Winter 2007-2008: Frequent small snow events and lots of mixed precipitation
(Click on any image to see larger image)
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
The overall North American
upper air pattern during the winter of 2007-2008 was largely influenced by a
strong La Nina event in the
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.
30-31 December 2007
13-14 January 2008
26-27 February 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
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
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
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
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,
Figure 5. Plume diagrams for
Figure 6. Plume diagrams from 09Z 29 February SREF for
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
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
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
Figure 11. Skew-T displays of upper soundings from a) 00Z
10 December for
Figure 12. BUFKIT precipitation forecasts from the 12Z
February 1 GFS for a)
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.
Figure 14. a) Temperature forecasts at 925 Mb from 12Z
31 January GFS and
Figure 15. Skew-T from
Figure 16. Plume diagrams for
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.
Hudson-Mohawk convergence event of
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.
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
Figure 19. Satellite images from 2345 UTC 1 January a)
infrared and b) water vapor. Note the
upper trough axis approaching the northeastern
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
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
Figure 22. Plume diagrams for
Figure 23. Visible Satellite imagery from 1531 UTC. Note the north-northwest to south-southeast
oriented band of cloudiness from southern
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
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
There were two potential
high wind events, one on
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
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
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.
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
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.
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
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.
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.
Figure 32. Area Forecast Discussions from NWS
from a) around 22 December, and b) around 430 AM 23 December. Albany, NY
Figure 33. Non-Precipitation Warning for High Winds issued from NWS
at a) 330 PM 22 December, and b) 447 AM 23 December. Albany
Figure 34. Zone Forecast Products from NWS
from 335 AM 23 December for a) the Catskill area, and b) the Capital District area. Albany
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
Figure 36. Skew-T plots from 12Z 9 January for a)
and b) Albany, NY . Note the strong low-level winds in both plots, with westerly winds at Buffalo, NY where the cold front was just tracking through. Buffalo
Figure 37. Skew-T plots from 00Z 10 January for a)
and b) Albany, NY . Note the nearly dry adiabatic lapse rate below 800 Mb at Buffalo, NY 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 Albany , and the winds were weakening through the boundary layer. Buffalo
Figure 38. Storm reports for 9 January from a) the
, and b) our LSRALY. Note that convective wind events occurred in Storm Prediction Center and Buffalo ’s areas, that evolved into a longer term gradient wind event as it tracked east. Binghamton
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.