Winter 2007-2008:  Frequent small snow events and lots of mixed precipitation

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The winter of 2007-2008 featured many challenging storms, with relatively light snows, and an abundance of mixed precipitation, particularly ice.  Due to the number of events during the winter, it was decided that a general evaluation of the forecast process and guidance sources for 2 broad categories of events would be addressed, rather than individual storm post-mortems.  These two broad categories are:  snow events, and mixed precipitation events, of which there were nearly even numbers of each.  Two special categories will also be featured, since they were significant events, but don’t fall into the main two categories.  These special events are:  Hudson-Mohawk convergence and high wind events.  Many of the snow and ice events featured amounts bordering on advisory and warning criteria, making Winter Storm Watch, Warning and Advisory decisions difficult.  The upper flow across North America for much of the winter was nearly zonal, resulting in relatively fast-moving systems that were not well-resolved in sources of guidance.  The timing and strength of systems were not very well resolved until 12-24 hours prior to the onset of the events, and even then, there was considerable spread in the guidance, contributing to relatively low-confidence forecasts.



The overall North American upper air pattern during the winter of 2007-2008 was largely influenced by a strong La Nina event in the Pacific Ocean.      A –NAO and +PNA were noticeably absent all winter, contributing to the lack of deep cold air over the northeastern U.S., and a broad mean upper trough over central North America.  Some informal discussions and considerations within the meteorological community suggested the winter could be similar to 1970-1971, in which the mean surface storm track was just inland of coastal northeastern U.S.  This was expected to result in below normal snowfall for the I-95 corridor.  However, a look at Albany, NY climatological records, suggested a potential record amount of snow for interior NY and interior New England, since 1970-1971 was one of the snowiest on record for Albany, NY.  Of course, analogs are not perfect, but the surface storm track was largely an inland northeastern U.S. storm track much of the 2007-2008 winter, resulting in ≥90” of snow for Glens Falls, NY through the Adirondacks to Burlington, VT, and points east through interior VT, NH and ME, where near record snows fell.  Albany, NY was just far enough south, that the mixed precipitation suppressed the snowfall and Albany received normal seasonal snow at around 61”.  The I-95 corridor did receive below normal snowfall due to the inland surface storm track.


Evaluation of snow events


The following is a list of the events that were mostly snow for the majority of the NWS Albany forecast area during the winter of 2007-2008.  It should be noted that very few events were all snow for the entire NWS Albany forecast area, so a subjective determination was made in creating the list of predominant snow events for the majority of the NWS Albany forecast area.  Disclaimers will be noted in events that featured periods of mixed precipitation in some areas.  Selected graphics will be presented illustrating the typical upper air and surface features for this event category, along with examples from many sources of guidance, and an evaluation of the value/performance of the guidance.


13 December 2007

16 December 2007 – period of sleet in much of the region

30-31 December 2007

1 January 2008

13-14 January 2008

22 February 2008

26-27 February 2008

29 February-1 March 2008


There were actually two types of snow events this past winter.  One type was the traditional Miller-B pattern, with a secondary storm tracking along the coast of Long Island and southeastern New England.  This pattern featured the upper system tracking just south of the region, and a relatively strong surface high anchored to the north, keeping the low-level cold air over the region. 



Figure 1.  Mean Sea Level Pressure and surface plot for 1800 UTC 16 December.  Note the redevelopment of a secondary surface low pressure center just south of Long Island.  However, also note the strong primary low pressure center over western NY.  The strong primary low pressure center was a recurrent feature throughout the winter, which was associated with relatively strong upper dynamics that tracked just west of our region, at times providing enhanced low level warm advection and multiple precipitation types.  In this case on 16 December, there was an extensive region of sleet that affected the region, in addition to 7+ inches of snow.



a) b)  

Figure 2.  SREF U and V Wind anomalies at 850 hPa from a) 09Z 14 December, valid 00z December 17 and b) 03Z 13 January, valid 12Z 14 January.  The low-level wind anomalies did reach the threshold for historical winter storms (U winds exceeding -4 SD) a few times this winter, but the upper-level systems were always too progressive, never cut off from the steering flow anomalies at 250 Mb were almost always <2.5 SD).



Figure 3.  Plume diagrams from the 12Z 14 December MREF and the 09Z 14 December SREF, showing the relatively little spread in MREF members, and larger spread in the SREF members, which was typical of many events during the entire winter.  Note the period of mixed precipitation indicated in both plumes.  There was considerable skill in identifying mixed precipitation events, but forecasted amounts of each individual precipitation type showed much less skill.


The other type was a quick shot of boundary layer warm advection due to a quick-moving upper impulse tracking around the periphery of a primary upper system centered just north of the Great Lakes.  The quick movement prevented long-duration boundary layer warm advection that could have warmed low levels above freezing, which would have changed the precipitation type to freezing rain, sleet or rain.


There were also two upper jet regimes, some events characterized with our region in the right entrance region, others with our region in the left exit region, with no real preference of one regime over the other.  However, this does further support the well-recognized pattern of upper jet structure we expect in the higher impact events.


The important aspect of the predominant snow events was the inconsistency in the guidance, whether it was deterministic NAM, GFS, ECMWF or the ensembles (SREF and GFSensemble).  In fact, based on guidance for each event, many of the warning-level snow events were thought to be confined to our southern forecast area, only to have the axis of heaviest warning-level snow build north into the Mohawk Valley, Capital District, Lake George and Saratoga Regions, and southern VT as the event was occurring. 


Figure 4.  Probability of 0.50” liquid equivalent precipitation in 24 hours from a) 00Z 10 December MREF, valid 12Z 14 December, and b) 06Z 10 December MREF, valid 12Z 14 December.  Note the evolution in spread in 2 consecutive MREF runs, and the expansion of the higher probabilities to the north.  In many of the warm advection cases this winter where the precipitation band was expected to be east/west-oriented, the precipitation shield, and also the heavier precipitation often extended further north than guidance suggested, resulting in the Capital District, Lake George and Saratoga Regions receiving the upper ranges of guidance-forecasted precipitation.



Figure 5.  Plume diagrams for Albany, NY from the a) 00Z 10 December MREF, and b) 21Z 11 December SREF.  Note the multiple events in the plume diagrams, which increased the difficulty determining clustering and values of forecasted precipitation for various precipitation types.  The frequency of events through the winter and multiple precipitation types being forecasted at various locations complicated the analysis of the plume diagrams, and consequently resulted in low confidence forecasts for some events.




Figure 6.  Plume diagrams from 09Z 29 February SREF for a) Albany, NY, b) Saranac Lake, NY, and c) Monticello, NY.  Note the clustering at lower precipitation amounts at in northern areas such as Saranac Lake, NY and higher amounts at southern locations such as Monticello, NY.  The sources of guidance in many of the events during the winter showed these tendencies.  Keep in mind, that Glens Falls had much more snow this season than Albany and points south.  Also keep in mind that snow to liquid ratios are not considered in any guidance QPF, so less liquid equivalent precipitation likely resulted in more snow in some events, especially north of the Mohawk Valley and Capital District.




Figure 7.  Probability of snow, rain, ice pellets and rain from the 09Z 14 December SREF, valid 18Z 16 December.  The SREF precipitation type probability displays were helpful in determining precipitation types in many events this winter.  Note the indication of >50% probability of ice pellets in much of the region during the 16 December event, which did occur, in addition to 7”+ of snow.



Figure 8.  Probability of 1”+, 4”+, 8”+ and 12”+ of snow from the 21Z 12 January SREF valid 18Z 14 January.  This guidance was helpful in determining potential snowfall amounts in pure snowfall cases, but because of how probabilities are calculated, based on many ensemble members, there was a tendency to slightly underestimate snowfall amounts, and confine them to a smaller area than what eventually occurred.






Figure 9.  Regional radar reflectivity from a) 2356 UTC 30 December, b) 0256 UTC 31 December, c) 0557 UTC 31 December, and d) 0858 UTC 31 December.  Note the banding early in the event, evolved into a prominent upper deformation zone, with high reflectivity bands extending north into the Adirondacks, Lake George and Saratoga regions.


Evaluation of mixed precipitation events


The following is a list of the events that were mixed snow, sleet, freezing rain and rain for the majority of the NWS Albany forecast area during the winter of 2007-2008.  Selected graphics will be presented illustrating the typical upper air and surface features for this event category, along with examples from many sources of guidance, and an evaluation of the value/performance of the guidance.


2-3 December 2007

9-10 December 2007

1 February 2008

6-7 February 2008

12-13 February 2008

4-5 March 2008

8-9 March 2008



Figure 10.  Radar reflectivity and MSLP for 1200 UTC 3 December.  Note the primary surface low pressure center over northern NY is stronger than the secondary redevelopment south of New England.






Figure 11.  Skew-T displays of upper soundings from a) 00Z 10 December for Albany, NY, b) 00Z 10 December for Buffalo, NY, c) 12Z 10 December for Albany, NY and d) 1200 UTC 10 December for Buffalo, NY.  Note the wind profile evolution at each site, and the associated thermal advection between 925 Mb and 700 Mb.  Also note the warm layer at and above 800 Mb, which does not show up in traditional 850 Mb plots.  Analysis of upstream soundings for wind profiles and thermal advection can help with short term evaluation of evolution of precipitation type.





c) d)


Figure 12.  BUFKIT precipitation forecasts from the 12Z February 1 GFS for a) Albany, NY, b) Saranac Lake, NY, c) Glens Falls, NY, d) Poughkeepsie, NY.  Note the decreasing amounts of freezing rain and ice pellets toward southern locations, but all locations suggest some period of rain.  Warning level ice is forecasted only for Saranac Lake, but determining areal extent of potential warning level ice proved extremely difficult for all mixed precipitation events.




Figure 13.  Mean MSLP from a) 00Z 31 January GFS Ensemble (with GFS Ensemble members overlayed) valid 06Z 2 February and b) 09Z 31 January SREF valid 06Z 2 February.  Note the remarkable agreement with the ensemble means and the individual members.  Also note the subtle but important difference in the surface low track, with the SREF further northwest.  This was a consistent bias all winter, however, when mixed precipitation was being forecasted, deterministic and ensemble guidance did forecast the surface feature to track inland of the coast.


a) b)


Figure 14.  a) Temperature forecasts at 925 Mb from 12Z 31 January GFS and NAM, and b) ageostrophic wind barbs and isotachs from the 06Z 31 January GFS between 1000-850 Mb.  Note the 925 Mb temperatures well below freezing for much of the forecast area, and the very strong north to northeast low-level flow keeping the low-level cold air in the region.  Also note the southerly ageostrophic flow in OH/WV/PA that is tracking toward our region, which was common in many of the mixed precipitation events, and resulted in precipitation transitions from frozen to freezing and liquid precipitation across much of the forecast area.


a) b)


Figure 15.  Skew-T from Albany NY from a) NAM and GFS 12Z 31 January forecast, and b) observed.  Note the difference in the forecasts and the observed, highlighting the subtle but significant differences in forecasts and real-time conditions.  The boundary layer warmed quicker in this case.






Figure 16.  Plume diagrams for Albany NY from the a) 09Z SREF 5 February, b) 09Z 6 February, c) 12Z 5 February and d) 12Z 6 February.  Note the difference in forecasts not only between the MREF and SREF, but from earlier to later model runs.  The amounts of each precipitation type changed significantly from model run to model run, contributing to relatively low confidence forecasts in precipitation types and areal extent of each precipitation type.



Figure 17.  Precipitation probability from 09Z 13 February SREF, valid 15Z 13 February.  These SREF precipitation probability displays were helpful in forecasting where and when various types of precipitation would fall.  However, due to the nature of ensembles, some of the finer details were smoothed out, creating uncertainty in areal extent of precipitation types.


Evaluation of Hudson-Mohawk convergence event of 2 January 2008


The Hudson-Mohawk Convergence phenomenon is a Master’s Thesis topic, researched by Mike Augustyniak.  The figures below support the fact that the snow event that occurred on the morning of 2 January was a Hudson-Mohawk Convergence event.  These phenomena are extremely difficult to forecast more than 3 to 6 hours prior to occurrence, especially when trying to determine the intensity and areal extent of the snow.  The pattern supporting these events can be resolved in forecast guidance 12 or more hours in advance, but due to the mesoscale and local scale nature to the phenomena, the intensity and areal extent are most important when forecasting the sensible weather.  If possible, it is best to highlight the possibility of an event in the Area Forecast Discussion with as much lead time as possible, then look at mesoscale and local scale data to pinpoint the locations and amounts 1 to 6 hours prior to potential advisory or warning level amounts.  Some weak Hudson-Mohawk Convergence events occur nearly every winter, and are often below advisory or warning criteria, are usually weak, and brief, associated with exiting, diminishing precipitation at the end of a storm.  However, sometimes >4” of additional snow can occur locally, which has a significant impact on road crews, Albany airport, and schools, especially when it seems like a “surprise” to them, and little to no snow is falling outside the Capital District area.  This event occurred 6-12 hours after the 1 January snowstorm departed.  The snow maximum was 3-5” from Clifton Park in Saratoga County to North Colonie in Albany County, while around 2” fell toward Troy and Brunswick.  Little to no snow fell outside of this area.


a) b)


Figure 18.  Plot of 850 Mb heights, contours, wind barbs, temperatures and dewpoints from a) 00Z 2 January, and b) 12Z 2 January.  Note the upper low slowly exiting the northeast, and by 12Z the trough axis extends through interior New England and the mid-Atlantic.  This upper energy is usually associated with enhanced low-level moisture and instability. 


a) b)


Figure 19.  Satellite images from 2345 UTC 1 January a) infrared and b) water vapor.  Note the upper trough axis approaching the northeastern U.S., that would eventually track through our region through the morning of 2 January.



a) b)


Figure 20.  Early morning a)12Z Skew-T sounding from Albany, NY and b) 00Z 2 January WRF forecasted wind barbs and MSLP valid 14Z 2 January.  Note in the 12Z sounding that there was a nearly saturated layer through 700 Mb, with a significant depth of the cloud at -12ºC to -18ºC, ideal for dendritic snow growth.  The winds and MSLP field over the region showed subtle convergence with north to northeast winds in northern NY and VT, and north to northwest winds in central and southern NY, and southern New England.


a)  b)


Figure 21.  Snow accumulation valid 23Z 2 January from a) 00Z 2 January, and b) 12Z 2 January.  Note the WRF focused the snow maxima along terrain, such as the Helderbergs along the Albany/Schoharie County border, and along the Taconics just east of the Hudson River.  Recall that the observed snow maximum was 3-5” from Clifton Park in Saratoga County to North Colonie in Albany County, while around 2” fell toward Troy and Brunswick.  


a) b)


Figure 22.  Plume diagrams for Albany, NY from a) 00Z 02 January MREF, and b) 21Z 01 January SREF.  Note each ensemble hinted at a low liquid equivalent QPF event for Albany during the morning of 2 January.



Figure 23.  Visible Satellite imagery from 1531 UTC.  Note the north-northwest to south-southeast oriented band of cloudiness from southern Saratoga County through Albany and Rensselaer Counties.


a)  b)


Figure 24.  Convergence in the surface wind field at 14Z 2 January in a) surface plot, and b) MSAS wind and MSLP plot.  Note the north to northeast winds north of Albany into VT, and the northwest winds from Albany and points west and south, providing some of the mesoscale low-level convergence.  Local channeling of wind down the Hudson and Mohawk Rivers, converging around the Capital District enhances the convergence, resulting in stronger upward motion and the extremely localized snow events around Albany.


a) b)


Figure 25.  KENX radar reflectivity images from a) 0924 UTC 2 January, shortly after the heavier snow began, and b) 1633 UTC 2 January, when the snow was rapidly diminishing.  Note the orientation of the band and location of the reflectivity maxima remained very consistent throughout the event.



Evaluation of high wind events of 23 December 2007 and 9 January 2008


There were two potential high wind events, one on 23 December 2007, and one on 9 January 2008.  The 23 December event was forecasted to be a warm advection, southerly wind event, while the 9 January event was a cold advection, west to northwesterly wind event.  Southerly winds were forecasted to gust at 65-90 Kt in many locations above 1500 feet in our forecast area.  The 23 December event produced only isolated wind damage to a few high elevations, with peak winds in most locations in the forecast area below Wind Advisory criteria. 


The 9 January event produced more widespread strong winds, with much of the region experiencing peak winds in the Wind Advisory range.  The 9 January event was well anticipated by northeastern U.S. NWS offices, noted in all guidance sources (not shown) and local NWS Area Forecast Discussions (not shown.  The forecast confidence in the 9 January event was likely due to past events such as 17 February and 29 October 2006, compared to largely convective wind events such as 1 December 2006.  Based on storm reports across NY, there were some convective wind events before the convection dissipated, but only in Buffalo and Binghamton’s forecast areas, then it evolved into a pressure gradient event. 


The general lessons from these two events are:


1.  It is difficult to experience widespread high wind events associated with warm advection, especially southerly winds, unless there is a consensus from most guidance sources that dry adiabatic lapse rates will be realized to the surface, which is rare due to frequent surface-based inversions during the winter in warm advection situations.  Based on studies from NWS Burlington, VT, southeast and east winds can produce significant winds in the Green Mountains, due to some upslope processes, but rarely southerly winds.


2.  We should match up the NPW forecasted winds with the ZFP forecasted winds as best as we can.  In the 23 December event, the NPW suggested up to 90 MPH wind gusts, while the ZFP suggested up to 105 MPH.


3.  Strong pressure gradients must be associated with a storm system with a well-defined eastward movement through or just north of the Great Lakes, otherwise, the tightest pressure gradient will not track into our region.


4.  The highest probability for high winds are associated with cold advection, as the boundary layer cools rapidly, the lapse rate between the surface and boundary layer trends to dry adiabatic, resulting in the most efficient mixing of winds from aloft to the surface.


a) b)


Figure 26.  Mean Sea Level Pressure from MSAS at 12Z 23 December and initialized Mean Sea Level Pressure from 00Z 24 December MREF and anomalies.  Note the lack of eastward movement of the surface low pressure and associated pressure gradient.


a) b) c)


Figure 27.  Forecasts from a) 12Z 22 December of GFS ensemble mean 850 Mb winds valid 00Z 24 December, b) 12Z 23 December WRF 30 AGL winds valid 23Z 23 December and c) 00Z 23 December WRF surface winds valid 23Z December.  Note the very strong 60 Kt+ winds forecasted in the GFS Ensemble mean, suggesting most of not all members were forecasting these strong southerly winds.  The SREF depicted a very similar wind forecast (not shown).  The WRF forecasts were very similar to the NAM and GFS operational forecasts at all levels, but provided more detail in the 30 AGL and surface winds forecasts.  The 850 Mb, 30 AGL and surface wind forecasts were consistent from run to run in all operational forecast models.  Note the significant difference between the 30 AGL forecasts and the surface wind forecasts, illustrating the forecast models were resolving some sort of low-level inversion preventing much of the wind from mixing to the surface in this warm advection situation.


a) b)


Figure 28.  Winds and anomalies (color shaded) from a) 09Z 22 December SREF valid 00Z 24 December, and b) 00Z 24 December GEFS valid 00Z 24 December.  Note the forecasts from the SREF and the observed from the GEFS were similar, suggesting consistency and accuracy to the forecasts from guidance for V wind anomalies to range from 4 SD to >5SD from normal.


a) b)


Figure 29.  MSAS mean sea level pressure and pressure tendency at 18Z 23 December and 21Z December.  Note the rise/fall couplet continuing to track east through western NY and PA, but the surface low pressure center was in the western Great Lakes.  The pressure gradient was not tightening over much of the northeastern U.S.


a) b)


Figure 30.  Radar reflectivity from 22Z 23 December and 03Z 24 December.  Note the weakening of the convection associated with the surface pressure rise/fall couplet in figure 3, implying the loosening of the pressure gradient and weakening low-level forcing.


a)  b) c)


Figure 31.  Forecasted reflectivity from the a) 12Z 22 December WRF valid 15Z 23 December, b) 12Z 23 December WRF valid 15Z 23 December and c) observed radar reflectivity from 15Z 23 December.  Note the similarity, with the WRF resolving the upslope rain in the Catskills and the oncoming frontal rains in western areas.

a) b)
Figure 32.  Area Forecast Discussions from NWS Albany, NY from a) around 4 PM 22 December, and b) around 430 AM 23 December.
a) b)
Figure 33.  Non-Precipitation Warning for High Winds issued from NWS Albany at a) 330 PM 22 December, and b) 447 AM 23 December.
a) b)
Figure 34.  Zone Forecast Products from NWS Albany from 335 AM 23 December for a) the Catskill area, and b) the Capital District area.


a) b)


Figure 35.  Surface plot and mean sea level pressure from a) 12Z 9 January and b) 00Z 10 January.  Note the strong 988 Mb surface low pressure center tracked along the U.S./Canada border and the tight pressure gradient tracked through much of New York and northern New England after the cold front tracked through.


a) b)
Figure 36.  Skew-T plots from 12Z 9 January for a) Albany, NY and b) Buffalo, NY.  Note the strong low-level winds in both plots, with westerly winds at Buffalo where the cold front was just tracking through.
a) b)
Figure 37.  Skew-T plots from 00Z 10 January for a) Albany, NY and b) Buffalo, NY.  Note the nearly dry adiabatic lapse rate below 800 Mb at Albany after the cold front tracked through, suggesting much of the strong winds at the boundary layer were mixing to the surface.  The pressure gradient was relaxing around Buffalo, and the winds were weakening through the boundary layer.
a)Map of 080109_rpts's severe weather reports b) 
Figure 38.  Storm reports for 9 January from a) the Storm Prediction Center, and b) our LSRALY.  Note that convective wind events occurred in Buffalo and Binghamton’s areas, that evolved into a longer term gradient wind event as it tracked east.  
Thanks to the Storm Prediction Center, E-Wall at Pennsylvania State University, University Corporation for Atmospheric Research, Forecast Systems Laboratory, Eyewall server from NWS State College, PA and Pennsylvania State University, and the National Centers for Environmental Prediction for images.