The Valentine’s Day Snowstorm of 2007
This storm occurred while the NAO was neutral, which had
been the NAO phase since the beginning of February (Fig. 1A). The PNA was positive, and had been in this phase
since mid January (Fig. 1B). The PNA was
not very strongly positive, but positive enough so that the teleconnection with
Polar Vortex centered around
Figure 1. Observed and forecasted NAO (left) and PNA
potential for a significant storm became evident 5 to 7 days before the storm,
around 7-9 February. As has been typical
this winter, the GFS and ECMWF solutions diverged in terms of timing, track and
strength of the system, but both models suggested a Miller-B type storm
redeveloping off the
the storm was expected in 3 to 5 days, the ECMWF and GFS began to converge
toward a common timing track and strength of the storm. However, the GFS was still suggesting a more
southerly track and colder air in the northeastern U.S., but differences with
the ECMWF were much less. Even with the
differences in the long-range forecast guidance, there was a decent consensus
that a southern stream upper system would phase with a northern stream upper
low dropping southeast out of
Figure 2. A) 03Z 11 February SREF 850 hPa forecasted U wind anomalies (top) and V wind anomalies(bottom) valid 12Z 14 February. B) 18Z 11 February MREF 850 hPa forecasted U wind anomalies (top) and MSLP anomalies (bottom) valid 00Z 15 February.
Figure 3. A) 15Z 12 February SREF probability for 1.00” liquid equivalent and spread valid 06Z 15 February. B) 12Z 12 February MREF probability for 1.00” liquid equivalent and spread valid 06Z 15 February.
Figure 4. Plume diagrams for
consensus from all sources of guidance, the GFS,
on the water vapor (Figs. 5A-D), infrared and visible satellite pictures (Figs.
6A and 6B) of the storm as it tracked out of the eastern Pacific across the southwestern
U.S. and into the southern plains, this was a very well-developed system
(evident in 500 hPa analyses in figs. 7A-D) with associated convection
suggesting downstream upper ridging would likely produce a more northwestern
storm track and precipitation transition zone.
As the storm lifted out of the lower
Figure 5. Water Vapor satellite imagery with 15 minute lightning plot overlay for A) 1200 UTC 11 February, B) 1145 UTC 12 February, C) 2045 UTC 13 February and D) 1945 UTC 14 February.
Figure 6. Visible satellite imagery with 15 minute lightning plot overlay for A) 2045 UTC 13 February and B) 1945 UTC 14 February.
BUFKIT, SREF/MREF probabilities for 1.00” QPF, and Plume diagrams showed QPF
ranges unusually high for a nearly all snow event for the
Figure 7. 500 Mb heights, contours, wind barbs and temperatures on A)1200 UTC 12 February, B) 1200 UTC 13 February, C) 1200 14 February, D)1200 UTC 15 February.
Figure 8. 250 Mb wind barbs, isotachs and streamlines for A) 1200 UTC 14 February and B) 1200 UTC 15 February.
What was learned from this event?
unusual consistency seen in each 15 member ensemble and the operational models
resulted in the highest level of confidence possible for issuing Heavy Snow and
Winter Storm Watches during the early morning of 12 February. Heavy Snow and Winter Storm Warnings were
issued during the early morning of 13 February.
The Winter Storm Warnings were issued from the southern Catskills
through the POU area,
Collaboration with surrounding offices through 12Planet was smooth as there were few if any disagreements on the forecast evolution. HPC WWD graphics were helpful, and defined the axis of heaviest snow very well. It was felt, however, that their snowfall amounts were a little low, and zone of freezing rain was too far north and west, into our forecast area. The Grid editing and formatters worked well, with no problems noted, and snowfall amounts in the text consistent with Grids.
As far as snowfall amounts, the SREF and MREF plumes were in unusual agreement for 1.25” to over 2.5” of liquid equivalent, and the probabilities for 2.00” in 36 hours were above 50% for a significant portion of the forecast area. We thought 18” to 36” of snow was realistic for areas where mostly snow was predicted, and 10” to 20” in areas that were expecting varying degrees of sleet mixed in. All parameters pointed to unusually strong frontogenesis and banding (Figs. 9A and 9B), and wherever the band or bands set up, there could be more than 36”. During the storm, the SPC issued timely and helpful mesoscale discussions and graphics outlining precipitation types, regions and intensities (Figs. 10A-C).
Figure 9. Base reflectivity from A) KENX at 1759 UTC 14 February, and B) a mosaic of composite reflectivity at 1700 UTC February 14.
Figure 10. Mesoscale graphics from the SPC highlighting precipitation types, locations and intensities during the storm impact.
forecasts of this magnitude are rare, but the consensus from all sources of
guidance was rare, and the pattern recognition supported the extreme snowfall
amounts. The consensus on the forecasted
storm track from 2 separate 15 member ensembles and operation model runs,
tracking the storm through
Figure 11. 0000 UTC 15 February A) MSAS NWS MSLP and B) NAM12 initial 500 hPa heights and MSLP. Note the surface low over interior cape Cod and an upper low over eastern NY.
The NAM/SREF precipitation transition zone worked out well, as areas from the Capital District through the Berkshires and southern VT did see a brief period of sleet (see KALY soundings, Figs. 12A and 12B). So, using satellite imagery, radar imagery and Dynamic Tropopause information, it was recognized that there would be enhanced downstream upper ridging and warming further north and west, affecting the location of the precipitation transition zone.
Figure 12. KALY sounding from A) 1200 UTC 14 February
and B) 1800 UTC February. Note the 1200
UTC sounding is completely below freezing, but the 1800 UTC sounding just
touches the 0C isotherm around 800 hPa.
A period of sleet was reported in
Blizzard Warnings were issued during the morning of 2/14, based on potential wind gusts to 35 MPH during the last few hours of the heaviest snow. The last band of snow lingered until around in the Capital District as the upper deformation zone and TROWAL tracked through. The last band of snow also looked like it was enhanced by the local river convergence zone (Fig. 13).
Figure 13. Base reflectivity from KENX 0430 UTC 15
February. Note the enhanced band
oriented north-south through the mid
The snow to liquid ratios were nearly 10:1, even with surface temperatures over much of the region between 5˚F and 20˚F. There was a warm layer aloft around -5˚C to -10˚C that probably reduced the dendritic growth at least in the Capital District. The ratios were likely higher within the enhanced band that developed northwest of the Capital District. There was one primary enhanced band (Figs. 9A and 9B), not multiple bands, this could be researched.