9 August 2013 Training Convection Flash Flooding Event
Waves of training convection along a stalled frontal boundary during the early morning and afternoon hours on 9 August 2013 led to flash flooding across southeastern and northeastern portions of the NWS Albany CWA. A Flash Flood Watch was in effect for portions of eastern New York and northwest Connecticut, coincident with where the heaviest rainfall occurred.
A stalled cold front and wave of low pressure provided the focus for showers and thunderstorms beginning during the overnight hours on the 8th and continuing through the morning and early afternoon hours of the 9th. The cold front stalled across northwestern portions of the NWS Albany CWA as the flow aloft was relatively weak (20-35 kt) and parallel to the frontal boundary. Warm and moist air underneath the right entrance region of an anticyclonically-curved upper-level jet was pumped northward ahead of the front, with surface dewpoints in the upper 60s to low 70s and PWAT values around the 99th percentile (2-2.25 inches) above the normal August PWAT value. K index values between 35 and 40 indicated that thunderstorms with very efficient rain processes were likely. The 12 UTC ALB sounding revealed a tall, thin CAPE profile of around 1500 J/kg expected by the afternoon hours (LIs were also between -1 and -4 indicating an unstable, but not explosively unstable, environment supportive of continual regenerative updrafts). Freezing levels were also relatively high ~13 kft and midlevel lapse rates were moist adiabatic. The static stability profile revealed a slight decrease with height, supportive of favorable parcel ascent. Aiding the potential for training convection were weak 850 hPa and 500 hPa winds (10-25 kts and 25-35 kts respectively) as well as a weak cloud-bearing wind (17 kts) collocated with a low-level theta-e ridge across southeastern portions of the NWS Albany CWA. All of these thermodynamic indices along with the synoptic environment were similar to local conceptual models created by office staff, which has proven to be useful throughout the years.
The severe threat was marginal, with only 20-25 kts of 0-6km shear amidst the marginal instability environment. However, with the anomalously high moisture in place, an isolated damaging wind gust could not be ruled out from precipitation loading within the updraft. The weak midlevel lapse rates severely limited the hail potential as well, and the tornado threat was low as well given the lack of sufficient shear and weak wind profile.
Global model guidance QPF from the NAM, GFS, ECMWF, and GGEM all indicated a relative max in QPF amounts of only around 1-1.5 inches across northwestern portions of the NWS Albany CWA not the southeast. There was also significant variability in QPF placement between model runs. The global model guidance also suggested that this rainfall would be of a persistent, stratiform nature, with relatively light rainfall rates over an extended period of time. GFSEnsemble and SREF guidance did show areas of 1+ in 24hrs across southeast portions of the NWS Albany CWA for multiple model runs, but there were no probabilities for 2 in 36hrs. These ensemble predictions suggested a soaking summer rain, but not any flooding. Mesoscale models such as the HRRR and local WRF hinted at the potential for late morning/early afternoon convection, although at much less of an impact than what actually occurred.
With all of these factors in place, a Flash Flood Watch was issued by the midnight shift during the early morning hours of 9 August that covered all of western New England and eastern New York (excluding the Adirondacks). This region was placed in the watch as model QPF guidance, FFG, and best thermodynamic indices were located across this region.
Convection occurred mainly in two phases. The first occurred with the passage of a weak upper-level shortwave trough which produced a stratiform rain shield across much of the NWS Albany CWA. As this shortwave exited to the southeast, along with the slightly stronger midlevel wind field associated with it, the wind profile weakened allowing for a speed convergence max to set up across the northern Taconics, western Berkshires, and southern Vermont. With weak orographic ascent also present, the rain shield began to develop a quasi-stationary region of heavier showers that quickly produced 1-3 inches of rain across southern Vermont and western Massachusetts. This prompted the issuance of a Flash Flood Warning for northern Columbia, eastern Rensselaer, southeast Washington, northern Berkshire, Bennington, and Windham Counties. Williamstown, MA was particularly hit hard with reports of flooded basements and road washouts in and around the town. Just a couple hours later, the second phase began as the cold front stalled and a wave of low pressure developed along it. This wave of low pressure helped spawn renewed convection across portions of the Catskills eastward into Dutchess and Litchfield Counties. This convection quickly obtained a training nature allowing rainfall totals to quickly pile up. A FLS was initially issued for southeast Ulster, Dutchess, and Litchfield Counties before being upgraded to a Flash Flood Warning at 819am (valid until 1215pm), mainly as a prediction based on the atmospheric conditions, reports upstream and the anticipation that the heaviest rain would continue to track across the same areas.
Observations from Orange County (NWS Upton CWA) depicted over 3 of rain fell in two hours (~8am-10am), with LSRs describing vehicle rescues. This area of heavy rain with embedded thunder was moving northeast across Ulster, Dutchess, and Litchfield counties, further supporting the issuance of the FFW. Verification of the warning became difficult as the rain occurred over a region often sparse of reports. Several calls were made to local law enforcement and emergency management to try and find verification. Local emergency managers in Dutchess and Litchfield Counties volunteered to drive around to look for flooding. Ham radio operators also were very helpful during the event as they continually monitored radio chatter as well as deploying to look for flooding. Despite these efforts from local partners, verification initially remained difficult, despite both KENX and KBGM STP/STA indicating 3-5 across the warned area. FFMP also supported flash flooding, as several basins had exceeded 3-hr and 6-hr FFG. Satellite sounder imagery also proved as a useful nowcasting tool indicating PWAT values 200% above normal. KOKX was difficult to interpret as the STP/STA had not been reset from the past 3 days. Radar reflectivities indicated low echo centroid (LEC) storms, with high rainfall rates of around an inch per hour given the reflectivity values of 50-55 dBZ.
Due to the lack of reports, the FFW was allowed to expire for Ulster, Dutchess, and Litchfield Counties, despite continued convection across these regions. A possible reason for the delayed response in flash flood reports could have been attributed to the local drought and dry antecedent soil conditions across these areas, as these areas had gone roughly a week without any measurable rainfall. Main stem river flooding also did not occur, despite the heavy rainfall across the area. No more than 10 minutes afterwards, a call was received from a local emergency manager of flash flooding ongoing in the town of Torrington with multiple roads closed and vehicle rescues underway. A local emergency manager in Dutchess County called as well relaying reports of flash flooding he had seen from driving through the area. A new FFW was reissued for the same area after receipt of these reports. A third FFW was later issued for Ulster, Dutchess, and Litchfield Counties at 518pm, after receiving a spotter report of flash flooding in the town of Gardiner and renewed indications of flash flooding depicted by radar trends and FFMP.
Although the severe threat was minimal, there was a report of 5 trees down around 530pm in the town of Beacon Hills (extreme southern Dutchess County). Radar data did not support the issuance of a SVR, with velocity values only indicating 20 kts ~7.5 kft from KENX and one pixel of 31 kts ~5.9 kft AGL from KOKX. While precipitation loading may have led to an isolated wet microburst, these trees may have fell simply from moist soils from the 3-5 of rainfall that fell.
The potential for heavy rain was highlighted well in several NWS Albany products. The HWO mentioned the threat for thunderstorms with locally heavy rainfall on 7 August, and also mentioned the threat for poor drainage flooding of low lying and urban areas on 8 August. The AFD mentioned the threat for heavy rainfall and a slow-moving frontal system several days prior to the event. An email was sent to area emergency managers during the afternoon on 8 August highlighting the risk for locally heavy rain and potential poor drainage flooding. Facebook was also used as a method of reaching out to the general public.
Model guidance did not predict this event well. QPF amounts of 1-1.5 inches were severely underestimated from what actually occurred (some locations in Dutchess County received over 7 of rain despite measurement errors from the KPOU ASOS) Sub-gridscale mesoscale processes, such as orographic lift and a weak speed convergence zone across the lower Hudson River Valley likely played a large role in the actual training convection that occurred. Events such as this one highlight the need for continual improvement in model QPF forecasting, and how the use of conceptual models and the nowcasting environment (6-24hrs) can ultimately be the deciding factor.
Social media proved to be a valuable resource during the flash flooding event. Facebook and Twitter were used to reach a variety of audiences, with Facebook posts being liked and shared and Twitter posts being retweeted. Facebook and Twitter were also used to relay critical weather information, such as the issuances of the Flash Flood Warnings and PNSs. Local spotter reports descriptively described the event mentioning specific rain rates, locations, and flooding.
Day 1 0100 UTC Convective Outlook (left) and Day 1 1200 UTC Convective Outlook (right). Southeast portions of the NWS Albany CWA were outlined in a See Text from SPC for a marginal severe threat.
Day 1 1200 UTC Convective Outlook text mentioning the isolated wind threat.
Day 1 1200 UTC SPC wind probabilities. Southeast portions of the NWS Albany CWA were only highlighted with a 5% outlook.
1200 UTC sounding from Albany, NY (ALB) depicting a tall, thin modestly unstable environment, along with anomalously high moisture and weak winds supportive of slow-moving, efficient warm rain processes.
1200 UTC HPC surface analysis (left) and at 2100 UTC (right) depicting the slow progression of the frontal boundary as a wave of low pressure rode up along the front.
24-hr rainfall valid at 1200 UTC on 9 August (left) and 10 August (right). The training nature of the convection can be seen across northeast and southeast portions of the NWS Albany CWA.
Four panel of model QPF for 9 August with the GFS (top left), NAM80 (top right), ECMWF (bottom left), and NAM12 (bottom right) all displacing the max in QPF.
Four panel of 850 hPa (top left and right) and 250 hPa (bottom left and right) winds depicting the weak, convergent low-level wind field and right entrance region of the upper-level jet.
Satellite sounder imagery depicting PWAT values 200% above normal.
Four panel comparison of STA and STP from KENX, KOKX, and KBGM depicting the highest rainfall totals of 3-5 across Ulster, Dutchess, and Litchfield Counties.
NOUS41 KALY 100302
PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE ALBANY NY
1102 PM EDT FRI AUG 09 2013
THE FOLLOWING ARE UNOFFICIAL OBSERVATIONS TAKEN DURING THE PAST 12
HOURS FOR THE STORM THAT HAS BEEN AFFECTING OUR REGION. APPRECIATION
IS EXTENDED TO HIGHWAY DEPARTMENTS...COOPERATIVE OBSERVERS...SKYWARN
SPOTTERS AND MEDIA FOR THESE REPORTS. THIS SUMMARY IS ALSO AVAILABLE
ON OUR HOME PAGE AT WEATHER.GOV/ALBANY
********************STORM TOTAL RAINFALL********************
LOCATION STORM TOTAL TIME/DATE COMMENTS
TORRINGTON 4.96 116 PM 8/09 SPOTTER
HARWINTON 3.05 641 PM 8/09 SPOTTER
FALLS VILLAGE 3.00 1107 AM 8/09 TRAINED SPOTTER
CANAAN 2.50 508 PM 8/09 WEATHERNET6
LANESBOROUGH 3.40 417 PM 8/09 WEATHERNET6
SAVOY 3.40 1050 PM 8/09 WEATHERNET6
ALFORD 2.80 545 PM 8/09 WEATHERNET6
PITTSFIELD 2.66 504 PM 8/09 WEATHERNET6
PRESTON HOLLOW 4.02 1011 PM 8/09 WEATHERNET6
RAVENA 3.00 518 PM 8/09 WEATHERNET6
FEURA BUSH 3.00 957 PM 8/09 WEATHERNET6
GUILDERLAND CENTER 1.10 511 PM 8/09 WEATHERNET6
COLONIE 0.72 906 PM 8/09 WEATHERNET6
COHOES 0.67 1151 AM 8/09 WEATHERNET6
CHATHAM 3.20 1024 PM 8/09 WEATHERNET6
ANCRAMDALE 2.99 556 PM 8/09 WEATHERNET6
NORTH CHATHAM 2.72 547 PM 8/09 WEATHERNET6
KINDERHOOK 2.10 350 PM 8/09 SPOTTER
CRARYVILLE 2.09 536 PM 8/09 WEATHERNET6
TAGHKANIC 2.03 557 PM 8/09 WEATHERNET6
LIVINGSTON 1.92 1043 PM 8/09 WEATHERNET6
HUDSON 1.75 608 PM 8/09 WEATHERNET6
POUGHKEEPSIE 7.84 710 PM 8/09 TRAINED SPOTTER
RED OAKS MILL 5.56 610 PM 8/09 SPOTTER
LAGRANGEVILLE 5.35 213 PM 8/09 SPOTTER
MILLBROOK 4.87 428 PM 8/09 WEATHERNET6
RHINEBECK 4.05 437 PM 8/09 SPOTTER
SALT POINT 2.55 1048 AM 8/09 TRAINED SPOTTER
PERTH 0.72 449 PM 8/09 WEATHERNET6
ELKA PARK 2.85 558 PM 8/09 WEATHERNET6
GREENVILLE CENTER 2.80 425 PM 8/09 WEATHERNET6
CATSKILL 1.90 1056 PM 8/09 WEATHERNET6
CAIRO 1.70 537 PM 8/09 WEATHERNET6
FREEHOLD 1.45 500 PM 8/09 SPOTTER
EARLTON 0.88 1208 PM 8/09 WEATHERNET6
WELLS 0.25 533 PM 8/09 WEATHERNET6
INDIAN LAKE 0.10 1206 PM 8/09 WEATHERNET6
FONDA 0.89 503 PM 8/09 WEATHERNET6
AMSTERDAM 0.77 443 PM 8/09 WEATHERNET6
EAST NASSAU 3.50 555 PM 8/09 TRAINED SPOTTER
SPEIGLETOWN 0.80 427 PM 8/09 WEATHERNET6
TROY 0.78 501 PM 8/09 WEATHERNET6
CENTER BRUNSWICK 0.61 432 PM 8/09 WEATHERNET6
SARATOGA SPRINGS 0.60 518 PM 8/09 WEATHERNET6
CHARLTON 0.40 442 PM 8/09 WEATHERNET6
MILTON 0.32 525 PM 8/09 WEATHERNET6
MALTA 0.32 1109 AM 8/09 WEATHERNET6
EDINBURG 0.18 1212 PM 8/09 WEATHERNET6
NISKAYUNA 0.68 200 PM 8/09 SPOTTER
NORTH BLENHEIM 3.98 1207 PM 8/09 TRAINED SPOTTER
JEFFERSON 3.80 509 PM 8/09 WEATHERNET6
FULTON 3.25 600 PM 8/09 WEATHERNET6
SUMMIT 3.11 447 PM 8/09 WEATHERNET6
RICHMONDVILLE 2.72 447 PM 8/09 WEATHERNET6
HUNTERSLAND 2.01 619 PM 8/09 WEATHERNET6
MIDDLEBURGH 1.83 428 PM 8/09 WEATHERNET6
SCHOHARIE 1.60 1138 AM 8/09 WEATHERNET6
ULSTER PARK 4.11 530 PM 8/09 WEATHERNET6
ESOPUS 3.66 415 PM 8/09 WEATHERNET6
KINGSTON 2.85 1052 PM 8/09 WEATHERNET6
PHOENICIA 2.46 529 PM 8/09 WEATHERNET6
WILLOW 2.38 349 PM 8/09 SPOTTER
BRANT LAKE 0.02 458 PM 8/09 WEATHERNET6
COSSAYUNA 0.25 543 PM 8/09 WEATHERNET6
WOODFORD 1.86 603 PM 8/09 WEATHERNET6
LANDGROVE 0.17 440 PM 8/09 WEATHERNET6
PNS issued depicting rainfall amounts that fell between 12am and 11pm 9 August.
Watch/Warning/Advisory page from the NWS Albany webpage during the height of the event depicting the Flash Flood Watch and Flash Flood Warning.
Facebook (left) and Twitter (right) posts highlighting the use of social media in conveying the flooding threat.
PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ALBANY NY
634 PM EDT FRI AUG 09 2013
0842 AM FLASH FLOOD WILLIAMSTOWN 42.71N 73.20W
08/09/2013 BERKSHIRE MA EMERGENCY MNGR
WATER ON ROADS AND FLOODED BASEMENTS.
1200 PM FLASH FLOOD LAGRANGEVILLE 41.65N 73.76W
08/09/2013 DUTCHESS NY TRAINED SPOTTER
CREEK OVERFLOW ACROSS ROAD. WATER 6-12 INCHES DEEP.
1218 PM FLASH FLOOD WINSTED 41.93N 73.07W
08/09/2013 LITCHFIELD CT EMERGENCY MNGR
SEVERAL CITY STREETS CLOSED DUE TO FLASH FLOODING.
1240 PM FLASH FLOOD SALISBURY 41.98N 73.42W
08/09/2013 LITCHFIELD CT TRAINED SPOTTER
FIRE RESCUE DISPATCHED TO VEHICLE STRANDED IN
FLOODWATERS ON TOWNHILL ROAD.
0100 PM FLASH FLOOD DOVER PLAINS 41.75N 73.58W
08/09/2013 DUTCHESS NY 911 CALL CENTER
A FEW ROADS CLOSED DUE TO FLOODING.
0100 PM FLASH FLOOD WAPPINGERS FALLS 41.60N 73.92W
08/09/2013 DUTCHESS NY 911 CALL CENTER
A FEW ROADS CLOSED IN TOWN.
0115 PM FLASH FLOOD RIVERTON 41.96N 73.02W
08/09/2013 LITCHFIELD CT PUBLIC
6-10 INCHES OF WATER MOVING ACROSS RTE 20.
0116 PM FLASH FLOOD TORRINGTON 41.84N 73.13W
08/09/2013 LITCHFIELD CT TRAINED SPOTTER
MOTORISTS BEING RESCUED AND SEVERAL CITY STREETS CLOSED
DUE TO FLASH FLOODING.
0545 PM FLASH FLOOD GARDINER 41.68N 74.15W
08/09/2013 ULSTER NY 911 CALL CENTER
A ROAD WAS CLOSED DUE TO FLOODING.
EVENT NUMBER ALY1300383 ALY1300384 ALY1300387 ALY1300385 ALY1300388
ALY1300389 ALY1300390 ALY1300386 ALY1300391
LSRs listing flash flood reports.