The “Albert Clipper” snow of 7 March 2007 By Richard H. Grumm National Weather Service Office State College,PA 16803

1. INTRODUCTION In the strong troughs that often A relatively well forecast “Alberta accompany clippers, the air is generally Clipper” brought snow to most of south- cold and supports snow. In addition to central Pennsylvania on 7 March 2007. the general pattern of cold air, the An Alberta Clipper is defined (Glickman vertical thermal profile is also typically 2000; Hutchinson 1995; Thomas and cold too favoring dendritic crystal Martin 2006) as a fast moving low growth (Mitchell, 1988). These systems pressure system that moves southeast out tend to lack the intrusion of warm air of Canadian Province of Alberta through aloft, typically between 850 and 700 hPa the Great Plains, Midwest, and/or the which limits snowfall by producing a Great Lakes during the cold season. The near freezing or melting level which can majority of these cyclones pass through limit snow crystal growth (Stewart or north of the Great Lakes and only 1992). Stewart and King (1987) about 10 percent of this storms track identified the impact of the elevated south of the Great Lakes (Thomas and warm layer affecting snow amounts and Martin). A less used term for lee precipitation type. Thus clipper systems cyclones originating in Canada is known which tend to lack a significant elevated as a "Saskatchewan Screamer" or warm layer and are often associated with Saskatchewan Schooner" which simply temperatures of -10 to -20C in the originated in the in the Province of crystal growth region can produce high Saskatchewan. The term Clipper and snow to water ratios. Alberta Clipper are more commonly applied to these northern stream systems. Forecasting snow amounts is an in exact science which relies heavily on a single The low pressure area is usually mode or an ensembles of model QPFs. accompanied by light snow. Though rain Once a best estimate QPF is determined, is possible, especially near and south of or a range of likely QPFs, a snow to the cyclone track with warmer air ahead water ratio needs to be established and of the cyclone. Alberta clippers also may applied to the QPF ranges. Ideally, this produce strong winds, and may usher in could be done on each ensemble member arctic air. In general, these systems at discrete time steps and an ensemble of produce light precipitation but snow amounts could be used. A occasionally these northern stream simplified method would be to apply waves can pack more punch. The East snow ratios based on the profiles to a Coast Storm of January 2005 was a range of ensemble QPFs. This simpler Clipper which underwent rapid method is demonstrated here. development along the East Coast and developed into a major winter storm. Figure 1 SREF forecasts initialized at 2100 UTC 6 March 2007 showing c) Mean sea level pressure and anomalies and 850 hPa temperatures and anomalies. The left side shows the 00-hour forecasts valid at 2100 UTC 6 March and the right side shows the forecasts valid at 1200 UTC 7 March.

This paper will provide and overview of plotted using GrADS (Doty and Kinter Clipper low and attendant snowfall of 7 1995). March 2007. The National Centers for Environmental Predictions (NCEP) Model and EPS data were retrieved in global ensemble forecast system near real-time from NCEP and archived (GEFS:Toth and Kalnay 1997,Toth et al. for re-display using GrADS. A mix of 1997, Buizza et al. 2005) and the NCEP traditional ensemble displays, as short-range ensemble forecast system described by Sivillo et al. (1997) and (SREF) are used to diagnose and address Grumm and Hart (2001) were used. The snow forecast issues related to this latter method displayed the ensemble event. mean relative to the departure of the field in standard deviations (SDs) from 2. METHODS normal. The method to compute and develop the climatogoligical data from Snow fall reports were obtained in near- which the departures were computed is real time from spotters and the National described by Hart and Grumm (2001). Weather Service Cooperative observing program and public information In addition to planview images of statements. All plots of snowfall were ensemble displays, time series of spaghetti plots were d displayed using a.

b.

Figure 2 SREF plumes for a point near State College from forecasts initialized at a) 2100 UTC 6 March and b) 0300 UTC 7 March 2007. Data show 3-hourly precipitation by member (gray) and accumulated precipitation by time (see color keys). plume diagrams. Plumes of precipitation Other ensemble displays included by precipitation type and temperature probabilistic guidance as originally forecasts are presented. described by Sivillo et al (1997).

Figure 3 As in Figure 1 except SREF forecasts initialized at 0300 UTC 7 March 2007 showing (left) 00hour forecasts and (right) 12 hour forecasts valid at 1500 UTC 7 March 2007. 3. RESULTS Lakes and the Northeastern United States. a. Storm Overview Forecasts valid at 07/1200 UTC showed Figure 1 shows the SREF 00-h and 15 the weak “clipper” over Illinois and the hour forecasts of the surface pressure weakening anticyclone to the east. A and the 850 hPa temperatures. A strong second strong anticyclone with more surface anticyclone was over the region arctic air was building in behind the before the snow. Mean sea-level anemic clipper. The 850 hPa pressure (MSLP) was +1 to +2 SDs temperatures showed the cold air over above normal with a small area of 2 to 3 the eastern United States and Great SD above normal MSLP values over the Lakes. Though not shown, surface eastern United States. At 850 hPa, temperatures were mainly in the teens temperatures were well below 0C with over Pennsylvania during the period of temperature anomalies well below snowfall. normal over most of the eastern Great Figure 4 SREF plumes from forecasts initialized at 0300 UTC 7 March 2007. Upper panel shows 2m temperature forecasts, middle panels shows 850 hPa temperatures and lowere panel shows 700 hPa temperatures.

Though not shown, the 850 hPa winds easterly jet (Stuart and Grumm 2006) were weak. The more traditional strong associated with major East Coast storms Figure 5 As in Figure 4 except near Somerset Pennsylvania. was not present with this system. The indication that there would not be heavy lack of a strong easterly jet was a good precipitation with this event. a.

b.

Figure 6 As in Figure 4 except plume near Somerset, Pennsylvania from forecast initialized at 0300 UTC a) 6 March and b) 7 March 2007. hourly instantaneous precipitation The plume diagram of precipitation indicated that the snow would begin amounts and type for State College are around 06-0900 UTC and end between shown in Figure 2a. These data indicated 15 and 1800 UTC. The time of that all SREF members were maximum accumulation was forecast to forecasting snow. The precipitation type be between 09 and 1500 UTC. was not a significant forecast concern. GEFS members (not shown) also Figure 3 shows the 07/0300 UTC SREF indicated an all snow event. The 3- forecasts valid at 07/0300 UTC and Figure 7. SREF forecasts initialized at 0300 UTC 6 March showing QPFs. Upper panels shows the probability of 0.10 inches or more QPF and the mean 0.10 contour. Lower panels show the ensemble mean QPF (shaded) and the 0.10 contour from each member. The valid times are for the 12-hours ending at 0000 UTC 7 March, 1200 UTC 7 March and 1800 UTC 7 March 2007. 07/1500 UTC. Similar to the previous 1 to 4 inches of snow with a high time period and forecasts, the surface probability around 2 inches. anticyclone was over the Mid-Atlantic region at 07/0300 UTC with below The thermal plume diagram in Figure 4 normal 850 hPa temperatures. By shows the forecast 2m, 850 hPa and 700 07/1500 UTC, the surface low was hPa temperatures at a point near State forecast to be over Kentucky with below College. Temperatures from the surface normal 850 hPa temperatures over most to 700 hPa were forecast to remain well of the region. The cyclone was weak below freezing. The 850 and 700 hPa and thus had weak winds and weak temperatures during the period of warm advection, limiting the northward snowfall were forecast to be in the -15 surge of warm air. and -14C range respectively. The clouds, as determined by the NAM suggested, The plume diagram is shown in Figure indicated the ascent and moisture 2b. These data are quite similar to those between these two layers suggesting shown in Figure 2a. All the precipitation maximum crystal growth rates. These with the Clipper was forecast to fall as data implied that higher snow ratios snow; the ensemble mean storm total were indicated. A 20:1 ratio would have snow QPF was 0.21 with a maximum of implied 2 to 8 inches of snow with a around 0.42 and a minimum of 0.10. All mean to 4 inches of snow. Observations the snow was forecast to fall between in and around State College suggested 4 0900 and 1500 UTC. Peak snowfall was to 5 inches was observed with around forecast be centered on 1200 UTC. A 0.20 liquid. traditional 10:1 ratio would have implied Farther south, model QPF data suggested higher amounts of QPF with comparable c. Precipitation data thermal profiles (Figure 5). Precipitation type plumes from forecasts initialized at The COOP data for precipitation ending 06/0300 and 07/0300 UTC showed all at 07/1200 and 08/1200 UTC show that snow with around 3 to 6 inches using the 0.16 and 0.06 inches of precipitation fell mean QPF and a 10 to 20:1 ratio. The at State College (Fig.10). Most of this range at the high end approached 4 to 8 was observe before 07/1200 UTC and inches. This area received 6-8 inches of was about 0.22 total precipitation. fluffy snow. Somerset received 0.25 and 0.10 inches for a total of 0.32 inches of QPF. b. QPF Forecasts

Figure 7 shows the QPF in 12 hour 4. CONCLUSIONS periods from the 06/0300 UTC SREF. These data show a high probability, 60- A relatively well forecast “Alberta 70% chance, of 0.10 inches of QPF with Clipper” brought snow to most of south- the developing wave over Minnesota and central Pennsylvania on 7 March 2007. Wisconsin, as the wave increased the An Alberta Clipper is defined (Glickman probability of 0.10 inches of QPF 2000; Hutchinson 1995; Thomas and increased to over 90% over Michigan. Martin 2006) as a fast moving low The growing area of precipitation and a pressure system that moves southeast out high probability of 0.10 inches or more of Canadian Province of Alberta through increased over Pennsylvania and the Great Plains, Midwest, and/or the northern Maryland for the period ending Great Lakes during the cold season. at 07/1800. The QPF patterns show the Though not shown, all NCEP models rapid progression of the wave and the and ensemble forecast systems forecast attendant precipitation shield. this system quite well. The emphasis here was on the SREF forecasts. The probability of over 0.30 inches in 24 hours was considerably lower as shown As shown, the surface features and the in Figure 8. The focus of over 0.30 850 hPa thermal features were well inches was over southwestern forecast 24-48 hours before the event. Pennsylvania at the 50-60% level. Though not shown the event was well forecast 2-4 days in advance by the Figure 9 shows the 24-hour QPF GEFS. The plumes showed the timing forecasts for the 06/2100 UTC SREF. and the type of precipitation. The These forecasts were wetter and the temperature plumes showed little probability of greater 0.30 inches of QPF variation implying all forecasts were was over 90% in southwestern quite similar, implying that the NCEP Pennsylvania. Though not shown, lower models were able to forecast the system probability categories also showed quite well. The temperatures were cold higher probabilities. Clearly, at shorter too, supporting an all snow event. The ranges the uncertainty was, and typically cold temperatures also implied the is, lower. And thus, the forecasts were potential for high snow to water ratios, more accurate than previous forecasts. which were observed with this system. ratios as sleet and small snowflakes The key find here is that when the served to limit the snow ratios. plumes show all snow there is a higher likelihood that snow totals could be The snow fall map (Figure 11) shows greatly affected by snow growth rates. that compared to the precipitation This was the first significant snow event amounts, snowfall to liquid ratios were of the winter of 2007 in Pennsylvania near 20:1 in most locations. The SREF where snow to water ratios were QPFs were quite good but the snow to significant. Previous storms had 10:1 or water ratios produced more snow than a lower ratios. In those events the GEFS traditional 10:1 ratio. The all snow and SREF suggested that precipitation scenario and cold 700 hPa temperatures type issues were significant. In those implied there would be high ratios, mixed events the fact that precipitation which were forecast in Pennsylvania. type issues were prevalent suggested normal to lower than normal rations, It is apparent that precipitation type which were observed in those events. As forecasts can serve a guide as whether an example, the high probability of ice effort should be put into determining pellets (sleet) in the forecasts for 13-14 snow ratios. When snow is the only February suggested warm air aloft near preciptitation type, the problem with the region of maximum dendritic snow to water ratios can be an important growth. In that event most areas saw 7:1 issue and needs to be addressed. Ideally,

Figure 9 As in Figure 8 except from SREF forecasts Figure 8 As in Figure 7 except for 0.30 inches of QPF for the initialized at 2100 UTC 6 March showing the 24 hour QPF 24 hour period ending at 1800 UTC 7 March 2007. ending at 2100 UTC 7 March 2007. ratios should be compute for each period Meteor. Monogr., No. 44, Amer. Meteor. of QPF from each ensemble member to Soc., 280 pp. arrive at an ensemble range and mean of snowfall based on thermal parameters Martner,B.E, R.M. Rauber,R.M. and QPF in each member. Rasmussen,E.T. Prater, and M.K. Ramamurthy, 1992: Impact of a 5. REFERENCES destructive and well observed cross- country winter storm. Bull. Amer. Buizza, R., P.L. Houtekamer, Z. Toth, Meteor. Soc.,73,169-172. G. Pellerin, M. Wei, Y. Zhu, 2005: A comparison of the ECMWF, MSC and Mitchell,D.L. 1988: Evolution of Snow- NCEP Global ensemble prediction Size Spectra in Cyclonic Storms. Part I: systems. Mon. Wea. Rev. 133, 1076- Snow Growth by Vapor Deposition and 1097. Aggregation, JAS,45, 3431–3451.

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b.

Figure 10. COOP data showing observed liquid equivalent precipitation (inches) for the 24 hours ending at 1200 UTC a) 7 March and b) 8 March 2007.

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