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.  

20130809 0100 UTC Day 1 Outlook Graphic20130809 1200 UTC Day 1 Outlook Graphic 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.

 

20130809 1200 UTC Day 1 Damaging Wind Probabilities Graphic

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.

 

000

NOUS41 KALY 100302

PNSALY

CTZ001-013-MAZ001-025-NYZ032-033-038>043-047>054-058>061-063>066-082>084-

VTZ013>015-101502-

 

PUBLIC INFORMATION STATEMENT

SPOTTER REPORTS

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

                     RAINFALL           OF

                     /INCHES/   MEASUREMENT

 

CONNECTICUT

 

...LITCHFIELD COUNTY...

   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

 

MASSACHUSETTS

 

...BERKSHIRE COUNTY...

   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

 

NEW YORK

 

...ALBANY COUNTY...

   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

 

...COLUMBIA COUNTY...

   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

 

...DUTCHESS COUNTY...

   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

 

...FULTON COUNTY...

   PERTH                 0.72   449 PM  8/09  WEATHERNET6

 

...GREENE COUNTY...

   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

 

...HAMILTON COUNTY...

   WELLS                 0.25   533 PM  8/09  WEATHERNET6

   INDIAN LAKE           0.10  1206 PM  8/09  WEATHERNET6

 

...MONTGOMERY COUNTY...

   FONDA                 0.89   503 PM  8/09  WEATHERNET6

   AMSTERDAM             0.77   443 PM  8/09  WEATHERNET6

 

...RENSSELAER COUNTY...

   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 COUNTY...

   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

 

...SCHENECTADY COUNTY...

   NISKAYUNA             0.68   200 PM  8/09  SPOTTER

 

...SCHOHARIE COUNTY...

   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 COUNTY...

   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

 

...WARREN COUNTY...

   BRANT LAKE            0.02   458 PM  8/09  WEATHERNET6

 

...WASHINGTON COUNTY...

   COSSAYUNA             0.25   543 PM  8/09  WEATHERNET6

 

VERMONT

 

...BENNINGTON COUNTY...

   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.