22-23 August Nor’easter – Minor flooding and winds

 

A strong upper low was predicted to drop southeast out of Canada and become nearly stationary over the interior northeastern U.S., and guidance was suggesting some extreme rainfall across the northeastern U.S. associated with the system.  This system was resolved well by long range guidance 3 or more days prior to the development of the upper low, and guidance was consistent in predicting 3-7 inches of rain over a large portion of New York and New England.  Wind anomalies at 850 hPa and 250 hPa were similar to what we observe when predicting significant snowstorms in the winter.  However, because of climatology in the summer, less wind at 850 hPa results in higher anomalies, so no widespread wind problems were anticipated associated with this system.  Usually, with such extreme precipitation expected, fairly widespread moderate to even severe river flooding could be predicted, but the dry weather during the summer resulted in dry antecedent conditions and very low river levels, so the impact of the heavy rain was expected to be minor at best.  There were some minor flooding problems, but also some strong winds and power outages, especially in the southern Green Mountains that was not necessarily anticipated.  Wind gusts in the southern Green Mountains were around 35 Kt in area surface observations, but the persistent long term nature of the gusts may have been what caused some tree damage and power outages in southern Vt.  The following images and loops will help to explain what happened and there will be some speculation as to why it happened.

 

  

 

Above:  Four panel displays of 500 hPa heights and vorticity (left), 850 hPa winds (center) and Quantitative Precipitation Forecast (right), initialized at 12Z 21 August for the NAM, GFS, ECMWF and GGEM.  This particular 12Z 21 August model run was chosen because it was representative of other model runs before and after this particular run.  Note the broad consistency in all the guidance as to the location and rate of development of the upper low.  The 850 hPa wind forecast was a bit less consistent, as were the locations and magnitudes of the QPF maxima.  However, again, in a broad sense, most of the northeastern U.S. could expect very heavy rain and some areas some persistent winds.

 

 

 

Above:  Winds and anomalies at 850 hPa from the 15Z 21 August SREFvalid 09Z 23 August (left) and 12Z 21 August GEFS valid 12Z 23 August (right).  Note the U wind anomalies suggested highly anomalous easterly winds into southern VT, which should have been used as a signal that some wind problems were possible in southern VT.

 

   

 

Above:  SREF probabilities for 2 inches in 36 hours (left and left center), 3 inches in 36 hours (right center) and 4 inches in 48 hours (right).  Some unusually high probabilities for 2”-4” of rain were predicted, contributing to a higher than usual confidence for extreme rainfall.

 

   

 

Above:  GEFS probabilities for 2 inches in 36 hours (left and left center), 3 inches in 36 hours (right center) and 4 inches in 48 hours (right).  Some unusually high probabilities for 2”-4” of rain were predicted, contributing to a higher than usual confidence for extreme rainfall.

 

 

 

Above:  Upper air sounding for Albany, NY (ALB) at 00Z 23 August.  Note the winds of 40 Kt or greater just above the surface, suggesting the potential forstrong winds in higher elevations around Albany, which was being realized in southern VT.

 

 

 

Above:  Water vapor satellite imagery with lightning overlay on 22 August (left) and 23 August (right).  Note how the system evolved very similarly to what the guidance suggested.

 

 

 

Above:  Visible satellite imagery with lightning overlay on 22 August (left) and 23 August (right).  Note how the system evolved very similarly to what the guidance suggested.

 

 

Above:  Radar reflectivity loop for 22-23 August.  Notice the persistent areas of heavier rainfall in New York during the 2 day period.

 

 

Above:  Stage 4 total precipitation for 22-23 August.  Note where the heaviest rain fell, and compare to the previous radar reflectivity loop.

 

   

 

Above:  Pictures of tree damage from Manchester, VT (left and center left), and Sunderland, VT (right center and right).  These pictures confirm that persistent long-term winds of 20-25 Kt and gusts up to 40 Kt can produce wind damage.