Sept 15, 2012: More Strong Storms For Tuesday

Forecast Highlights:

- Chilly temperatures continue through Sunday night
- Cold front to produce strong storms, heavy rain Tues night
- Temperatures remain near average for longer range

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Note: The post date is from Sunday, 9/16 since it was posted after midnight. This post belongs to Saturday, 9/15's night update. Sunday's update will be posted on Sunday night.


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Observations for Saturday, September 15 have not been posted yet.


Tonight - Monday: Dry; Chilly Start, Then Warmer

A trough remains in place over the region behind last night's cold front, with a high pressure in place and 850mb temperatures near 4-7C, resulting in chilly temperatures for the region. Mostly clear skies will continue tonight with lows dropping into the low to mid 40s inland and the upper 40s to low 50s for the rest of the area except for NYC, in the mid to upper 50s. Mostly sunny skies will continue through Sunday with highs in the low to mid 70s across the area. Overnight lows will be similar to those of tonight, although slightly warmer, with highs on Monday warming up into the mid to upper 70s as a warmer air mass spreads in and 850mb temperatures warm up to near 10C.


Monday Night - Wednesday: Storm Produces Strong Storms, Heavy Rain

The next significant weather event to affect the region will take place on Tuesday into Wednesday. A pattern change is currently unfolding which will be discussed in more details in the next section, as the North American pattern becomes more amplified with ridging developing over the western US and Alaska as well as near Atlantic Canada. A trough will drop into the central US, moving towards the region with moisture drawn from the Gulf of Mexico up the East Coast ahead of a strong cold front. There is some uncertainty with the exact scenario, although it appears at this time that there will be two main rounds of storms. A round of scattered storms, locally heavy, is likely around Tuesday morning, mainly before 12-2 PM. Following these storms, drier conditions are likely for the late afternoon into the early overnight hours with the heaviest rain staying to the west of the area, although scattered showers/storms may still be possible especially north/west of NYC. Temperatures will peak in the mid 70s across most of the area.

The cold front will approach overnight, moving through on Wednesday morning, with temperatures barely dropping or remaining steady overnight in the upper 60s to low 70s across most of the area. While not much instability is expected, favorable wind shear and a strong low level jet will result in the potential for strong/low-end severe thunderstorms with the cold front overnight, especially after 2-4 AM, with strong wind gusts the main risk. Windy conditions are also likely prior to the arrival of the main storm line. With precipitable water values approaching 2 inches, the potential is there for heavy rainfall locally above 1-2 inches with the cold front. Widespread severe weather to the extent of last Saturday (9/8) is not expected. Storms are expected to persist into Wednesday morning, especially from NYC and further east, with clearing skies later into the day with highs generally in the low to mid 70s. Stay tuned for more information for Tuesday and Tuesday night's outlook.


Thursday - Longer Range: Pattern Change Expected

As previously mentioned, a pattern change will take place starting from the middle of this week. An increasingly amplified pattern will develop with more ridging towards the western US and Alaska as well as Atlantic Canada and the Davis Strait, which will result in a more persistent trough pattern in between, with the trough axis near the central US, where below average temperatures are expected along with frequent troughs. The 12z ECMWF, posted to the left for Friday, 9/21 from the Pennsylvania State University e-Wall, shows the pattern expected with the ridging near Atlantic Canada and the western US with a strong trough in between. As the trough axis will remain over the central US, temperatures are likely to average out to generally close to average for the area through the end of the month, with occasional transient cool spells but with the core of the cold staying west/NW. This pattern favors near-above average precipitation mostly focusing towards the eastern half of the US. This pattern is remarkably different than that observed during last year's fall, when the pattern was much more progressive with less amplitude, with a lack of sustained troughs in the US and persistent ridging near the western US and Greenland.

Temperatures will briefly cool down following Wednesday morning's cold front, with cooler temperatures returning for Wednesday night and Thursday. With a SW flow still in place and the cold front failing to progress much to the east of the region, the cool air mass will lift back into Canada as a warmer air mass spreads in again with temperatures warming up by the late week into next weekend. Uncertainty increases by next weekend with another storm potential which each model handles differently, although the potential is there again for rain towards late next weekend or early next week. Stay tuned for more information on the longer range.