National Weather Service-Pennsylvania State University Weather Events Abstract: West Coast Heavy Precipitation Event of January 2012 by Richard H. Grumm National Weather Service State College PA 16803 A deep trough over northern western North America and a ridge over the eastern Pacific brought a strong 250 hpa jet and a surge of deep Pacific moisture into the western United States from 18-22 January 2012. Reports of over 16 inches of precipitation were observed in the mountains of southern Oregon. Many areas of California had heavy rain or snowfall. The combination of the cold air over western Canada and Alaska and the surge of moisture produced snow in Washington State. Before the ice storm began, the Seattle area had 10-20cm of snowfall. Heavy snow impacted the higher elevations. Farther south and at lower elevations heavy rain dominated. Strong Pacific jets and moisture surges are often associated with El Niño conditions. However, such events can and clearly do occur during El Niña winters. This event was observed during a La Niña winter. This paper will document the West Coast heavy precipitation event of January 2012. The NCEP GEFS precipitation forecasts show that the GEFS did well in predicting the potential for heavy rainfall and the impact of the Pacific moisture plume into the mountains of the western United States.
1. INTRODUCTION A strong Pacific jet (Fig. 1) brought several surges of high precipitable water (PW) air into the West Coast of North America from 18 to 22 January 2012 (Fig. 2). Strong Pacific Jets and surges of high PW air are common features associated with many West Coast heavy rainfall events (Junker et al. 2009; Junker et. al 2008). The surges of high PW air, often termed atmospheric rivers (AR: Neiman et al. 2008, Ralph et al 2006, Ralph et al. 2005; Ralph et al. 2004) have been shown to be important in heavy rainfall events in California and along the West Coast in general. Grumm and Hart (2001) and Graham and Grumm (2010) demonstrated how standardized anomalies could be used to identify significant or high impact weather events. Junker et al. (2008) specifically showed the value of standardized anomalies in identifying heavy rainfall patterns in the western United States and the mountains of northern California. They identified the key pattern to include a deep trough off the coast and a strong ridge to the south. The height anomalies in the trough were generally -2 to - 3σ and the anomalies in the ridge were typically on the order of +1σ above normal. The key parameters however were the anomalies in the PW field and the 850 hpa moisture flux anomalies (MFLUX). Junker et al. (2009) showed how forecasts of anomalies provided forecasters additional confidence in predicting heavy rainfall in the western United States. The total liquid equivalent precipitation (QPE) in 48 hour periods from 1200 UTC 16 January through 1200 UTC 24 January (Fig. 3) show the enduring nature of this precipitation event. These data show that the heavy precipitation moved southward in time as the PW plume impinging upon the West Coast moved southward (Fig. 2). Clearly, the heaviest precipitation fell between 1200 UTC 18-20 January 2012 when in excess of 256 mm (10 inches) fell in southern Oregon. Total QPE in this area during the height of the event exceeded 450 mm (18 inches) during the total time of the event (Fig. 4). A key issue is forecasting heavy precipitation events. Probabilistic forecasts offer the best tools to predict an event like this. Junker et al. (2009) showed the value of forecasts and ensemble forecasts of heavy rainfall events in the western United States. The value of re-analysis climate data (R-Climate) in the forecast process has been demonstrated by Grumm (2011) and Stuart and Grumm (2006). These studies show how forecasts of significant departures of key parameters can aid in predicting high end weather events, such as Russian heat wave of 2010 (Grumm 2011), East Coast Winter Storms (Stewart and Grumm 2006), and record rainfall events (Grumm 2011). This paper will document the pattern and the R-climate anomalies associated with the historic West Coast precipitation event of 17-22 January 2012. The focus is on the R- Climate based standardized anomalies as a tool to both analyze and predicted this and similar events. The forecast section of this paper will show the value of using climate data and internal ensemble prediction system climate data to better anticipate and predict extreme weather events. 2. Methods and Data
The NCEP GFS is used to re-produce the conditions associated with the event to include the large scale pattern. The standardized anomalies are displayed in standard deviations from normal as in Hart and Grumm (2001) and are computed using the climatology from the NCEP/NCAR global reanalysis data (Kalnay et al. 1996). The focus is on the pattern and anomalies associated with the storm. The value of EFS and anomalies with EFS data are presented. Ensemble data shown here are from the NCEP Global Ensemble forecast system which is run at 75km in horizontal resolution. A comparison of the NCEP GEFS 55km QPF products will be shown here. These data show the value of higher resolution in the spatial distribution of QPF and the role model resolution plays in the amount of QPF produced by a forecast system. The emphasis here is on products which may aid in predicting heavy rainfall events. The overall pattern is described using the 27.5km NCEP GFS 00-hour forecasts. The pattern and standardized anomalies followed the methods outlined in Hart and Grumm (2001) and the GFS 00-hour forecasts were used to establish the pattern and standardized anomalies. The term R-Climate is used in reference to analysis and forecast which use re-analysis climate data to diagnose or forecast the departures from normal. Rainfall and precipitation estimates were in gridded form using the Stage-IV data (Lin et al. 2005) in 6-hourly increments. This facilitated examining 6, 12, 24, 48 and storm total amounts in any desired time window. Hourly data were available but were not used in this study. For brevity times are presented as day and hour in the format 17/1200 UTC and 18/0000 UTC which would be 1200 UTC 17 January 2012 and 0000 UTC 18 January 2012 respectively. Fully qualified dates are limited to comparative data from times outside of January 2012. 3. Large scale pattern i. The large scale pattern The 500 hpa pattern over the western Pacific and western North America (Fig. 5) shows the anomalous trough over southwestern Canada (Fig. 5a) and the strong ridge over southwestern Mexico as the event developed. This pattern is generally similar to the composite 700 hpa pattern for northern California heavy rain events shown by Junker et al. (2008:Fig.4a). During the peak period of heavy rainfall (Fig. 3b&c) a deep trough developed off the West Coast with a ridge to the east (Fig. 5d-f), similar to the 700 hpa composite pattern for heavy rain along the West Coast shown by Junker et al. (2008:Fig. 4d). The 250 hpa jet (Fig. 1) developed in the strong gradient shown between the relatively high heights to the south and lower heights to the north. Unlike the 500 hpa heights (Fig. 5), the 250 hpa heights (not shown) had a ridge over the southwestern Pacific though most of the event with +1σ anomalies over the eastern Pacific and most of the intermountain west, east of the trough axis. The strong Pacific jet (Fig. 2) developed in this strong gradient between the ridge to the south and the trough to the north. Initially, the strong 250 hpa jet (Fig. 1a-b) came ashore in Washington State. The second strong jet came ashore farther south (Fig. 1d-f) mainly in southern Oregon and northern California.
The strong flow about the ridge over the eastern Pacific brought a plume of high PW air into western North America. The deep moisture with +2 to +3s PW anomalies was located well west of the Coast at 17/1200 UTC (Fig. 2a) and raced eastward with the strong westerly flow, impacted the coastal zone by 18/1200 UTC (Fig. 2b). Despite PW values in the 25-30 mm range, the PW anomalies were on the order of +2 to +3σ in Washington and Oregon. The PW surge to south on 20/1200 UTC showed +3 to +4σ anomalies in that region. By 22/1200 UTC relatively dry air was present over most of the western United States. ii. Regional pattern The regional view of the PW and PW anomalies into the West Coast from 18/1200 UTC through 19/1800 UTC (Fig. 6) show the focused plume of high PW air into the region. The 850 hpa winds peaked near 5σ above normal at 18/1200 UTC (not shown) off the Oregon coast. The high winds and high values of moisture produced high values of 850 hpa moisture flux (Fig. 7) and moisture flux anomalies near the higher 850 hpa winds peaked over 6s above normal along the Oregon Coast at 18/1200 UTC (Fig. 7a). Though difficult to view, moisture flux anomalies of 4-5σ were present most of the time and at times 850 hpa moisture flux anomalies peaked over 5σ above normal. The heavy rains in California were also accompanied by a surge of high PW air (Fig. 2) and this resulted in very high values of 850 hpa moisture flux with moisture flux anomalies well over 6s above normal at 21/0600 UTC (Fig. 8f). As the Pacific energy and moisture came ashore, there was an abnormally cold air mass located of western North America. The 850 hpa temperatures (Fig. 9) and 700 hpa temperatures (not shown) indicated a deep cold column of air with below 0C temperatures and -1 to -2σ temperature anomalies. This led to snow early in the event in Washington and Oregon. The Seattle area had 10-20 cm of snow fall before the event transitioned to a freezing rain event. At lower elevations, snow was observed in Portland Oregon, though temperatures rapidly rose above freezing in northern Oregon between 18/1200 and 19/0000 UTC (Figs. 9b-d). The 6-hourly data (not shown) indicated a rapid rise of temperatures shortly after 18/1800 UTC over much of northern Oregon and southern Washington. The Pacific air overwhelmed the cold air which eventually limited snowfall to the higher elevations of California, Oregon and Washington. iii. Forecasts The QPFs from the 17/0000 and 17/1200 UTC GEFS at 75 km resolution and the forecast from the 55km GEFS-PARA for 75mm and 100mm of QPF in 48 and 72 hours are shown in Figures 10 & 11 respectively. These data show that resolution matters the higher resolution GEFS produces higher probabilities of both 75 and 100 mm of QPF and generally higher QPF. Note the 100mm probability in southern Washington in the 55km GEFS which is smaller in the 75km GEFS. The finer resolution clearly shows the impact of terrain, thus the QPF maximum near terrain features is more focused. Similar forecasts from 18 January 2012 are shown in Figures 12 & 13. These data show higher probabilities in the 75 and 100 mm QPFs from the coarser GEFS, especially for 100mm of QPF. The coarser resolution GEFS produced a large area of higher QPFs. Similar data from 14-19 January were examined and showed the same basic trend for terrain focused heavy rainfall in all GEFS 75 and 55 km cycles. The 16 January
forecasts are shown as a longer range forecast example. These data show the consistent probability of heavy rainfall along the West Coast and the more focused areas of heavy rainfall in the higher resolution GEFS55 initialized at 16/0000 UTC (Fig. 14). iv. Observations The precipitation pattern in 48-hour windows was shown in Figure 3 and an event total estimate was shown in Figure 4. Viewed in 24 hour windows (Fig. 15) the distinct terrain focus and the periods of heavy accumulation are quite evident. Over 256mm of precipitation fell in southwestern Oregon between 28/1200 and 29/1200 UTC (Fig. 15a) with around 128mm in the following 24 hour period (Fig. 15b). The focus of the heavy precipitation moved south and westward for the period ending at 21/1200 UTC (Fig. 15c) and the final period showed a dramatic decrease in the precipitation across the region. These data show the favored locations of the precipitation (Figs. 3, 4&15) with the terrain. As indicated by the GEFS and GEFS55 there was a local maximum in southern Washington near Mount Adams, the second tallest peak in Washington State. The GEFS55 likely resolves this feature better than the GEFS at 75km and thus may explain the more localized maximum in many of its forecasts in southern Washington. In northern California the heavier QPE amounts (Fig. 4) were focused in the Cascades around Mt Shasta and Mt Eddy. Secondary maximum ran down the northern portions of the Coastal Range and into the Sierras. The secondary maximum in the Sierra appeared focused along the Nevada border from near White Mountain to Mt. Whitney (Fig. 15c). Several GEFS55 runs attempted to show secondary maximum inland implying higher resolution terrain impacted the precipitation forecasts (for example Fig. 14c&f). 4. Conclusions A strong Pacific jet (Fig. 1) brought several surges of high precipitable water (PW) air into the West Coast of North America from 18 to 22 January 2012 (Fig. 2). Strong Pacific Jets and surges of high PW air are common features associated with many West Coast heavy rainfall events (Junker et al. 2009; Junker et. al 2008). The surges of high PW air, often termed atmospheric rivers (AR: Neiman et al. 2008, Ralph et al 2006, Ralph et al. 2005; Ralph et al. 2004) have been shown to be important in heavy rainfall events in California and along the West Coast in general. In this case the surge of high PW air brought some extremely heavy rainfall and snowfall to the mountains of the West Coast. The data shown here reinforce the concepts demonstrated by Grumm and Hart (2001) and Graham and Grumm (2010) in identifying potential high impact weather events. The anomalous PW plume with 2 to 3σ above normal PW anomalies moving into the West Coast is often a good signal for heavy rainfall. The large, often 4 to 6σ 850 hpa moisture flux anomalies (Fig. 8) are another strong signal associated with record precipitation events (Bodner et al. 2011; Junker et al. 2008). This event clearly had some very high 850 hpa moisture flux anomalies. These large anomalies combined with the high probability of 75 and 100mm of QPF in the NCEP GEFS are good reinforcing data to aid in predicting high impact precipitation events. Junker et al. (2009) suggested using the QPF along with anomalies to gain confidence in predicting heavy rainfall events and
potentially record heavy rainfall events. This case shows the value of using these two methods to confidently predict higher end precipitation events. The NCEP GEFS probability of 75 and 100 mm of QPF were shown from the operational GEFS at 75km and several runs of the GEFS55. The GEFS55, with higher resolution, appeared to produce the heavier QPF values over smaller regions and more focused toward the terrain features. The implications here are that the 20km difference in resolution allows the forecast system to better resolve some of the key terrain features. Thus, in northern California the GEFS55 focused the higher QPF amounts closer to the higher terrain while the GEFS tended to show heavy rainfall focused along most of the length of the Cascades. Near the higher terrain, the GEFS55 produced 175mm of QPF in northern California (Fig. 16) while the coarser GEFS produced about 150 mm (Fig. 17). These data imply the importance of resolution on the impacts of both model resolution and terrain. One could theorize that the operational GEFS if replaced by the GEFS55 would show smaller but more focused regions of heavy rainfall and might produce locally higher QPF amounts. 5. Acknowledgements Albany MAP for discussions on the event in the T+8 to T-2 days time period. 6. References Bodner,M.J., N.W. Junker, R.H. Grumm, and R.S.Schumacher 2011: Comparison of atmospheric circulation patterns during the 2008 and 1993 historic Midwest floods. NWA Digest, accepted for publication Sept. 2011. Graham, Randall A., and R H. Grumm, 2010: Utilizing Normalized Anomalies to Assess Synoptic-Scale Weather Events in the Western United States. Wea. Forecasting, 25, 428-445. Grumm, R.H. 2011: New England Record Maker rain event of 29-30 March 2010. NWA, Electronic Journal of Operational Meteorology, EJ4. Grumm, R.H 2011: The Central European and Russian Heat Event of July-August 2010.BAMS, 92, 1285-1296. Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16,736 754. Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, 2426 2442. Junker, N.W, M.J.Brennan, F. Pereira,M.J.Bodner,and R.H. Grumm, 2009:Assessing the Potential for Rare Precipitation Events with Standardized Anomalies and Ensemble Guidance at the Hydrometeorological Prediction Center. Bulletin of the American Meteorological Society,4 Article: pp. 445 453 Junker, N. W., R. H. Grumm, R. Hart, L. F. Bosart, K. M. Bell, and F. J. Pereira, 2008: Use of standardized anomaly fields to anticipate extreme rainfall in the mountains of northern California. Wea. Forecasting,23, 336 356. Lin, Y. and K.E. Mitchell, 2005: The NCEP Stage II/IV hourly precipitation analyses: development and applications. Preprints, AMS 19 th Conference on Hydrology, San Diego, CA. Paper 1.2.
Neiman, P.J., F.M. Ralph, G.A. Wick, J. D. Lundquist, and M. D. Dettinger, 2008: Meteorological characteristics and overland precipitation impacts of atmospheric rivers affecting the west coast of North America based on eight years of SSMI/satellite observations. J. Hydrometeor., 9, 22-47. Ralph, F. M., G. A. Wick, S. I. Gutman, M. D. Dettinger, C. R. Cayan, and A. B. White, 2006: Flooding on California s Russian River: The role of atmospheric rivers. Geophys.Res. Lett., 33, L13801, doi:10.1029/2006gl026689. Ralph, F. M., P. J. Neiman, and G. A. Wick, 2004: Satellite and CALJET aircraft observations of atmospheric rivers over the eastern North Pacific Ocean during the winter of 1997/98. Mon. Wea. Rev., 132, 1721-1745. Ralph, F. M., P. J. Neiman, and R. Rotunno, 2005: Dropsonde observations in low-level jets over the northeastern Pacific Ocean from CALJET-1998 and PACJET-2001: Mean verticalprofile and atmospheric-river characteristics. Mon. Wea. Rev., 133, 889-910. Stuart N. A., and R. H. Grumm, 2006: Using wind anomalies to forecast east coast winter storms. Wea. and Forecasting, 21, 952-968..
Figure 1. GFS 00-hour forecasts of 250 hpa winds (kts) and 250 hpa wind anomalies in 24 hour periods from a) 1200 UTC 17 through f) 1200 UTC 22 January 2012. Return to text.
Figure 2. As in Figure 1 except for GFS 00-hour precipitable water (mm) and precipitable water anomalies. Return to text.
Figure 3. Stage-IV rainfall data showing the total liquid equivalent precipitation (mm) in 48 hour periods including the periods ending at 1200 UTC a) 18 January, b) 20 January, c) 22 January, and d) 24 January 2012. Contours as per the color bar. Return to text.
Figure 4. As in Figure 3 except for total liquid equivalent for the period of 1200 UTC 18-22 January 2012. Return to text.
Figure 5. As in Figure 1 except for 500 hpa heights (m) and height anomalies in 24 hour periods. Return to text.
Figure 6. As in Figure 2 except for GFS precipitable water and precipitable water anomalies in 6-hour increments from a) 1200 UTC 12 January through f) 1800 UTC 19 January 2012. Return to text.
Figure 7. As in Figure 6 except for GFS 850 hpa moisture flux and moisture flux anomalies in 6-hour increments from a) 1200 UTC 18 January through f) 1800 UTC 19 January 2012. Return to text.
Figure 8. As in Figure 6 except for GFS 850 hpa moisture flux and moisture flux anomalies in 6-hour increments from a) 0000 UTC 20 January through f) 2100 UTC 19 January 2012. Return to text.
Figure 9. As in Figure 8 except for 850 hpa temperatures and temperature anomalies in 12 hour increments from a) 0000 UTC 18 January 2012 through d) 1200 UTC 20 January 2012. Return to text.
Figure 10. GEFS forecasts of quantitative precipitation (QPF) showing in upper panels, the probability of 75mm or more QPF in 48 hours ending at 1200 UTC 21 January 2012 and the ensemble mean 75mm contour. Lower panels show the ensemble mean QPF and each members 75mm contour. Data from NCEP a & d) NCEP 75km GEFS initialized at 0000 UTC 17 January 2012, b &e) NCEP 75km GEFS initialized at 1200 UTC 17 January 2012, and c &f) NCEP 55km parallel GEFS initialized at 1200 UTC 17 January 2012. Percentages as per the color key in the upper panels. Shaded QPF is in the color bar. Return to text.
Figure 11. As in Figure 11 except for 100 mm in the 72 hour period ending at 1200 UTC 22 January 2012. Return to text.
Figure 12. As in Figure 11 except for forecasts from a & d) NCEP 75km GEFS initialized at 0000 UTC 18 January 2012, b &e) NCEP 75km GEFS initialized at 1200 UTC 18 January 2012, and c &f) NCEP 55km parallel GEFS initialized at 1200 UTC 18 January 2012. Percentages as per the color key in the upper panels. Shaded QPF is in the color bar. Return to text.
Figure 13. As in Figure 12 except for 72 hour accumulations for the period ending at 1200 UTC 22 January 2012. Return to text.
Figure 14. As in Figure 13 except for forecasts from a & d) NCEP 75km GEFS initialized at 0000 UTC 16 January 2012, b &e) NCEP 75km GEFS initialized at 1200 UTC 16 January 2012, and c &f) NCEP 55km parallel GEFS initialized at 0000 UTC 16 January 2012. Percentages as per the color key in the upper panels. Shaded QPF is in the color bar. Return to text.
Figure 15. As in Figure 3 except for Stage-IV 24 hour accumulated precipitation for the 4 periods ending at a) 1200 UTC 19 January 2012, b) 1200 UTC 20 January 2012, c) 1200 UTC 21 January 2012, and d) 1200 UTC 22 January 2012. Return to text.
Figure 16. NCEP GEFS55 initialized at 1200 UTC 17 January 2012 showing QPF ending at 1200 UTC 20 January 2012 including a) probability of 6mm in 6 hours, b) probability of 12mm in 12 hours, c) probability of 25mm in 24 hours and d) the probability of 50mm ending in 24hours. Probabilities as in the color key, ensemble mean QPF contours every 25mm. Return to text.
Figure 17. As in Figure 16 except for operational 75km GEFS. Return to text.