Sept 30, 2011: Latest Pattern Outlook

This discussion, posted on Saturday morning, covers the outlook for this weekend, next week and next weekend. The other pages were not updated today, and will be updated on Saturday evening.

Today’s Outlook:

The outlook for today remains the same from last night’s update. Partly sunny skies with a SSW wind are expected, with high temperatures reaching the lower to mid 70s inland, mid to upper 70s in the immediate NYC area, and the mid 70s across most of Long Island and southern Connecticut.

Weekend Storm Outlook: Rain Returns

The outlook for this weekend has turned much more complicated over the last several days than first thought, as an upper level low will sit over the region for this weekend through early next week, extending the already ongoing period of mostly cloudy and unsettled conditions into the first week of October. The models have had issues with the handling of this cut off low pressure, mostly showing it staying to the north/east of the region, either offshore or over Canada, but trends over the last 24-36 hours have placed this low much further south/west than first thought, over the NYC area into Pennsylvania.

As the upper level low moves into the area tonight, a wave of low pressure will form east of the Mid Atlantic tonight, bringing rain up the coast from SE Virginia into New Jersey and Connecticut. Light to moderate rain will develop across the area after 12 AM and will continue through Saturday morning, with 1/4 to 1/2 inch of rain expected to fall across  the area tonight. As the low pressure tilts towards the NW while moving over NYC on Saturday, the heavier bands of rain will push further northwest, affecting New York and Pennsylvania, while the NYC area sees occasional showers. These scattered showers will continue through Sunday before becoming more isolated by Sunday night. Between Saturday and Sunday night, an additional 1/4 to 1/2 inch of rain is expected to fall across the area, bringing the storm total to 1/2 to 1 inch across the area.

As the storm will trap the cold air mass over the Mid Atlantic, sub-freezing 850 mb temperatures are expected in that region, which combined with precipitation, will produce light to moderate snow over the higher elevations of West Virginia. The potential is even there for light accumulations should the snow fall heavily enough.

Weekend Temperature Outlook: Cooling Down, But No Major Cold

Due to the cut-off low pressure, the previously expected strong cold spell has been eliminated as well. With the cut-off low moving into NE Pennsylvania and southern New York, the cold air mass will be trapped over the Mid Atlantic, where chilly high temperatures in the 50s to lower 60s are expected along with a chilly NW/WNW wind. With the storm to the west of the area, however, a south wind is expected, with SE winds over New England pulling in a mild air mass from offshore, with high temperatures reaching the 60s to lower 70s in parts of New England.

With the NYC area expected to be in between the warm and cold air masses, temperatures will be chilly in the area, but will not be as cold as mentioned over the last few days. On Saturday, high temperatures are expected to peak in the lower to mid 60s inland and in the mid to upper 60s across the rest of the area. On Saturday night, low temperatures will drop into the mid to upper 40s west of NYC, and in the lower to mid 50s from NYC and further east. With cloudy skies and scattered showers on Sunday, high temperatures will reach the mid to upper 50s in NW NJ/SE NY, lower 60s in the immediate NYC area, and the mid 60s east of NYC, in Long Island and southern Connecticut. A south, potentially SSE or SSW wind is expected for Sunday. Sunday night will bring chilly temperatures as well, dropping into the lower to mid 40s inland, mid 40s to lower 50s in the immediate NYC area, and the lower to mid 50s in Long Island/S CT.

The reason why temperatures will not be as cold as first expected is not due to a weaker cold spell than expected. The expected intensity of the cold when entering the Ohio Valley has not changed from what it was several days ago. What has changed was the scenario, with the cut-off low, previously expected to stay  east of New England, now expected to stay west of NYC. If the previously expected scenario where the cut off low stays east of the region or even over New England had verified, then the expectation for high temperatures reaching the lower to upper 50s across the area with lows in the upper 30s to mid 40s would likely have verified; even the models showing that scenario were much colder, showing highs in the 40s west of NYC. With the cut-off low now expected to end up to our west, however, the NW wind was replaced with a S/SW wind bringing in warmer temperatures, and even though the models are trending warmer, most of them are still going colder, showing highs in the 50s across most of the area, which based on the latest expectation, may be too cold.

Next Week’s Outlook: Staying Chilly, Then Warming Up

When the cut off low was expected to stay to the east of the region, the expectation was there for the cold spell to exit with a much warmer air mass moving in by early next week. Due to the new expectation for the cut-off low, however, the warm air mass will fail to quickly build into the region, and with the upper level low merging back into the main flow, another strong trough will dig into the region, but this time, due to the upper level low moving further east, the trough will only affect New England with the colder temperatures for Tuesday into Thursday, with the NYC area in the edge of this trough.

The cut off low will remain stationary over the region for Monday, with high temperatures expected to reach the upper 50s to mid 60s across the area. By Tuesday, a strong high pressure will move in from Canada, which will bring mostly sunny skies along with dry conditions. This high pressure will stick around through the weekend, keeping mostly sunny skies and dry conditions in place, making this the first prolonged period of dry conditions since the middle of September. The temperatures, however, will vary, ranging from a chilly mid week to a warm weekend.

High temperatures on Tuesday are expected to reach the mid to upper 60s across most of the area, with lower to mid 60s inland. Colder overnight lows are expected for Tuesday night, dropping into the lower to upper 40s away from NYC. High temperatures will stay in the lower to upper 60s across the area on Wednesday, and will slightly warm up by the late week, reaching the upper 60s to lower 70s by Friday. The warm air mass is not expected to reach the region at its peak intensity, with the warmest temperatures staying to the west of the region, but regardless, temperatures will warm up next weekend, reaching the 70s across the area. Stay tuned for more information on next week and next weekend’s outlook.

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