09/3/14 Update: An official report on this event was done by the National Weather Service in Upton,NY. It includes 1.08″ of rainfall that was recorded in just 9 minutes! The moist nature of airmass that was being transported over Long Island was extremely anomalous as well. Precipitable water values 2.38″ from climatology data was reported over Wallops,VA, Since 1963, 99% of precipitate water values were below this value. My original report:

Very heavy showers and thunderstorms drenched parts of LI  overnight Tuesday and early Wednesday morning. Islip MacArthur Airport, NY received 13.27 inches of rainfall by time it was over. Another thunderstorm later in the afternoon dropped another .30″ of rainfall. For total rainfall of 13.57″ out of this event. That total breaks the all-time record rainfall in 24 hours in New York State. Which was 11.60 inches in Tannersville, NY on Aug. 28-29 2011, during hurricane/tropical storm Irene. For records at Islip Airport, 13.51″ of rain was recorded for August 13th. Which far, far exceeded the record highest rainfall for the date of 0.91″. A rainfall total of 14.03″ for the month so far, also beats the wettest August on record 13.78″ back on August 1990. It’s also very close to the all-time monthly record of 14.07 back in October 2005. The historic nature of these records are tempered somewhat by records only going back to 1984 here. However, at Central Park, which records go back to 1869, only three months have been wetter than at Islip for the month so far. Some other towns not too far away saw double-digit rainfall totals as well. Such as Holbrook with 12.57″ and Bay Shore at 11.35″. More heavy rainfall totals can be found on National Weather Service Upton,NY Public Information Statement.

The most extraordinary note about Islip’s rainfall yesterday, was how much rainfall fell in a very short period. Islip between 5am and 7am, reported 5.34″ in first hour, then 4.37″ in the second hour. For a total of 9.71″ in just 2 hours alone! Rainfall rates like that are very uncommon for this area and more typically seen in tropical cyclones. At this time, I can’t find any information on record rainfall rates for this region. Nor can I recall any storm producing that 4″ to 6″ per hour rainfall rates. This produced a significant amount of flash flooding on roadways and neighborhoods in that area.

There was also a very wide range of rainfall totals across the Tri-State area. More than I originally expected myself. Most prolific rainfall totals, where just along and north of a coastal front that extended from a secondary and triple-point surface low further down the coast into Eastern Long Island. Along and just north this front, deep moist convergence and surface frontogenesis was most enhanced. A middle to upper disturbance juxtaposed with this surface system, also enhanced divergence aloft. Precipitable water values were near 2.00″ and MUCAPE near 500 J/kg. This is resulted in thunderstorms with extreme heavy rainfall rates training over from Southern NJ to West-Central Long Island. Winds were also gusting over 40mph near the shores as well. Highest report was a 57mph thunderstorm wind gust near Robert Moses State Park in Long Island.

With an extreme amount of dynamics focused over these regions, this took alot of energy for heavy rainfall for areas farther inland and further east. Which resulted in far less rainfall. While in my last post, I mentioned there could be localized higher totals with any training t-storms anywhere, I didn’t discuss the possibility that the secondary/triple-point low could rob areas further inland of moisture and dynamics. Most model guidance didn’t not these handle these apsects as well. Therefore the rainfall totals shown on the models where far off for most areas. But that it is not really unusual. It’s always difficult for the models, resolve differences in convective parameters and forcing mechanisms. When accessing heavy rainfall potential, it’s better look for and focus more on synoptic features present on the models. Such as the low-level jet and precipitable water values. That all being considered, I don’t think anyone could of reasonably argued for more than 10 of rain for parts of the area, from a non-tropical system that was relatively progressive for our area.