Oct 10-11, 2013 Storm Updates

Below, a few storm updates will be posted on the coastal low affecting the area today and on Friday. Updates are being posted both on the blog and on Twitter.

Yesterday’s Forecast Discussion
7-Day Forecast


8:30 AM: Rain Remains South, West Of Area

1158As of 8 AM, the coastal low is located near the coast of extreme SE Virginia. Minimum pressure is near 1010 mb, and the low is very slowly drifting north. It will continue to remain stationary or drift north until later today when another high pressure will enter New England and suppress it to the south.

Since yesterday evening’s update, rain ultimately reached the coast from offshore, but set up in a band training over southern NJ and Delaware into Pennsylvania, where totals over 5 inches have been recorded. To the north of this gradient, however, no rain reached the area, as a result of continued subsidence over the region with dry air aloft and also related to persistent convection well offshore. While the rain bands will slowly make their way north, they will fail to settle over the area as most of the model guidance indicated, with scattered showers possible late this morning and early this afternoon especially near and south of NYC before the storm retreats south. Yesterday’s revised rain outlook, despite siding lower than model guidance, was still too high; at this time, it appears most of the area should end up with less than 1/4 inch for the storm total. This was hinted at with some of last night’s models that initialized with correct conditions, such as the 18z 4k NAM which showed little to no rain over NYC.

Another update will be posted later this morning; otherwise the next forecast discussion will be posted tonight.

6:30 PM: Rain Forecast Slightly Lowered

2218As of 6 PM, the coastal low is located near eastern North Carolina. Minimum pressure is near 1010 mb, and the low is nearly stationary. It will remain stationary or slowly drift north until Friday morning, when a stronger high pressure will begin to enter New England and suppress the low back to the south.

Since the last update, most of the rain remained centered over Pennsylvania, with occasional drizzle affecting the NYC area; the model guidance incorrectly depicted an area of heavy rain forming along the New Jersey coast and affecting the area right now, which never formed. Looking at the current observations, the model incorrectly focused the rain near the coast; the persistent rainfall centered over Pennsylvania is enhancing subsidence over the eastern regions, which along with dry air aloft is inhibiting widespread rain development over the area. The latest high resolution models, such as the 12z RGEM and 18z 4k NAM, initialized with more accurate reflections of the current scenario with rain focused over Pennsylvania and not over the area; these runs end up keeping the heaviest rain west and south of central NJ, with little rain over the area, or in the case of the 4k NAM no rain over NYC.

While these models may be underestimating the rain a little too much, the current observations support an overall idea of occasional light-moderate rain spreading into the area tonight into Friday morning before retreating south, but with shorter duration and lower intensity than modeled. As such, I slightly lowered the rain outlook to less than 1/2 inch over southern CT and 1/2 to 1 inch for the rest of the area. Totals may be locally higher especially near and south/west of NYC.


3:30 PM: Rain Falling West Of Area

northeastAs of 3 PM, the coastal low is located near the eastern tip of North Carolina. Minimum pressure is near 1009 mb, and the low is very slowly drifting to the north. It will continue on this path until Friday as a high pressure in New England slides east, before retreating back south as another high pressure builds into New England from the north.

Areas of light to moderate rain affected the area this morning, with radar estimates showing less than 1/4 inch, but have since shifted to the west. Occasional showers will continue across the region through Friday before shifting to the south of the area. Rain amounts remain in question, however. Most model guidance at this time depicts the area receiving at least 2 to 3 inches of rain, with totals locally up to 5 inches; the model guidance is not always accurate, however, and comparing today’s model runs to the latest conditions show differences, especially near the New Jersey coast, where most models, including the ECMWF, showed a band of heavy rain which enters the area in the afternoon and continues into the night and Friday. As the latest radar shows, however, the heaviest rain is centered over Pennsylvania, with little to no rain off the NJ coast.

While today won’t feature heavy rainfall, more widespread areas of moderate rain are likely later tonight into Friday. I am continuing to side with lower than modeled rain totals for this outlook with slightly lower totals than yesterday’s outlook, with 1/4 to 1/2 inch in Connecticut and SE NY, and at least 3/4 to 2 inches of rain for most of the area, locally higher over 2 inches near and south of NYC; this range is to account for areas of heavier banding in parts of the area that will verify closer to the higher totals, with other areas towards the lower end of this range. These totals may be generous, however, and could have to be lowered further if current trends continue. I will continue to monitor the storm, and this post will be updated should any aspect of the outlook change.

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