August 22 Final Storm Forecast

Over the last few days, I have mentioned that a heavy rain event is possible for Sunday night and Monday, and while it was certain that heavy rain would fall in central New York, it was uncertain if the heavy rain would extend into the area or if it would stay to our north. The models did not have a good handle of this storm, ranging from as little as no rain in NYC on the short term NMM model to as much as nearly 4 inches of rain on yesterday’s GGEM run. The models yesterday started showing a secondary low pressure enhancing the rain over the area, and this scenario now seems likely, however it is uncertain where the low pressure forms and where the heaviest rain falls.

Below is my storm discussion and final forecast map. Note that as it is still uncertain where the heaviest rain falls, some slight revisions to the forecasts could be made during the storm updates that I will post this afternoon.

Storm Final Forecast / Rain Map

Click on the map to view it in a larger size.
Storm Set Up: What we are currently seeing is a low pressure located on the border of NW PA and SW NY, with its cold front currently near west Pennsylvania, and a weak low pressure is forming near northeastern North Carolina. The low pressure in NW PA will continue to move ENE, and once it reaches central New York, which should be around 2 PM, the low pressure will start to transfer its energy to a developing low pressure SE of NYC, with heavy rain forming near and east of the immediate NYC metro. This low pressure is then expected to weaken throughout Tuesday while another low pressure forms to its south, staying well east of the area while moving NNE towards Nova Scotia, which for the longer range, will help draw in a chilly air mass into the area on Friday.

Precipitation Forecast: With the low pressure moving into central New York, the heaviest rainfall will be located until this afternoon towards south/central New York, and south of the storm, non-severe showers and thunderstorms, locally heavy, will affect the area during the afternoon into the evening hours. With this part of the storm, at least 1/2 to 1 inch of rain is likely for most of the area, with locally higher amounts up to 1.5 inches in the heavier showers.

As the secondary low pressure takes over, the heavy rain will end across central/northeastern New York, with heavy rain forming near the low pressure. It is still uncertain where the low pressure will be exactly located, with the storm’s heavy rain forming either near the immediate NYC area, over Long Island and Connecticut, or offshore. The rain will then gradually become lighter in the western parts of the area overnight, with light rain for Monday, while Long Island and S CT will likely continue to see moderate to heavy rain.

Forecast Rain Totals / Rain Map: At this time, at least a total of 1 to 2 inches is possible for most of the area, with 2-3 inches for Long Island/S CT and places well north of NYC. These rain totals will possibly change, stay tuned for more updates this afternoon and/or in the late morning in case the storm’s forecast changes.

Note: This rain map has an unusually low confidence level, due to an unusually large model spread, and it is possible that the scenario may end up being different from what this map shows.

Other Notes: Tropical Depression 6 has formed in the eastern Atlantic Ocean last night, and more details on that, as well as the longer range and the cooler temperatures that will return to the area by Friday, will be posted with the next full update tomorrow in the early morning.

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