29 July 2009 Flooding and Isolated Severe Weather

(Click on any image to see loop or larger image)


On 29 July, an upper level impulse tracked northeast around the periphery of a mean upper trough, through the eastern U.S. Subtropical and tropical moisture was associated with the system, along with highly anomalous precipitable water values (2-3 SD, or 150-200% of normal). Operational guidance from the deterministic NWP models (GFS, NAM and ECMWF) and ensembles provided good consistent run-to-run output suggesting a heightened potential for a flooding event, and some isolated severe weather. Extensive flash flooding occurred, but thankfully, there were no fatalities.


Storm Prediction Center Upper Air Analyses


Note the very little movement to the upper low north of the western Great Lakes. An upper level jet segment did track northeast of New England, putting our region in the right entrance region. There was a very distinct wind shift at 850 hPa, which likely provided some low level forcing to trigger and sustain convection.


500 hPa 12Z July 29 500 hPa 00Z July 30 300 hPa 12Z July 29 300 hPa 00Z July 30



850 hPa 12Z July 29 850 hPa 00Z 30 July


Satellite loops


Water Vapor Note the upper impulse tracking out of the midwest and Ohio Valley, with the tropical connection.



Visible loop Note the strongest convection tracked through southern NY into New England. However, convection and heavy rain, with very little lightning, trained through central and eastern New York, into southern Vermont and the Berkshires.



Visible loop 1 Visible loop 2 Visible loop 3


Satellite Precipitation Estimates Note the highlighting of the high precipitable water values and the features contributing to the heavy rain.



SPENES graphic SPENES text1 SPENES text2


Data and Forecasts


The prospects for severe weather were limited, as seen in the observed CAPE, even with high Theta-E values over the region. As seen in the satellite imagery, there was extensive cloudiness, preventing much surface warming.



Observed LAPS CAPE MSAS surface Theta-E


High Resolution WRF predicted radar composite reflectivity


The High Resolution WRF did not resolve the timing, placement and mode of convection as well as it does for severe weather, possibly due to the complex instability and extreme moisture profiles in the atmosphere over the region.



GFS, NAM and ECMWF rainfall forecast loops, consecutive runs 00Z 28 July through 12Z 29 July


All the guidance showed the axis of heaviest rain over central and eastern NY into New England. The ECMWF and NAM tended to predicted higher rainfall amounts, while the GFS and ECMWF were able to place the axis of heaviest rainfall the best, as the NAM trended the heaviest rain over the Adirondacks in later runs, which was too far northwest. Did 00Z runs provide better guidance than the 12Z runs? That is open to interpretation.





Ensemble Guidance and Anomalies


The GFS and GEFS showed an anomalous southerly 850 hPa jet, at 3-4 SD above normal. PWAT values were 2-3 SD above normal. Probabilities for 1 inch of rain in 24 hours from both the GEFS and SREF were confined to a very small area, with the 00Z 29 July GEFS suggesting high probabilities in Vermont. The mesoscale nature to convection likely affected the ability for the ensemble mean to resolve high probabilities of heavy rainfall. This is evident in the spreads in the GEFS and SREF, showing virtually all members with 1 inch of rain or more, but the placement of the heavy rain cores is not consistent in each ensemble member. The 00Z 29 July GEFS showed the best consistency in all the ensemble members.



GFS 850 winds GEFS 850 winds GEFS PWAT GEFS prob. 1 inch GEFS prob. 1 inch (next run)



SREF 850 winds SREF PWAT SREF prob. 1 inch


Radar Loops


KENX Loop from 1700-1900 UTC of reflectivity, Urban and Small Stream and Flash Flood Warnings, and Severe Thunderstorm Warning Polygons



KENX Loop from 1900-2100 UTC of reflectivity, Urban and Small Stream and Flash Flood Warnings, and Severe Thunderstorm Warning Polygons



KENX Loop from 2100-2300 UTC of reflectivity, Urban and Small Stream and Flash Flood Warnings, and Severe Thunderstorm Warning Polygons



KENX Loop from 2300-2355 UTC of reflectivity, Urban and Small Stream and Flash Flood Warnings, and Severe Thunderstorm Warning Polygons



Individual plan view base velocity radar images and images from the Four-Dimensional Storm Investigator (FSI) between 1907-1920 UTC


The severe weather potential was limited, and the mode of convection was rather shallow, compared to the -20C levels. So, most of the severe weather potential came as straight line winds and isolated tornadoes. Rotating thunderstorms tracked through the eastern Catskills and mid Hudson Valley, and there were some isolated reports of trees and wires down, but no widespread severe weather. There were impressive values of base velocity over 70 Kt at times, and a fairly deep core of strong winds on the FSI, but most of the strongest winds were not reaching the surface. The height of the radar beam from KENX and surrounding radars was preventing analysis below 6000-7000 ft above ground level.









Precipitation estimates from KENX, MPE and Stage 3 from the River Forecast Center Note the general similarities, so the radar provided fairly accurate rainfall estimates.



12Z 29 July Soundings


Note the instability across the region, and the precipitable waters are anomalously high. The K Index is not especially high, but still indicating respectable moisture.





18Z 29 July Soundings






Storm Prediction Center Outlook



Storm Prediction Center Probabilistic Outlooks for tornadoes, hail and wind



Storm Prediction Center Mesoscale Discussions






Severe Thunderstorm Watch



Albany Area Forecast Discussions


Short term discussion and Hydrology Discussion



Near term discussion and Hydrology Discussion



Near term discussions





Surrounding Area Forecast Discussions NWS Burlington, VT, NWS Binghamton, NY, Taunton, MA and Upton, NY












Severe weather reports for individual NWS offices, courtesy of The Iowa Environmental Mesonet division of Iowa State University Department of Agronomy


NWS Binghamton, NY



NWS Albany, NY



NWS Burlington, VT



NWS Taunton, MA



NWS Upton, NY



Flooding and damage photos from northeastern Columbia County and southeast Rensselaer County, around Stephentown, NY (Courtesy of Tim Scrom and Brian Frugis, NWS Albany, NY)













This picture courtesy of Tim Melino




Any questions or suggestions, please e-mail Neil.Stuart@noaa.gov