until Sunday and Monday as the upper low finally exited into and through southeastern Canada.

 

a)b)

 

Figure 4.  Mean Sea Level Pressure forecasts from the 12Z 1 March NAM12 and GFS40 at a) 30 hours valid 1800 UTC 2 March and b) 36 hours valid 0000 UTC 3 March.

 

a)b)

 

Figure 5.  850 Mb wind barbs (kt) and U and V winds (shaded) for a) 30 hour Short Range Ensemble Forecast valid 1800 UTC 2 March and b) 30 hour Medium Range Ensemble Forecast valid 1800 UTC 2 March.

 

a)b)

 

Figure 6.  Plot of 850 Mb wind barbs, heights and isotachs (shaded) for a) 1200 UTC 2 March and b) 0000 UTC 3 March.  Note the 90 kt maximum along and the North Carolina Coast at 1200 UTC 2 March and east of Maine at 0000 UTC 3 March.  Also note 50 kt winds into the northeastern U.S. at 1200 UTC 2 March.

 

Forecasting issues:

 

It was clear several days in advance of the storm, that mixed precipitation would affect most of the region.  Defining regions for each precipitation type, accumulations, and timing transitions would be critical to the forecast.  Contributing to the difficulty of the forecast was the wide variety of model solutions for forecasted temperature profiles throughout our region during the storm.  The one high-confidence aspect of the forecast was high QPF, with guidance consistently forecasting around 2” of liquid equivalent in southern areas and .75” to 1.5” liquid equivalent in central and northern areas (Figs. 7a-b). 

 

a)b)

 

Figure 7.  Quantitative Precipitation Forecasts from the 1200 UTC 1 March a) NAM12 and b) GFS.

 

The forecast models were also very inconsistent in the days leading up to the event, with regard to low-level temperatures (850 mb and 925 mb), which were crucial in determining precipitation type.  Three to four days prior to the event, the GFS was the “colder” model, while the NAM was forecasting “warmer” temperatures.  Then, one to two days prior to the event, they flip-flopped drastically, with the NAM depicting colder temps in the low levels, while the GFS was suddenly warmer.  This discrepancy between the operational models also carried over to the ensemble solutions, which were divided into two camps just prior to the event:  the colder MREF, and warmer SREF.  This made forecasting precipitation types a very difficult challenge, especially one to four days in advance of the system.  It also became evident that due to uncertainty, long lead times would not be attainable for this event.

 

There was a general consensus by the afternoon of 28 February, that areas from the southern Catskills through Poughkeepsie and northwestern CT would see mostly rain.  The consensus from the majority of guidance sources forecasted temperatures at least in the lower 40s, adding to concerns for potential flooding due to snow melt, rainfall, blocked storm drains from plowed snow, and the outside chance for ice jams, so a Flood Watch was issued. 

 

There was also a general consensus by the afternoon of 28 February, that the Adirondacks, Lake George Area and southern VT had the best chance for mostly snow, with possibilities of sleet mixing with at times.  Converting the liquid equivalent QPF using a ratio of 8-10:1 and accounting for some sleet as well, warning level snow and sleet was expected over this region.  A Winter Storm Watch was issued.

 

The low confidence part of the forecast was the precipitation type and transitions across the Mohawk and Schoharie Valleys into the Capital District, northern Catskills and Berkshires.  Would there be warning level snow, sleet, or freezing rain, or a combination (Figs 8a-c)?  A Winter Storm Watch was issued for these areas on the afternoon of 28 February, with the understanding that it would be updated to a Warning or Advisory once the details about precipitation types became clearer.

 

a)

b)

c)

 

Figure 8.  Plume diagrams from a) 06Z 1 March MREF and b) 15Z 1 March SREF and c) 18Z MREF.  Note the sleet as the predominant precipitation type in most of the ensemble members.

 

By early on 1 March, several runs of the operational models and ensembles had shown some loose consistency on the expected evolution of the system approaching our region, and the evolution of the sensible weather in the broad mixed precipitation zone.  The important Canadian surface high centered just south of Hudson Bay was setting up with its axis just east of Montreal and Quebec City, nosing south into northern and central New England.  All sources of guidance were indicating that as the new surface low developed along the triple point of the occluded front, warm front and cold front, and tracked across southern New York and New England, strong warm advection associated with the strong 850 Mb jet would push temperatures above freezing in a shallow layer around 850-800 Mb from the western Mohawk Valley to the Capital District and Berkshires (Figs 9a-f and 10 a-d), with deeper warm advection further south, more proximate to the “fist” of the 850 Mb jet core. 

 

a)

 

b)

c)

d)

e)

f)

 

Figure 9.  850 Mb temperatures from the 1200 UTC 1 March NAM12 for a) 18 hours valid 0600 UTC 2 March, b) 21 hours valid 0900 UTC 2 March, c) 24 hours valid 1200 UTC 2 March, d) 27 hours valid 1500 UTC 2 March, e) 30 hours valid 1800 UTC 2 March and f) 33 hours valid 2100 UTC 2 March.  Note the temperatures above freezing as solid contours and below freezing as dashed contours.

 

a)b)

c)d)

 

Figure 10.  850 Mb temperature spread and mean contours from a) 15Z 1 March SREF valid 15Z 2 March, b) 15Z 1 March SREF valid 18Z 2 March, c) 12Z 1 March MREF valid 12Z 2 March, and d) 12Z 1 March MREF valid 18Z 2 March.

 

The surface temperatures were forecasted to reach just above freezing along and south of the Mohawk Valley and Berkshires during the last few hours of precipitation.  However, the forecasted temperatures at around 925 Mb were forecasted to be below -2C through the entire precipitation duration in this region (Figs 11a-d).  The operational model soundings (Figs. 12a-e), thermal profiles, Bufkit data (Figs. 13a-f), and SREF and MREF Plumes (as shown in figure 8) suggested a few inches of snow changing to 1-3” of sleet for the Mohawk Valley and Capital District to the Berkshires, ending as a few hours of cold rain with surface temperatures just above freezing.  There were a few ensemble members suggesting freezing rain to rain.  The surface-based cold layer was forecasted to be shallower over the western part of the Mohawk Valley suggesting a possible prolonged period of freezing rain (suggested by the 925 Mb and 850 Mb temperature forecasts in figures 9 and 10). 

 

a)

b)

c)

d)

 

Figure 11.  925 Mb temperatures from the 1200 UTC 1 March NAM12 for a) 18 hours valid 0600 UTC 2 March, b) 24 hours valid 1200 UTC 2 March, c) 30 hours valid 1800 UTC 2 March, and d) 36 hours valid 0000 UTC 3 March.  Note the temperatures above freezing as solid contours and below freezing as dashed contours.

 

a) b)

c) d)

e) f)

 

Figure 12.  1200 UTC 1 March model forecast soundings for Albany, NY from a) NAM12 18 hour forecast valid 06Z 2 March, b) GFS40 18 hour forecast valid 06Z 2 March, c) NAM12 24 hour forecast valid 12Z 2 March, d) GFS40 24 hour forecast valid 12Z 2 March, e) NAM12 30 hour forecast valid 18Z 2 March, and f) GFS40 30 hour forecast valid 18Z 2 March.  Note in a-d that the isotherm is touching the 0C isotherm, and warmer than 0C in e and f.

 

a)b)

c)d)

e)f)

 

Figure 13.  BUFKIT precipitation overviews from a) 1200 UTC NAM for Albany, NY, b) 1200 UTC GFS for Albany, NY, c) 1200 UTC NAM for Glens Falls, NY, d) 1200 UTC GFS for Glens Falls, NY, e) 1200 UTC NAM for Poughkeepsie, NY, and f) 1200 UTC GFS for Poughkeepsie, NY.  Blue is snow, orange is sleet, red is freezing rain and green is rain.

 

The progress of the HPC WWD forecasts reflected the low confidence in the precipitation type forecast and location of the precipitation transition zone (not shown).  The ice forecast for central and eastern NY was generally ¼” to ½” and the zone shifted north and south with each forecast.  Mostly snow was forecasted along and north of the Mohawk Valley through the Adirondacks, Lake George and Saratoga regions into southern VT. 

 

During the early morning hours of 1 March, a Winter Storm Warning was issued for the Adirondacks, Lake George Region and southern VT, where 7” or more of a snow and sleet combination was expected.  This was based on a high confidence of mostly snow and sleet for this region, and the anticipated liquid equivalent QPF converted to snow using 8-10:1.  Even with the relatively wet snow expected, ensemble and operational model output over the course of 2 or more days were showing 850 Mb U wind anomalies of -4 SD or less over northern New England and northeastern New York (Figs. 5a-b), suggesting high probability of exceeding warning level snows where all snow was expected.  The 250 Mb U wind anomalies of -2.5 SD or less were displaced north of the western Great Lakes, associated with the main upper low, which was forecasted to track just north of our region, so a prolonged precipitation event was not expected, as the upper deformation precipitation would not affect our region.  The Winter Storm Watch was maintained over the Mohawk and Schoharie Valleys, Capital District, Berkshires and northern Catskills, due to very low confidence on whether any one precipitation type or combination would reach warning criteria.

 

Throughout the day on 1 March, it was clearly evident that there was widespread severe convection associated with the system, even into the cold sector into the Great Lakes area.  This suggested that downstream upper ridging and dynamic tropopause processes would result in the system tracking closest to the guidance depicting the further north and west track, and would favor warmer air at the boundary layer.  Surface temperatures were a dilemma, though, with the strong Canadian surface high pressure center continuing to build into our region.

 

The decision during the afternoon of 1 March was to upgrade southern Herkimer County to a Winter Storm Warning for mostly ice, upgrade the Berkshires for snow and sleet at higher elevations, and convert the rest of the Watch area to a Winter Weather Advisory for a few inches of snow changing to an inch or two of sleet, and a few hours of freezing rain or rain before ending.  All the Warnings and Advisories were collaborated with surrounding offices and were reasonably consistent.  Extra staffing was planned for the midnight shift on 2 March with the evening intern able to stay over on the midnight shift to do climate and/or a 06Z sounding if necessary, and an extra forecaster was scheduled.

 

What happened:

 

A band of snow developed along the leading edge of the upper-level warm advection that tracked northeast into our region during the late afternoon and evening of 1 March (Fig. 14).  The low levels were so dry that most of the precipitation did not reach the ground.  During the late evening, just prior to midnight, the leading edge of the deeper moisture rapidly spread north and east across our region (Fig. 15).  Precipitation began as a brief period of snow and sleet from the Mohawk Valley and Schoharie Valleys through the Catskills, Poughkeepsie area and northwestern CT.  The precipitation began as snow in the eastern Adirondacks, Lake George Area, southern VT and the Berkshires. 

 

Figure 14.  Regional radar mosaic of composite reflectivity valid 2200 UTC 1 March. (Courtesy NCAR)

 

Figure 15.  Regional radar mosaic of composite reflectivity valid 0400 UTC 2 March.  (Courtesy NCAR)

 

The precipitation quickly changed to rain over the southern Catskills and Poughkeepsie area, as freezing rain and an occasional mix with sleet became the predominant precipitation type across the Mohawk and Schoharie Valleys, northern Catskills, Capital District, Berkshires and parts of northwestern CT between midnight and sunrise.  Based on this rapid precipitation transition, it was decided around 1:30 AM on 2 March to lower Winter Storm Warnings in southern Herkimer and the Berkshires to Winter Weather Advisory.  This change was still well collaborated with surrounding offices, where Advisories and Warnings continued adjacent to southern Herkimer County.

 

Snow mixed with sleet after sunrise in the Adirondacks and Lake George area, while sleet and freezing rain pushed to GFL and southern VT.  Freezing rain and sleet continued in the Mohawk and Schoharie Valleys into the Capital District, Berkshires and northwestern CT through the morning, becoming just rain in many areas before noon.  Much of the Adirondacks and higher elevations of southern VT had mostly snow and sleet throughout the storm.  Freezing rain and sleet persisted in favored, protected regions of the Mohawk and Schoharie Valleys, Capital District, Berkshires and northwestern CT through much of the morning of 2 March (Figs. 16a-b).  Our Winter Weather Advisory was cancelled in Litchfield County, CT at around 4:30 AM 2 March, while NWS Taunton converted their Advisory in CT and MA to a Freezing Rain Advisory.  NWS Upton maintained their Winter Weather Advisory in southern CT.  The cancellation of the Advisory in Litchfield County, CT was not consistent with Winter Weather and Freezing Rain Advisories that surrounded Litchfield County from our surrounding offices.  However, NWS Upton cancelled their Advisories for southern CT around 8 AM 2 March.  Our office addressed the localized continuation of freezing rain and sleet in Litchfield County, CT with Special Weather Statements.

 

a)

b)

 

Figure 16.  Regional radar mosaic of composite reflectivity valid a) 0900 UTC 2 March and b) 1600 UTC 2 March.  (Courtesy NCAR)

 

Hartford, CT media called shortly after 6 AM inquiring about our office canceling the Winter Weather Advisory for Litchfield County.  They called at least 2 more times before 8 AM and e-mailed pictures of ice in Litchfield County (Figs. 17a-d).  Looking at the pictures, it was difficult to see how much accumulation had occurred on objects, and whether accumulation was continuing, or if melting had begun.  Details on the discussion with Hartford, CT media and the decision-making process for canceling the Advisory will be provided in the lessons learned.

 

a)b)

c)d)

 

Figure 17.  Pictures from Litchfield County, CT sent by WFSB in Hartford CT, e-mailed to our office around 1200 UTC 2 March.

 

The combination of moderate to heavy rain and blocked storm drains required Urban and Small Stream Flood Warnings by mid morning for the Capital District, parts of the Mohawk Valley, Catskills, Taconics and northwestern CT.  Later in the day, rainfall runoff required a river flood warning for Brookfield, CT. 

 

In general, warning level snows, with some sleet, fell in the Adirondacks and higher elevations of southern VT.  Warning level ice accumulated in southern Herkimer, parts of Montgomery (Figs. 18a-b) and Litchfield Counties (Figs. 19a-b).  All other areas except the southern Catskills and Poughkeepsie area received advisory level snow, sleet and ice.

 

a)b)

 

Figure 18.  Ice pictures from the morning of 2 March from near Amsterdam, NY.

 

a)b)

 

Figure 19.  Pictures from Litchfield County, CT on the morning of 2 March.

 

Lessons learned:

 

  • Recognizing widespread convection associated with the system gave clues that despite the initial low-level dry, cold conditions, rapid and deep warming would take place, due to the downstream upper ridging and more northwesterly track of the system.  The 850 Mb jet forecast also hinted at a warmer scenario.  This suggested more freezing rain potential and less initial snow and sleet in the precipitation transition zone, as the warm layer would be deeper than models suggested.  Model guidance still needs to improve precipitation type forecasting, but until then, pattern recognition can help improve our precipitation type forecasts.  A local 06Z ALB sounding would have been helpful as well (Figs. 20a-b).
  • Monitoring mesoscale data through MSAS or LAPS can increase confidence in Warning/Advisory cancellation/continuation decision making.  Derived fields such as Ageostrophic Winds and Temperature Advection can provide important information on temperature trends, in addition to monitoring hourly MSLP analyses (Figs. 21a-c).
  • We must carefully try to determine the weather in locations between observation points.  Using MADIS data on the internet is helpful (Figs. 22, 23a-c, and 24a-c).

 

 

a)b)

 

Figure 20.  Soundings at Albany, NY from a) 0000 UTC 2 March and b) 1200 UTC 2 March.

 

 

 

 

 

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c)

 

Figure 21.  MSAS Ageostrophic wind barbs MSLP and temperature advection valid a) 04Z 2 March, b) 09Z 2 March and c) 12Z 2 March.

 

 

Figure 22.  Plot of observation points available from the FSL MADIS web site.

 

a)

b)

c)

 

Figure 23.  MADIS data (MSLP, temperatures, dewpoints, winds and precipitation) and trends from observation points in and upstream, based on surface wind flow, from southern Herkimer County around 0445 UTC 2 March for a) Poland, Cold Brook, Fort Plain and Hessville, b) Cooperstown, Milford and Ballston Spa, and c) Palatine Bridge, Colonie and Westmoreland.

 

a)

b)

c)

 

Figure 24.  MADIS data (MSLP, temperatures, dewpoints, winds and precipitation) and trends from observation points in and upstream, based on surface wind flow, from Litchfield, County, CT around 1230 UTC 2 March for a) Avon, Wolcott, Southington, and Sherman, b) Torrington, Thomaston, West Simsbury, Litchfield, c) New Hartford, Torrington2.