Sept 30/Oct 1 Storm Review, October 10 Update

I apologize for the delay in posting this review again, I have a busy schedule and was not able to finish this when I planned to, but I was able to finish my work on this summary and post it below.

Today’s update is a short one, and was posted in the bottom of this post. The 5-Day Forecast was updated only for the immediate NYC area.

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The late September to early October storm was a significant storm, bringing rainfall amounts as high as 10-15 inches for parts of the Mid Atlantic with damaging wind gusts affecting places further east, such as the NYC area. For days before the storm, it appeared that a potentially dangerous storm was on its way to the East Coast associated with a tropical system, leading to heavy rainfall amounts and strong winds.

As with every other forecast, there was uncertainty until the last minute. In this case, the models were inaccurate with the placement of the heavy rain axis until the last minute, placing it over the area while it ended up being even further west. As a result, the area only saw some heavy rain on the morning of September 30 and several hours of heavy rain in the morning of October 1, with 21 hours of dry and windy conditions in NYC when there could have been heavy rain and wind that whole time, leading to rainfall amounts reaching the 7-10 inch range.

Rain: Forecast vs. Actual Totals

Below, I posted a map comparing my forecast totals for the storm to the estimated actual totals. The actual total map is only an estimation based on radar estimations and reports, so it may be slightly off for some places, but it represents the main idea of which areas saw how much rain.

Click on the map to view it in a larger size if it is too small to read.

What was right: The general area of the heavy rain axis, while not completely on target, did highlight the general idea, with the western end from central/western Virginia into central PA, and the eastern side moving through New Jersey and NW Connecticut. The eastern edge, however, was too far east. I also correctly put some of the major cities that saw 5+ inches of rain in that range, including Washington DC, Philadelphia, and Allentown, PA.

What was wrong: The heavy rain axis was further east than the actual heavy rain axis. This, though, was not the main problem when compared to the eastern edge of the storm. I significantly underestimated how tight the rain gradient would be, thinking that it would rain most of the day on September 30 when it did not.

eastern Long Island and Connecticut, along with Cape May and SE Delaware, however, were the biggest bust zones. The first storm was too far west of those locations, and the second storm which was supposed to bring the heaviest rain into these areas was also too far west, leading to less than an inch of rain in those areas. As the storm started shifting east later on October 1st, parts of Long Island and Connecticut did see some moderate to heavy rain, but the storm was already moving east rather quickly and did not produce a long period of heavy rain as was originally expected.

Final Forecast: By the afternoon of September 30, it became clear that the forecast would not verify, and I put out a final map for the second storm only, lasting from that night into the overnight hours of October 1st. The dark green represents 3-5 inches, green represents 1-3 inches, and light green is less than 1 inch.

Despite the models still indicating 3 to 6 inches of rain across most of the area, I went along with the RUC, which correctly showed the storm further west than the other models, and that forecast was generally correct, expecting 3 to 5 inches inland, 1 to 3 inches for the immediate NYC area (though some places had more rain than that), and less than 1 inch for most of Long Island/southern Connecticut.

Storm Set Up: What Went Wrong?

Before the storm: When it became clear a few days before the storm that it would have a significant impact on the East Coast, the storm was expected to move right along the coast, if not slightly inland, leading to excessive rain totals of 4 to 8 inches from eastern Virginia through New Jersey/Long Island into southern New England, with strong winds for Long Island and SE New England.

On the day before the storm, some models such as the GFS caught on to the fact that the storm would be further west, moving through central VA/Pennsylvania, but the GFS has a bias where it does not handle tight rain cut off gradients correctly, and as a result had good placement of the heavy rain axis but included the immediate NYC area with heavy rain of 2-5 inches, which if I was to correct its bias to underestimate rainfall amounts, would have meant somewhere near 4-8 inches of rain for the area. The NMM and ARW short term models correctly predicted that there would be dry conditions for the area during parts of the day while most of the rain is to the west, but they still underestimated how far west the heavy rain would actually go.

During The Storm: On the morning of September 30, the storm already brought up to 3-5 inches of rain for the central Mid Atlantic, with a round of heavy rain moving through the area bringing 1-2 inches of rain, but after this round of rain moved through, the storm continued to move north to NNW, and did not switch direction to north/NNE early on like the models suggested, and combined with a tighter rain cut off line than expected, the first storm went through without giving a single drop of rain to most of the area.

By the afternoon hours, the short term model runs were suggesting that the second storm moves just off the coast, bringing the heaviest rain into all of NJ, New York City and SW Connecticut with 3 to 7 inches of rain, with Long Island seeing heavy rain but not as much. The GFS/NAM models remained consistent on their solutions that the heaviest rainfall is just west of NYC, but still underestimated the cut off line.

When working on my afternoon update, however, I noticed that as the second storm entered North Carolina, the short term models and even the NAM/GFS had the heaviest rainfall in eastern NC moving NNE, with rain returning into NYC by 8-10 PM, while the heaviest rain was in central NC moving north, if not NNW. The cold front to the west of the storm was still stationary, not pushing the storm east as the models expected, and as a result it appeared that the second storm would continue to move NNW and inland instead of moving north and NNE as the models expected. That led me to think that the second storm would also end up further inland than expected, with the heaviest rain towards eastern Pennsylvania.

The RUC also supported my thoughts, as when it came into range, it brought the storm just inland, with the heaviest rainfall in eastern PA/NW NJ, with several hours of moderate to heavy rain in NYC and a quick moving line of moderate rain affecting Long Island/southern Connecticut. Based on the RUC and my understanding of the scenario, I created a final rainfall map (which can be found above in the “Final Forecast” section), which ended up verifying for the main part. By the time that the cold front started pushing the storm east, it was already too late for any significant rainfall to affect the New York City area, and the scenario ended up being close to that of the RUC.

Summary:

As I said in the beginning of this post, there is at least some uncertainty until the last minute with storms. In this case, the storm pushing too far inland, the cold front failing to push the storm east until it was too late for 5+ inches of rain, and the underestimated cut off line for the rain led to the forecast failing for the area. Some parts of the forecast did verify, including placing eastern PA and Maryland/Virginia in the 5+ inch area, but not all parts of the forecast went as expected. As with previous storms, this shows the importance of looking at the latest observations to determine whether the forecast is verifying or not, and if any changes need to be made to the forecast.

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October 10: Colder High Temps On The Way
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This morning was a very chilly one, and as the forecast from Friday expected, low temperatures reached the 32 degree point in the interior, and some places even reached 30 degrees. Temperatures today warmed back up into the 60s and lower 70s, which is slightly above average. Tomorrow will also bring above average temperatures, but temperatures will start going down throughout the week, with another potential for near freezing temperatures in the interior on Tuesday night.

Tomorrow’s Outlook:

Tomorrow (Monday) will be a partly sunny day and the warmest day of the week. High temperatures will reach the lower to mid 70s for the immediate NYC area, upper 60s to lower 70s in the interior, and the upper 60s to lower 70s for Long Island/S CT. Winds will be from the WNW/NW.

Monday Night – Tuesday: Storm Expected, Light Rainfall

A weak storm that was first mentioned several days ago will move through the area on Monday night, bringing a steady light to moderate rain. Rainfall amounts will generally be near 1/4 inch, with amounts up to 1/2 inch possible. The rain will continue into Tuesday, when the storm will move out during the afternoon hours. Due to the cloud cover and rain, high temperatures will be quite chilly, in the lower to mid 60s, with upper 60s possible in the immediate NYC area.

Wednesday-Thursday: Chilly

On Wednesday, a trough moves into the area that was not expected at first. This solution is likely to verify, with high temperatures in the upper 50s to lower 60s inland and lower to mid 60s in the immediate NYC area, and lows on both Tuesday and Wednesday night potentially similar to those of this morning.

Thursday – Saturday: Storm Potential, Then Chilly

There is still some uncertainty with this time frame, however it is possible that a storm could affect the area, though rain is still expected in this time frame. Friday and Saturday are currently expected to be the coldest days.

Stay tuned for more details on this time frame.

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