Oct 21, 2012: Warmer Next Weekend, Then Stormy

Forecast Highlights:

• Much warmer this week; highs in 70s frequent
• Brief back door cold front brings clouds, some showers mid week
• Warmest temperatures are expected next weekend
• Changes for longer range: Analyzing a storm potential

Earlier this afternoon, I mentioned how changes in the forecast could be made for the longer range. The model guidance has been showing a significant storm affecting the region, but how likely is it that the storm unfolds this way? Click below to read the full post for more information.

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Tonight - Thursday: Warm, Then Briefly Cooler

Following the departure of the current trough, a warmer air mass will return to the region with 850 millibar temperatures near 10 degrees Celsius, allowing high temperatures to surge well into the 60s across the area on Monday along with mostly sunny skies. By Tuesday and Wednesday, however, a back door cold front will come close, leaving the tri-state area near a temperature gradient, with warmth in the Mid Atlantic and cooler temperatures in the Northeast. Tuesday is likely to be the warmer day with highs well into the 60s from NYC and further west and southwest, with low 70s likely in the immediate NYC area. Further north and east, more cloud cover and possible showers are expected with highs generally in the mid to upper 60s, perhaps a bit lower for some.

The outlook for Wednesday is a bit more uncertain, with a model spread regarding where the temperature gradient ends up. Currently, the front is likely to drop south of the area later on Wednesday into Wednesday night, with the warmest highs southwest of NYC and the coolest highs over Connecticut on Wednesday. The specific forecast temperatures at this time are likely to be in the mid 60s to low 70s range in northern NJ and NYC, low to mid 60s in SE NY and Long Island, and the upper 50s to low 60s in southern CT, although these numbers are subject to change. Scattered showers are possible north and east of NYC on Tuesday night into Wednesday night. Should the temperature gradient end up further north, the warmer temperatures would be more widespread, and mid 70s may be possible in the immediate NYC area. More widespread cooler highs are then expected for Thursday with partly cloudy skies with the front is south of the area as a high pressure drops south from Canada to eastern New England, with an onshore flow and highs generally in the upper 50s to mid 60s for eastern areas and mid to possibly upper 60s in western areas.


Friday - Beyond: Much Warmer, Then Possible Storm

As the New England high pressure continues to slide south of the area, a SW flow will resume for Friday into the weekend, with much warmer temperatures expected, warming back up into the upper 60s to mid 70s from NYC and further north/west, ending up slightly cooler for coastal areas towards Long Island and southern Connecticut. A cold front will approach the region from the central US by Saturday; while this cold front was originally expected to simply move through with colder temperatures behind it, the addition of a potential tropical cyclone into the equation has complicated the longer range outlook even more.

Today's model guidance has been very bullish with what would be a significant weather event for the region, with most models in agreement with showing a tropical cyclone phasing and retrograding into the United States as an abnormally strong hybrid/extratropical low. The model trend was led by the 0z CMC, which showed a very intense low pressure smashing into the Mid Atlantic and Washington DC, along with the 12z ECM that took a somewhat similar track, with both models showing a significant rain, wind and snow event. It is very important, however, to consider that this is still 8-10 days away, which can easily make a huge difference between what the models show and what actually happens. As such, it is premature at this time to call for a significant storm, although the potential is there, as discussed in more details below.

There are many factors affecting this time frame but one of several key features that led to the significant changes in the model guidance is Invest 99L, currently in the Caribbean, which is likely to develop into a tropical storm. Should it do so, it would be named "Sandy". 99L was initially expected to stay well offshore, but it has since trended further west. Typically, tropical systems in this time of the year originating in the Caribbean stay well offshore, although as shown to the left with the 12z GFS (image source: NCEP Model Analyses and Guidance), there will be a strong block over Greenland, along with a strong low pressure south of the block over the Atlantic and ridging to the north and east of 99L. While this is no guarantee that 99L will be blocked from taking the typical late October tropical cyclone path and staying well offshore, as that is still a very possible outcome, the presence of these features previously mentioned does increase the probability of another scenario in which 99L is closer to the United States, potentially affecting it directly. One of the main uncertainties is with the ridging northeast of 99L, as while the earlier GFS runs had strong ridging, thus blocking 99L from moving offshore, the 18z and 0z GFS runs have much weaker ridging to its NE which is less favorable for 99L to move up the coast and become a significant storm.

Assuming that 99L does in fact develop into a tropical cyclone, takes a further west path closer to Florida, and is blocked from moving out into the Atlantic, the next question is how it phases with an approaching shortwave in the central US. A strong trough will drop into the central US behind the cold front approaching the East, and solutions range from the CMC which stalls the front over the Ohio Valley as Invest 99L phases with the shortwave, to the 18z GFS which allows the front to progress to the NYC area before stalling, while 99L stays much further east in the open Atlantic. The evolution of this part of the forecast is also subject to change considering that the shortwave responsible for the phasing comes from the Gulf of Alaska, a region with poor sampling of data for the models and thus more difficulty for the model guidance to accurately forecast the final outcome.


Summary: Considering how the models are portraying this time frame, as well as the anomalous blocking expected, the potential is there for a moderate, possibly significant storm to affect the region, but considering the issues mentioned above as well as how far out this is, there is still plenty that can go wrong with this set up, instead leaving the region with not much more than a typical cold front coming through, and the possibility of such a historic weather event that would require the pattern to end up just right for the event to unfold as modeled is low at this time. At this time, a more likely outcome is where 99L stays further east, and has lesser impacts on the region, perhaps no direct impact at all, with a cold front containing some tropical moisture coming through.

Despite this uncertainty, however, it is given that there will likely be a somewhat stormier period going into the late weekend and early next week, but how stormy the time period ends up remains to be seen. Additionally, the recent changes complicate the pattern outlook going into November, and the pattern outlook originally scheduled for today has been postponed to Tuesday. Stay tuned for more information on the longer range outlook.