Oct 12, 2013: Quiet Pattern Continues This Week

Forecast Highlights:

gfs_namer_150_500_vort_htFollowing the coastal low late last week that drenched Pennsylvania but failed to extend north into the area as originally predicted, the relatively inactive pattern that was mostly interrupted by last week’s severe weather continues into this week, with little rain and temperatures generally slightly above average. Despite the absence of a stormier pattern, a stronger cool down is expected towards next weekend.




Observations for Wednesday through Friday, October 9-11, have been added in a separate post.



Tonight – Wednesday: Dry, Slightly Warmer Than Average

As the storm updates for the coastal low and observations for October 9-11 noted, the coastal low that stalled south of the region failed to produce widespread rain over the area, which can partly be attributed to dry air aloft and subsidence between convection well offshore and rain bands stalled over Pennsylvania. Temperatures warmed up into the 70s today for most of the area, but with a noticeably stronger east wind and cooler temperatures towards the late afternoon and evening as a high pressure entered the region from eastern Canada, bringing a cooler air mass along with it. This high pressure is also suppressing the coastal low pressure, which will drift south but overall remain nearly stalled off the coast of South Carolina. As ridging aloft over the region shifts east, the coastal low will begin to slowly return north, with slightly warmer temperatures for the early week as weak ridging builds in again ahead of a developing central US trough.

Mostly cloudy skies will continue tonight into Sunday morning, with a breezy NE wind around 10-15 mph, gradually decreasing. There is some slight uncertainty regarding how much cloud cover clears tomorrow; for tonight’s forecast, I am expecting partly cloudy skies by the afternoon and evening with highs in the mid 60s. Mostly sunny skies are expected for Monday and Tuesday with a continued light NE wind and highs in the upper 60s to low 70s. The next cold front will approach on Wednesday, which along with a southerly flow will pull moisture associated with the collapsed coastal low back north, with two areas of rain likely; one west of the area associated with the front, and another towards southeast New England and possibly eastern Long Island/CT with the moisture associated with the coastal low. The front is expected to end up near the East Coast on Thursday, but as the next section discusses, is likely to fail to quickly move through. The main question is how much rain reaches the area as opposed to staying east or west; at this time I included at least a 40% chance of showers, with the highest probabilities in eastern and western parts of the area, although especially with recent trends it would not be surprising if most of the rain stays away from the immediate NYC area.


Thursday – Next Weekend: Pattern Turns Colder

The first half of October has been marked by an absence of cold air masses over the region; during the first half of last year’s October, widespread sub-zero temperatures at 850 millibars were observed across the region, with highs peaking in the 50s in Central Park for a total of 7 days through 10/16, along with a low of 38 on the 13th. This month, however, Central Park has yet to fall below 62 degrees for a high, and the coolest low was 53 degrees. After another slightly warmer than average week, more significant changes in the synoptic pattern will develop for the late week and weekend, starting with a relatively strong block shifting west into Greenland before collapsing, setting up for a central based -NAO, and amplifying ridging in the northeast Pacific Ocean and western North America. In response to the ridge amplification to the west, a strong trough, currently in the Aleutian Islands, will dive southeast through central Canada, pulling an arctic air mass with 850mb temperatures near -10C southwards as well. This air mass will moderate by the time it reaches the US, but is expected to bring the most significant cool air mass of the fall so far.

The trough axis is likely to remain centered near the central US, where the most significant colder than average temperature anomalies are likely; the main question is how far east the cold air shifts, and whether it will be able to settle over the region especially considering hints of a weak southeast ridge signal with the high pressure staying south of the region, a setup which has not happened much this fall as the high pressures were typically to the north with a more northerly wind direction. Additionally, the model guidance often tends to speed up pattern changes, a scenario which was shown again as some of the runs several days ago already started the colder pattern by the late week or weekend, and it is a possibility again that the cold does arrive into the region, but delayed and not as cold compared to what was initially modeled. At this time, I am expecting the coldest temperatures and anomalies to stay further west, but with the colder air entering the region at times, such as next weekend and early next week, when the coolest temperatures so far this fall may be possible with widespread highs in the 50s possible. Temperatures may moderate towards the middle of next week, but with no major warm up likely at this time.

2 thoughts on “Oct 12, 2013: Quiet Pattern Continues This Week

  1. Anonymous Reply

    So this week brings a setup similar to that of the week of 9/16-9/22, with the suppressed coastal low coming back north up into Long Island with clouds and showers ahead of the next cold front?

    • NYC Area Weather Post authorReply

      Yes; there are some similarities to that setup, as the weakened low pressure remains stalled south of the ridging currently present north of the area, which slides east towards the mid week ahead of the next cold front, allowing showers to spread back north into at least eastern New England ahead of the frontal passage. The main differences, however, are that the cold front itself will be much weaker, and is unlikely to produce much rain in the area, and also with uncertainty regarding how far west these showers spread, which at this time are likely to mostly stay east of the area but could reach parts of Long Island and SE CT.

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