Oct 13, 2011: Colder Pattern Develops Saturday

Note: The 5-Day Forecast page was not updated tonight. The forecast will be updated on Friday afternoon.

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After the weak low pressure moving up the coast brought light rain to the area last night, reaching 1/4 inch across most of the area with locally higher amounts up to 1 inch in Long Island and southern Connecticut, today brought cloudy skies to the area with occasional drizzle. Temperatures ended up colder than expected due to the cloudy skies and drizzle, despite the warmer air mass over the region, with high temperatures reaching the lower to upper 60s across the area.

A stronger cold front will move through the region tomorrow, bringing an end to the prolonged period of above average temperatures. Scattered thunderstorms are expected tonight, but more widespread thunderstorms are expected for Friday afternoon and evening, some of them potentially getting close to strong/severe levels with gusty winds and heavy rainfall. Behind this storm, colder temperatures will return into the region with seasonable temperatures expected for the weekend, but the dry conditions won’t last for long as yet another storm will affect the region next Wednesday, bringing the risk for more moderate to heavy rain across the region.

Tomorrow’s Outlook: Thunderstorms Expected

With a stronger low pressure developing near the Great Lakes for tonight and tomorrow, a stronger cold front will move through the region tomorrow afternoon and evening, with a strong push of cold air behind the front. Ahead of the front, a warm air mass is expected, bringing warm temperatures for tomorrow again, reaching the upper 60s to lower 70s inland, lower to mid 70s in the immediate NYC area, and the upper 60s to lower 70s in Long Island and southern Connecticut.

Along with the warm air mass, instability is expected as well. Negative lift index values and bulk shear of up to 50 knots will support widespread thunderstorms across the region, and some of these thunderstorms could be strong, producing gusty winds and heavy rainfall. The heavy rain isn’t expected to affect the entire area, and some models target the immediate NYC area with the heavy rain while others keep NYC almost completely dry, but with the strong cold front, it is likely that at least parts of the area see locally heavy rain. Two rounds of rain are expected; the first one is expected tonight with scattered thunderstorms possible from NYC and further west, and the second one is expected during Friday afternoon and evening, bringing strong thunderstorms with heavy rain and gusty winds to parts of the area. When necessary, storm updates will be posted tomorrow afternoon and evening.

Weekend – Next Week: Colder Pattern Develops

As the strong low pressure moves towards Canada, it will pull in a strong cold air mass into the northern US and southern Canada. This low pressure will then stall over central Canada, trapping the cold air over southern Canada and the northern US. This is known as a Polar Vortex, or a PV, and especially during the winter, helps keep the cold air trapped over the region and increases the chances for cold and snowstorms to affect the Mid Atlantic and the Northeast. The coldest temperatures will stay to the north of the area, but regardless, near to slightly below average temperatures are expected between Saturday and Monday, with high temperatures in the upper 50s to mid 60s across the area and low temperatures in the mid 40s to lower 50s across most of the area, with lower to mid 50s in the immediate NYC area. Windy conditions are expected for Saturday with gusts up to 30-40 mph possible, and mostly cloudy skies are expected with scattered showers for Monday.

By the middle of next week, however, a more significant storm is expected to affect the region. As a low pressure approaches the Ohio Valley, it will begin to become negatively tilted while moving towards the coast, pulling a warmer air mass into the NYC area. The timing is still slightly uncertain, but mostly cloudy skies with scattered showers are expected for Tuesday, with high temperatures in the lower to upper 60s across the area, potentially reaching the lower 70s in the immediate NYC area in the warmer case scenario.

While there is more uncertainty by Wednesday, a secondary low pressure is expected to develop further southeast, with some models such as the ECMWF showing this happening either off the coast, with the secondary low developing into a strong low pressure over the Northeast, bringing the risk of snow to parts of the Northeast, and other models such as the GFS show this happening only slightly to the SE of the primary low, bringing a relatively short rain event followed by strong cold air. Based on the trends so far this fall, I am leaning closer to the ECMWF, with a moderate to potentially heavy rain event on Wednesday into Wednesday night followed by drier conditions returning for Thursday night as the low pressure intensifies while moving into the Northeast. Stay tuned for more information over the next several days regarding this storm’s impact on the area.

Next Weekend: Strong Cold?


As mentioned with the last few updates, a positive PNA and a negative NAO are expected to develop by the end of next week, and with the Polar Vortex sitting over central Canada keeping the cold air over southern Canada, in close proximity to the US, the storm during mid-late next week will be capable of pulling down this fall’s first widespread strong cold spell. Widespread sub-freezing 850 mb temperatures are expected over the central/north central US to the west of the storm, which will later spread into the region. There is some uncertainty with the intensity and location of the cold spell given the time range, but unless we see major changes with the expected set up for next week, there is a high probability that the NYC area sees widespread high temperatures in the 50s behind this storm, with low temperatures likely ending up in the 30s and 40s. If the intensity of the cold verifies, the potential may be there for flakes to fall across parts of the interior Northeast as well. Stay tuned for more information on the potential cold spell for the end of next week into next weekend.

Beyond next weekend, the smaller details of the forecast become more uncertain as this is in the longer range, but when looking at the overall picture, it does not appear at least as of now that the storm next week will become another Polar Vortex, and with the NAO and PNA trending back to neutral, the cold is not expected to remain stuck over the region, and is likely to weaken by the second half of next weekend with moderating temperatures. While it is unlikely that a strong and sustained warm spell returns into the region for the end of October, it does not appear at this time that the pattern supports any strong and sustained cold over the region, at least not yet. More information will be posted on the longer range over the next several days.

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