A series of arctic air masses entered the eastern United States during the month of February 2015. The second in a succession of these air masses was associated with a deep 500 hPa trough (Fig. 1). At 850 hPa a pocket of extremely cold air moved into the region. The 850 hPa temperatures ranged from -20 to -26C with 850 hPa temperature anomalies on the order of -2 to -4s over portions of the eastern United States (Figs 2-a-f). The coldest air “la crème de la crème” 1of North American cold with 850 hPa temperatures in the -32 to -34C ranged were focused over northern New England (Fig. 2c).
The deep trough (Fig. 1) and deep cold air (Fig. 2) were associated likely contributed to cyclogenesis (Fig. 3a). A modest low pressure over the Great Lakes spread snow to the east. As this feature and the strong frontal boundary reached the coast (Fig. 3b) rapid cyclogenesis ensued (Fig. 3c-e). The strong gradient between the deepening cyclone and strong anticyclone to the west produced strong winds. The strong easterly winds north of the surface cyclone (Fig. 4) contributed to the heavy snow eastern New England in areas in close proximity to the -3 to -4850 hPa jet (Figs. 4c-d).
Over Pennsylvania, light snow fell ahead of the developing cyclone over the Great Lakes. The total accumulated precipitation is shown in Figure 5. As the cyclone moved eastward and the arctic front swept across the State, it produced a line of convectively enhanced snow which included reports of thunder with the snow. Lightning was also observed along the line. The NCEP HRRR 2100 UTC 14 February analyzed radar (Fig. 6a) and forecasts from 5 previous cycles valid at 2100 UTC 14 February (Fig. 6b-f) show the convective line and simulations of this feature. The HRRR provided extremely useful guidance related to the passage of a very strong front with implied convective snowfall along the leading edge.2
This paper will document the pattern and anomalies associated with the arctic frontal passage of 14 February and the impacts of the arctic air mass on 15-16 February 2015. As with all high impact weather events, the forecasts and the communications of these forecasts are important. The 4th section will examine the forecasts produced by the NCEP ensemble forecast systems and the HRRR.
METHOD AND DATA
The large scale pattern was reconstructed using the Climate Forecasts System (CFS) as the first guess at the verifying pattern. The standardized anomalies were computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter 1995). For storm-scale details the 00-hour analysis from the hourly NCEP HRRR were used (see Figure 2).
The precipitation was estimated using the Stage-IV precipitation data in 6-hour increments to produce estimates of precipitation during the even in 6, 12, 24 and 36 hour periods. Snowfall was retrieved from National Snow Analysis website.
Snowfall data was obtained from both NWS public information statements and the National Snow site.
The NCEP SREF were retrieved and examined in real-time and archived locally. These data helped identify the different predictability horizons of the forecast systems. The NCEP EFS data may not reflect public forecasts or perceptions of the forecasts. Many forecasters use a diverse set of forecast tools and often lean on the European Center model and post processed forecast data.
The GEFS and NAEFS both forecast the event with relatively long lead-times. Due to archive issues the focus is on the GEFS. The 500 hPa forecasts from 6 1200 UTC cycles leading up to the event are shown in Figure 7. Forecasts from 7 to 12 February showed the deep trough with negative height anomalies over the eastern United States at 0000 UTC 15 February. As forecast length decreased, predictability increased and the anomalies became more negative as the mean showed a deeper trough.
The GEFS 850 hPa temperatures showed the surge of cold air in the afternoon and evening of 14 February (Fig. 8) and the extent of the cold air into the East by 1200 UTC 15 February 2015 (Fig. 9). These data too show the deeper cold air and larger anomalies as the forecasts converged on a similar solution. The 0000 UTC 15 February data show the timing of the front which was coincident with the snow squalls below. The GEFS forecast snow but shorter ranges forecasts address this. The focus here was on the forecasts of the arctic air.
The 52 member NAEFSBC forecasts of 500 hPa heights and 850 hPa temperatures (Fig. 10) showed that the NAEFSBC also forecast the deep trough and potential cold air on 15 February 2015 from both forecasts produced on both the 5 & 9 of February.
The NCEP SREF forecast the timing of the frontal system relatively well into the eastern United States (Fig. 11). The front came through State College at about 2200 UTC. The SREF also showed how intense the cold air would be over the eastern United States on 15 February (Fig. 12) a day where many record low-high temperature records were set3.
The SREF forecast the strong front and the eventual cyclogenesis along the frontal boundary (Fig. 13). The strong gradient between the cyclone and anticyclone produced strong winds, blowing snow, and wind chills well below 0F over large swaths of the eastern United States. The frontal system also was forecast to produce precipitation, in the form of snow. The SREF mean QPF and each member 6mm contour is shown in Figure 14.
The rapidly-updating HRRR is used to show the strong frontal passage over Pennsylvania and the simulations it produced of the snow squalls that occurred along the frontal boundary. Figure 6 showed some of the simulations in the reflectivity. The snow band produced about 1 inch of new snow over areas that received 1-2 inches of light snow during the 6-10 hour period prior to the frontal passage.
The 2m temperatures in the HRRR from 6 forecast cycles in 2-hour increments (Fig. 15) show the arctic air in western Pennsylvania at 2200 UTC and into east-central regions by 0000 UTC 15 February 2015 (Fig.16). The resulting intense snowbands in the HRRR simulated radar (Fig. 17) show that the HRRR produced intense convection for mid-February. There were some timing issues with the bands. However, the concept of convective snow with the frontal passage was reasonably well forecast by the NCEP HRRR.
The HRRR simulated around 1-3 mm of QPF with the snowbands over a 1 hour window and 1-4mm of QPF in a 2 hour window from 2000-2200 UTC (Fig. 18).
A strong arctic front brought unseasonably cold air into the eastern United States on 14-15 February 2015. This was the second of what would prove to be a series of arctic fronts during the month of February 2015. The this front was associated with a surface cyclone which produced heavy snow along the coast and lighter snow inland. Pennsylvania had a modest 1-4 inch snow event with about 1/3 of the snowfall associated with the arctic frontal boundary.
Behind the front temperatures fell to near or below record levels. Readings were well below zero from Pennsylvania into New England on 15-16 February. The high temperature in State College did not reach 10F on 15 February setting a new record low-high for the date. Chandlers Valley in northwestern Pennsylvania had a daily record low of -32F on 16 February (Table 1). As shown in Table 1 there was an incredible number of Stations with zero and sub-zero readings.
The strong frontal system and cold were well predicted with at least 9 days lead-time the NCEP GEFS and the NCEP-CMC NAEFS forecast systems (Figs. 7-10). The GEFS forecasts converged (Fig. 7-9) the anomalies became more negative. Less uncertainty and higher agreement was contained in the overall pattern and intensity of the standardized anomalies. The GEFS provided useful guidance with regard to the frontal system and the intense arctic outbreak.
As forecast length decreased, the NCEP SREF showed the intensity of the cold air and frontal system. It timed the frontal boundary and 2-6mm QPF well with the front and had signals related to the snow along the coast not covered here.
Finally, the NCEP HRRR did well showing the advance of the arctic air, the frontal passage, and the potential for convective snowfall with the frontal passage. We focused on the arctic front as it passed through central Pennsylvania and State College around 2200 UTC 14 February. The HRRR did well farther west and east. METAR reports from Indiana to New Jersey (not shown indicated this potent front affect a significant number of States and towns within those States. This case was a good success story for the HRRR in forecasting an intense frontal passage.
Thanks to the Albany MAP for METARS on snow, flowing snow and funnel clouds with snow. Elyse Hagner and John LaCorte for collecting reports on temperatures and temperature records.
Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS. Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H. Bredekamp, NASA, Washington, D.C., 209-219.
DeGaetano, A. T., M. E. Hirsch, and S. J. Colucci. 2002. Statistical prediction of seasonal East Coast winter storm frequency. Journal of Climate 15:1101–17.
Kahneman, D, 2011: Thinking Fast Thinking Slow. Farrar,Straus, and Giroux, NY,NY. 511pp.
Kalnay, Eugenia, Stephen J. Lord, Ronald D. McPherson, 1998: Maturity of Operational Numerical Weather Prediction: Medium Range. Bull. Amer. Meteor. Soc., 79, 2753–2769.
Roebber, P.J., M.R. Butt, S.J. Reinke and T.J. Grafenauer, 2007: Real-time forecasting of snowfall using a neural network. Wea. Forecasting, 22, 676-684.
Figure . CFSR 500 hPa heights (m) and standardized anomalies in 12 hour increments from a) 1200 UTC 14 February through f) 0000 UTC 17 February 2015. Return to text.
Figure . As in Figure 1 except for 850 hPa temperatures (C ) and standardized anomalies in 12 hour increments from a) 0000 UTC 15 February through f) 1200 UTC 17 February 2015 Return to text.
Figure . As in Figure 1 except for mean sea-level pressure and standardized anomalies in 12 hour increments from a) 1800 UTC 14 February through f) 0000 UTC 16 February 2015. Return to text.
Figure . As in Figure 3 except for 850 hPa winds and and u-wind standardized anomalies in 6 hour increments from a) 1800 UTC 14 February through f) 0000 UTC 16 February 2015. Return to text.
Figure . Stage IV QPE for the period of 0000 UTC 9-10 February and the period of the longer duration event from 0000 UTC 7 to 10 February 2015. Units are mm and plotted using Pygrib. Return to text.
Figure . NCEP HRRR forecasts of total reflectivity (dBZ) valid at 2100 UTC 14 February 2015 from HRRR initialized at a) 2100 UTC 21 February, b) 1800 UTC< c) 1500, d) 1200, e) 0900 and f) 0700 UTC 14 February 2015. Return to text.
Figure . NCEP GEFS ensemble mean 500 hPa forecasts valid at 0000 UTC 15 February 2015 showing the mean heights and height anomalies in 24 hour forecast cycle windows from a) 1200 UTC 7 February through f) 1200 UTC 12 February 2015. Return to text.
Figure . As in Figure 7 except for GEFS 850 hPa temperatures and temperature anomalies. Return to text.
Figure . As in Figure 8 except valid at 1200 UTC 15 February. This day saw many record low-high temperature records set in the eastern United States
Return to text.
Figure . As in Figures 7 and 8 except for NEAFBC BC valid at 0000 UTC 15 February showing forecasts from 1200 UTC 5 (left) and 1200 UTC 9 February 2015. Return to text.
Figure . As in Figure 7 except for NCEP SREF 850 hPa temperatures and anomalies valid at 2100 UTC 14 February 2015. SREF initialized every 6 hours from a) 0300 UTC 13 February through f) 0900 UTC 14 February 2015. Return to text.
Figure . As in Figure 11 except valid at 2100 UTC 15 February 2015. Return to text.
Figure . As in Figure 11 except for mean sea-level pressure at 0000 UTC 15 February. Return to text.
Figure . As in Figure 13 excerpt for SREF ensemble mean QPF and each members 6mm contour for the 24 hours ending at 1200 UTC 15 February 2015 Return to text.
Figure . NCEP HRRR 2m temperature forecasts valid at 2200 UTC 14 February 2015 from HRRR initialized every 2 hours from a) 2000 UTC through f) 1000 UTC 14 February 2015. Return to text.
Figure . As in Figure 15 except valid at 0000 UTC 15 February 2015. Return to text.
Figure . As in Figure 15 except for the HRRR simulate radar at 2200 UTC 14 February 2015. Return to text.
Figure . As in Figure 17 except for accumulated precipitation (mm) over the two hour window of 2000 to 2200 UTC 14 February 2015. Return to text.