1. INTRODUCTION. For brevity times are referred to in the format of 20/1800 for 20 August UTC. 3. RESULTS

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1 Heavy rains and precipitable water anomalies August 2010-Draft By Jason Krekeler And Richard Grumm National Weather Service State College, PA INTRODUCTION Heavy rain fell across the central Mississippi River valley (CMRV) and Northeast from the August 2010 (Fig. 1). Both areas were associated with above normal precipitable water (PW). The rainfall over the CMRV was associated with convection ahead of a slow moving cold front. The Northeast rain was associated with an upper level low that became cut-off as it move out of the Ohio valley. The heaviest rain fell across northern and central Missouri as a bowing line segment moved through the area and new convection continued to fire up in its wake until the cold front finally moved through. The heavy rains occurred where a plume of high PW values surged northward from the Gulf of Mexico. PW values reached 2.2 inches at 0000 UTC 21 August 2010 at Davenport, IA which was the 8 th highest value ever recorded. The rainfall in the Northeast was associated with the same frontal system as it moved east. The upper level low became cut-off over eastern Great Lakes and a surge of high PW values ahead of the cold front produced heavy rains across the Northeast. The 22/0000 UTC sounding at Pittsburgh showed PW values of 1.9 inches which exceeded +2SD. The low then moved over western New York and a strong southeasterly 850 hpa jet allowed a second surge of high PW air along the coast to filter into the interior Northeast. This paper will document the rainfall and the associated high PW which produced it. The goal is to demonstrate the value of anomalies in forecasting heavy rainfall events. 2. METHOD The pattern was reconstructed used the NCEP GFS and NAM. All data were plotted in GrADS (Doty and Kinter 1995). The severe weather data was overlaid on the JRA data. The higher resolution NCEP NAM is used to show the conditions during the event. The anomalies were computed from the NCEP/NCAR re-analysis data (Kalnay et al 1996) as describe by Hart and Grumm 2001 and Grumm and Hart Unless otherwise stated, the base data was the NAM and the means and standard deviations were computed by comparing the NAM to the NCEP/NCAR 30- year climatological values. Rainfall data was extracted from the 6-hour Stage-IV rainfall data. Storm total amounts and 6-hourly amounts at key times during the events are shown. For brevity times are referred to in the format of 20/1800 for 20 August UTC. 3. RESULTS i. Large scale pattern Figure 2 shows the 250 hpa winds (kts) and wind anomalies over the United States from 20/1200 UTC through 22/1800 UTC. A strong jet developed over the western US with a deep trough and an amplified ridge was building across the northern plains. Over this sequence of images, a trough deepened over the Great Lakes and a sharp trough was present over the Great Lakes by 22/1800 UTC (Fig. 2i). A subtropical ridge was present over the southeastern United States. South of this feature, a strong easterly jet was present over Cuba on the southern flank of the subtropical high (Fig 4). ii. Regional Patterns-Central

2 Figure 5 shows the mean sea level pressure over the eastern United States from 20/1200 UTC through 22/1800 UTC. A surface low moved out of the central plains and across the CMRV. This feature was over New York at the end of this sequence of images. Figure 6 shows the PW and PW anomalies over the Eastern United States from 20/1200 UTC through 22/1800 UTC. Despite the weak surface low, southerly flow ahead of the cold front enabled a surge of high PW air to extend up through the central Mississippi Valley and into the western Great Lakes. PW values of between 50 and 60 mm were common over Missouri and southern Iowa. These values equated to +2 to +3SD PW anomalies over the region. Using sounding data, the PW values reached 55.8 mm at 0000 UTC 21 August 2010 at Davenport, IA which was the 8 th highest value ever recorded. These sounding data imply that the NCEP model initializations captures the high PW values quite well. Figure 7 shows the 850 hpa winds and anomalies. These data show the southerly flow increasing west of the Mississippi River Valley. At 21/0000 UTC there were between +2 and +3SD southerly winds in close proximity to where the heaviest rain fell across northern Missouri. The high v-wind anomalies and high PW anomalies (Fig. 4) appeared to show the region of heavy rainfall. iii. Regional Pattern-East Figure 8 shows the 500 hpa heights over the United States from 20/1200 UTC through 22/1800 UTC. The pattern shows a trough that deepened over the eastern United States and eventually became cut-off from the main flow (not shown). The 850 hpa winds increased in speed and became southerly ahead of the cold front as it moved eastward across the Ohio valley. The PW values remained high ahead of the front and approached or exceeded +2SD over much of the Northeast during this event. The 22/0000 UTC sounding at Pittsburgh, PA showed PW values of 48.3 mm which exceeded +2SD. The Brookhaven, NY sounding at 23/0000 UTC reached 56.9 mm which also exceeded +2SD above normal. A secondary surge of high PW air occurred as the 850 hpa jet became southeasterly off the Atlantic Ocean (Fig 7g-i). This enhanced low-level jet (LLJ) may have ingested the higher PW air producing the high PW values observed Brookhaven, NY. iv. Rainfall The rainfall over the CMRV (Fig 1a) shows widespread 3+ inch rainfall totals with a maximum over 9 inches in northern Missouri. From 20/1800 UTC to 21/0000 UTC heavy rain fell across northern Missouri (Fig 3a). Based on radar imagery (not shown) a bowing line segment brought the first rainfall and quickly moved across Missouri. A new line of convection formed in western Missouri and moved southeast across the same areas as the previous line of convection. As the cold front began to push across Missouri the rain shifted southward. From 21/0000 UTC to 21/0600 UTC many locations saw as much as 4 inches (200 mm) of rain and most of central Missouri had an inch (25 mm) or more of additional rainfall (Fig 3b). The rainfall over the northeast shows a large area of over 1 inch (25 mm) and many areas of over 3 inches (75 mm) with a maximum of over 7 inches (175 mm). The heavier rainfall amounts were not visible in analyzed data. Table 1 summarizes maximum rainfall amounts from National Weather Service Public Information Statements. Rainfall began during the evening of the 22 August across western New York and western Pennsylvania. The rainfall then moved to northeast. Some areas saw over 1 inch (25 mm) in the period from 22/0000 UTC to 22/0600 UTC (Fig 2a). From 22/0600 UTC to 22/1200 UTC a large area of 1+ inch (25 mm) rain fell over western NY and north central PA (Fig 2b). This coincided with the surge LLJ. The southerly 850 hpa

3 winds increased to +3SD (Fig 6) above normal during this period of heavy rain. Ahead of the main precipitation shield over western NY a smaller area of rain moved over the Adirondack Mountains with some embedded convection. This area produced isolated areas of 3+ inches (75 mm) of rain. The rain continued to move slowly northeastward. Between 22/1200 UTC and 22/1800 UTC rain fell over NY, eastern PA, and southern New England (Fig 2c). The intensity seemed to lessen during this period but there were still areas of over 1 inch (25 mm) with isolated locations with 2 inches (50 mm) of rain. During the afternoon on the 22 nd the intensity picked up again. Heavy rain fell across the Adirondack Mountains and Lower Hudson Valley. These areas picked up between 1 and 3 inches (25 to 75 mm) of rain in a 6 hours period ending at 23/0000 UTC (Fig 2d). During this period the 850 hpa wind shifted to the southeast and the 850 hpa wind anomaly reached +3SD. The rain ended in Pennsylvania around 23/0000 UTC but continued in southern New England and Upstate New York (Fig 2). Heavy rain fell in the St. Lawrence Valley, southern CT and eastern Long Island, NY. Some of these areas recieved over 3 inches (75 mm) in this 6 hour period. The rain ended in many locations by 23/0600 UTC. However, isolated heavy rain continued over the northernmost areas of New York State. This area received an additional 1-3 inches (25 to 75 mm) of rain during this period. Overall, the Northeast saw heavy rain over a 60 hour period between 21/0000 UTC and 23/1200 UTC (Fig 1b). Over 1 inch (25 mm) of rain fell across northern PA, northern NJ, CT, southern MA, western VT, and nearly all of NY. There seemed to be two bands of heavier rain. One band began in central PA and went through southern NY, northern NJ, CT, and RI. This band saw 1-3 inches of rain with isolated 5+ amounts. The second band went from north central PA and extended through central and Upstate NY. This band was the heaviest and these areas received 1-3 inches through north central PA and central NY. Areas north of the Mohawk River received 2-5 inches with isolated 6+ amounts. A table of the top 20 rainfall totals for the northeast rain is list in Figure 10. v. Forecasts- Midwest The GFS QPF forecasts are shown in figure 9 and this forecast is valid for 21/1200z. The GFS did a fairly good job forecasting the heavy rain. The forecast initialized at 19/0000 UTC captured the large area of heavy rain across northern Missouri bit missed the heavy rain in central Missouri and missed the higher end of the precipitation amounts. The forecasts initialized at 20/0000 UTC and 20/1800 UTC showed a bulls-eye of higher amounts on the order of 96mm but failed to place it in the proper location. The NAM QPF forecasts (Fig 8) valid at 21/1200 UTC show that a couple of the runs produced an area of heavy rain along the Missouri/Iowa border which was a little too far north and the area of higher amounts was too small. The surge of high PW and enhanced southerly 850hPa jet was well forecast by the GEFS (Fig 9). The GEFS was similar to the NAM and GFS in placing the heaviest precipitation too far to the north along the IA/MO border. vi. Forecasts- Northeast The GFS had a good amount of variability from run to run leading up to the event (Fig 9). The first three runs show that the timing of the rain was late as the model had most of the rain falling over the Great Lakes and not over the Northeast by 22/1200 UTC. The model began to pick up on the speed of the system around 54 hours out but failed to show the possibility of heavy rainfall. By 36 hours prior to the event the model had the speed and potential for heavy rain well forecasted but the location of the heavy rain was too far north. By 24 hours prior to the event the model forecast was very accurate showing an area of 32 to 64mm across northern PA and southern NY by 22/1200 UTC.

4 The GEFS forecasts valid at 22/1200 UTC were similar to the GFS forecasts in that it was not until 24 hours prior to the event before the model began to produce accurate forecasts. Initially the model did not pick up on the surge of anomalous PW values into the region and therefore did not produce the heavy rainfall that was observed. Initially the model also misplaced the 850 hpa jet too far to the north and as a result also placed the heavy rain north of where it was observed. But by 24 hours prior to the event the 850 hpa jet position was well forecasted along with the surge of high PW air. The GEFS and GFS forecasts through the end of the event (23/1200 UTC) followed a similar pattern to the first part of the event. The model did not resolve the pattern well until 24 hours prior to the forecast being valid. Both models missed two areas of heavy rain in their forecasts. The heaviest rains were forecasted by the models to be across central NY State but actually fell in northern NY and southern NY. The St. Lawrence River valley in extreme northern NY saw heavy rains on the order of 3-6 inches yet the models failed to capture the full extent of it. The lower Hudson valley saw 2-6 inches yet the models failed to forecast this as well. The heavy rain was convective in nature during the day on the 22 nd. The cold front associated with this system was moving through eastern PA in the morning and sunshine broke out ahead of the front. Thunderstorms developed along the front and moved northeast through northern NJ, southern NY, and CT. The models likely missed this convection. 4. Discussion/Conclusions Heavy rain fell across the CMRV and Northeast from the August Both areas were associated with above normal PW. The rainfall over the CMRV was associated with convection ahead of a slow moving cold front. The Northeast rain was associated with an upper level low that became cut-off as it move out of the Ohio valley. While the rainfall amounts where in the same range, the timeframe of the rainfall was quite different. The CMRV rains fell over about 12 hours but the Northeast rains fell over 36 hours. The CMRV rainfall was from convection where as the Northeast rainfall was produced by a cut-off cyclone, more like a winter stratiform rainfall event, though there was embedded convection in the rainfall. High PW air over the Gulf States was transported northward by strong southerly flow ahead of the frontal system moving out of the plains. The cold front interacted with this stream of high PW air resulting in deep convection. The heavy rainfall was observed in close proximity to this surge of high PW over Missouri. This surge of high PW air and associated high PW anomalies was observed in the Davenport, IA sounding which showed PW values of 2.2 inches at 0000 UTC 21 August This was the 8 th highest value observed at this rawinsonde site. The 850 hpa jet associated with this plume of high PW air into Missouri and Iowa exceed +3SD above normal during the period of heavy st rain the 21. As the storm moved east the southerly flow ahead of it continued to transport anomalously high PW northward. The sounding from Pittsburgh showed PW values of 1.9 inches which exceeded +2SD at 22/0000 UTC. The 850 nd hpa jet increased overnight on the 22 to over +3SD in the area where the heaviest rains fell. The heavy rains coincided with the areas of anomalous PW and 850 hpa jet. While the models seemed to predict these features rather well the QPF forecasts for the CMRV were underestimated by the models. This is likely due to the sub-grid scale processes that cannot be resolved by the models which is related to the convective nature of the rainfall. The GFS fared better with the Northeast QPF forecasts. A number of the runs showed the potential for over 128 mm (~5 inches) of rain and even got the general location correct. However, the run 12 hours prior to the start of rainfall backed off on the higher end of the QPF. It did show a large area of over 32 mm that ended up being a very good forecast. But, the

5 location and amounts of the highest rainfall totals were largely missed in the forecast. The QPF forecasts for the Northeast were likely better than those for the Midwest due to the larger scale forcing involved in the event. Clearly, convective based precipitation is more difficult to predict than precipitation which is forced by larger scale processes (in the model grid scale). The absence of deep convection in the northeast may have added to the predictability of this event. The high PW values and strong winds in the CMRV and Northeast were clues to the potential for heavy rainfall. Clearly, as shown in Figures 4 & 5 the high v-wind anomalies and high PW anomalies (Fig. 6) appeared to delineate the regions of heavy rainfall. In the East the high PW is also a good signal for heavy rain and the slow moving nature of a cutoff cyclone should have been cause for concern as well. In each area, the models suggested rainfall but could not predict the upper end of the rainfall amounts with accuracy. Recognition of anomalous PW and 850 hpa jets by a forecaster will go a long way towards outperforming the model QPF forecasts in situations like these.

6 Figure 1. Total accumulated rainfall (mm) focused over the State of Missouri (upper) and New York (lower). Data show the accumulation period for each image. The Missouri data spans 1800 UTC 20 August through 1200 UTC 22 August The New York focused data shows the period of 0000 UTC 22 August through 1200 UTC 23 August 2010.

7 Figure 2. Fig 2. As in Figure 1 except for 6-hr accumulated rainfall ending at a) 22/0600 UTC b) 22/1200 UTC c) 22/1800 UTC and d) 23/0000 UTC. Fig 3. A in Figure 2 except for 6-hr accumulated rainfall ending at a) 21/000 UTC b) 21/0600 UTC c) 21/1200 UTC and d) 21/1800 UTC.

8 Figure 4. GFS analysis of 250 hpa wind and anomalies valid at a) 1200 UTC 20 August, b) 0000 UTC 21 August, c) 0600 UCT 21 August, d) 1200 UTC 27 August, e) 1800 UTC 21 August, f) 0000 UTC 22 August, g) 0600 UTC 22 August, h) 1200 UTC 12 August, and i) 1800 UTC 22 August 2010.

9 Figure 5. As in Figure 2 except showing mean sea level pressure and anomalies. Return to text.

10 Figure 6. As in Figure 2 except showing precipitable and anomalies. Return to text.

11 Figure 7. As in Figure 2 except for 850hPa winds and anomalies. Return to text.

12 Figure 8. As in Figure 2 except for 500hPa heights (m) and anomalies. Return to text.

13 Fig 7. GFS QPF forecasts Figure 9. NCEP GFS quantitative precipitation forecasts (mm) showng accumulated precipitation valid at 1200 UTC 21 August Forecasts initialized at a) 0000 UTC 19 August, b) 1200 UTC 19 August, c) 0000 UTC 20 August, d) 1200 UTC 20 August, e) 0000 UTC 21 August, f) 1800 UTC 20 August, g) 0000 UTC 21 August, h) 1200 UTC 21 August 2010.

14 Figure 10. As in Figure 7 except for the NCEP NAM initialized at a) 1200 UTC 19 August, b) 1800 UTC 19 August, c) 0000 UTC 20 August, d) 0600 UTC 20 August, e) 1200 UTC 20 August, f) 1800 UTC 20 August, g) 0000 UTC 21 August, h) 0600 UTC 21 August 2010 and i) 1200 UTC 21 August 2010.

15 Fig 9. SREF 850 winds (u Figure 11. NCEP GEFS forecasts from the 1200 UTC 20 August 2010 showing the probabilities of a) u-wind anomalies less than -2.5SDs below normal, b) v-wind anomalies greater than 2.5SDs above normal, c) precipitable water 2SD above normal and d) mean sea-level pressure less than -1.5 SDs below normal.

16 TOP 20 RAINFALL TOTALS FOR EVENT PHOENICIA NY 7.24 WEST SHOKAN NY 7.08 CONSTABLEVILLE NY 6.40 BOONVILLE NY 6.24 WINTHROP NY 6.07 WOODSTOCK NY 5.62 WELLS NY 5.57 TANNERSVILLE NY 5.50 BEARSVILLE NY 5.30 COLD SPRING NY 5.29 JAMESVILLE NY 5.29 FORT PLAIN NY 5.21 CHESTER NY 5.20 RUBY NY 5.18 WEST MONROE NY 5.17 OGDENSBURG NY 5.11 WANAKENA RANGER NY 5.11 PECK LAKE NY 4.82 HIGHMARKET NY 4.80 HARRISVILLE NY 4.75 Table 1. Top 20 highest rainfall amounts reported in the northeastern United States. Data are sorted by amount (inches). The town and state are shown. Return to text.