1-2 February snowstorm


This storm was quite a challenge to predict in the eastern U.S. A strong upper system in the central U.S. was tracking east and most sources of guidance and ensembles were suggesting the system weakening as it tracked into the northeastern U.S., before redeveloping and strengthening as it exited the northeastern U.S. Areas around Chicago, the Midwestern U.S. and Ohio Valley received over 2 feet of snow, including thunder snow and blizzard conditions, shutting down major highways in Chicago. A consensus in guidance 1 to 2 days prior to impact in the northeastern U.S. suggested 2 separate phases to the storm, with over 7 inches of snow possible in each phase, the warm advection phase and the upper deformation phase, with each phase separated by 6 to 12 hours. Total snowfall over a large area of the northeastern U.S. was expected to be 18 inches or more.


However, strong warm advection changed precipitation to sleet and freezing rain further north than expected, the precipitation ended sooner than expected, and the upper deformation snows were lighter than expected. Still, storm totals of around 1 foot of snow across a large part of the northeastern U.S. caused a significant impact, but far less than predicted. Analysis of data and guidance will highlight some of the possible reasons the storm did not evolve quite like guidance suggested, and some of the forecast model data that was misinterpreted by forecasters, that would have resulted in a more accurate and improved forecast. This storm was ranked as a 3 on the NESIS scale, mostly for the snow that fell from Chicago through the Ohio Valley, but also including the snows that fell in the northeastern U.S.


Above: 12Z 31 January GEFS 500 hPa heights forecast and 09Z 31 January SREF forecast valid 18Z 2 February. Note the positively tilted upper system tracking out of the Great Lakes, with the northeastern U.S. in the warm advection portion of the upper impulse. Above normal heights offshore implies the upper level system will continue to lift north into Canada, minimizing any upper deformation precipitation potential over the northeastern U.S.



Above: Loop of observed 500 hPa heights, showing how the upper pattern evolved. Notice the deamplification of the strong upper system as it tracked east out of the midwest and OH Valley.


Above: 12Z 31 January GEFS and 09Z 31 January SREF MSLP valid 18Z 2 February and 00Z 03 February, respectively. Note there was no strong, anchored surface high to the north, implying any cold air was not well anchored across the northeastern U.S. and any warm advection could increase chances for mixed precipitation.



Above: Loops of 850 hPa winds and temperatures from the GEFS (left 2 panels) and SREF (right 2 panels). Note the dominance of V wind anomalies over the northeastern U.S., implying warm advection, while the U wind anomalies remained well north. The GEFS 850 hPa temperatures showed the freezing line gradually building north, implying mixed precipitation possibly extending further north with each run.



Above: The precipitation type forecast from the GEFS and SREF showed the best probabilities for snow over northern areas, and the maximum probabilities for sleet and freezing rain extending north into central NY and New England.




Above: GEFS (top row) and SREF (bottom row) probabilities for 0.50 of liquid equivalent precipitation in 24 hours (left), and 12 hours (center left and center right), and 1.00 inch in 24 hours (far right). Note the importance of looking at QPF for different time ranges. The 24 hour probability masks the detail of the precipitation ending early, seen in the 12 hour graphics. Probabilities for 1.00 inch were quite low, implying probabilities for extreme snow amounts were also low.



Above: GEFS (left) and SREF (right) loops of plume displays. Note that there was a noticeable run-to-run consistency of a 2 stage event, with mixed precipitation during the second stage. Also note the total liquid equivalent precipitation was well over 1 inch of liquid equivalent precipitation, suggesting that any locations that received all snow could receive well over 1 foot of snow.





Above: Soundings from Albany, NY (top row) and Upton, NY (bottom row). Note the gradual warming below 700 hPa with time, with the strongest warming when the winds through the surface to 700 hPa layer turn to southerly. The boundary layer temperatures equaled or exceeded freezing at both Albany and Upton, resulting in snow changing to mixed precipitation at both locations.



Above: Loop of surface maps showing the surface low relatively weak as it tracked through and offshore the eastern U.S.



Above: Radar loops showing the evolution of precipitation during the 2 stages of the event.



Above: This storm has a preliminary rank of 3 on the NESIS scale.