Dec 16, 2011: Colder Weekend, Warmer Next Week

As a colder air mass started to move into the region today, colder temperatures were observed, peaking during the day in the mid 40s inland and the upper 40s to lower 50s across the rest of the area, although the 24-hour high temperatures were observed in the early morning hours, as temperatures overnight were steady in the 50s across the area, temperatures unusually warm for overnight hours during December - for instance, in Central Park, temperatures tonight were steady between 55 and 59 degrees. The average overnight temperature is 32 degrees, more than 20 degrees colder than tonight's temperature, and the average high temperature is 43 degrees.

With the cold air mass moving in, temperatures will briefly drop back to near to even slightly below average temperatures, reaching the upper 30s to lower 40s tomorrow and the mid to upper 30s on Sunday. These colder temperatures will only be brief, however, as temperatures warm back up into the 40s next week and potentially into the 50s on Wednesday. Another storm will affect the area in the middle of next week, but in addition to the rain, the potential is there for some light snow to fall in the interior parts of the area.


Weekend Outlook:

Saturday will bring partly cloudy skies across the area along with a NNW wind expected. A colder air mass will continue to move into the region, with 850 mb temperatures ending up near -10 degrees celsius, the coldest air mass to move into the region so far this winter. With the colder air mass, colder temperatures are expected across the area, reaching the upper 30s to lower 40s inland, lower 40s in most of the immediate NYC area, and lower to mid 40s in Long Island and southern Connecticut.

As a weak disturbance approaches the Great Lakes, mostly cloudy skies are expected overnight. The models over the last day underestimated the cold on Saturday night, and it now appears that Saturday night will bring the coldest temperatures, dropping into the upper 10s to lower 20s inland and the lower to upper 20s across the rest of the area except for NYC, where temperatures are expected to reach the upper 20s to lower 30s. Partly cloudy skies are expected for Sunday with slightly below average temperatures, peaking in the mid 30s inland and the mid to upper 30s across the rest of the area.


Monday - Wednesday: Warmer, Rain Returns; Some Interior Snow?

The trough in place over the weekend will quickly be pushed out on Monday, with a warmer air mass returning. As a result, warmer temperatures are expected, reaching the mid 40s inland and the mid to upper 40s across the rest of the area along with mostly sunny skies. The high pressure on Monday will quickly move out as a cold front moves towards the area on Tuesday, however, and clouds will increase on Monday night with mostly cloudy skies expected for Tuesday along with slightly cooler temperatures.

A low pressure in the central US will move towards the region, bringing more rain into the area. With some precipitation moving into the area when there is still some cold air, a mix of light rain and snow is possible in the northern and western parts of the area, although accumulations, if any, will be limited to the northernmost parts of the area. As a warmer air mass moves into the region with the low pressure, any frozen precipitation will change over to rain later into Tuesday night as steadier rain develops and temperatures slightly warm up towards the morning. Occasional showers are expected on Wednesday, and although exact temperatures are still uncertain, the potential is there for temperatures to reach the lower 50s in the immediate NYC area. Stay tuned for more information on the Tuesday-Wednesday storm.


Thursday - Next Weekend: Drier End To Week, Weekend Storm Possible

Drier conditions are expected to return into the region behind the mid week storm for Thursday and the first half of Friday, with temperatures slightly cooling down, back into the 40s. Towards the second half of Friday and into the weekend, however, uncertainty increases again as a potential storm approaches the region. The only consistency with the models is that the potential is there for a storm in this time frame; the location, timing and intensity of the storm continue to vary from run to run. During this time frame, the pattern remains the same as it has been so far, with a ridge still in place in the Southeast with a positive NAO/AO and no blocking. With the pattern in place, a nor'easter bringing a snowstorm to the I-95 corridor in the Mid Atlantic is very unlikely. There are some slight improvements in the overall set up in the US, and while this storm does not have a high or even a medium probability at this time of bringing any notable frozen precipitation event to the area, it does appear to have a slightly better chance of bringing some sort of frozen precipitation to parts of the area than other recent storms. While some frozen precipitation may be possible in the northern Mid Atlantic, including the area, places further north into the Northeast would be more likely to see a snowstorm than the NYC area. A snowstorm in parts of the NYC area is not completely impossible, but considering that the most important variables for a Mid Atlantic snowstorm including blocking, -NAO and strong western US ridging are missing, it will be difficult for such a scenario to happen. There is still \uncertainty on whether there will be a storm this weekend, as the timing on the models is inconsistent as well, with some showing a storm this weekend and others showing one later into the week, so while it's not certain that a storm will affect the region over the weekend, the possibility is there. Stay tuned for more information on the weekend outlook.


Brief Long Range Outlook: Looking at the pattern beyond next weekend, there is still no evidence of any large scale pattern change in the 15 day range, and likely beyond the 15-day range as well. The stratosphere remains very cold, with the cold air trapped in Canada along with a positive NAO/AO. Without the -NAO and the blocking, the brief troughs that move into the region are transient, quickly moving out with more ridging returning afterwards. One of the main parts needed to change the pattern to bring more winter-like conditions into the region includes a SSW, or a significant stratospheric warming event. A warmer stratosphere will help to displace the cold air from Canada, bringing more cold and snow into the region. During the previous two winters, height anomalies at 500mb were higher than average over Greenland and parts of Canada, displacing the colder air into the US. With the current pattern, however, there are negative height anomalies over Greenland and Canada, with warmer than average temperatures over the eastern US. Some models show slight warming in the stratosphere through the long range, but there is no SSW event apparent in the 15-day range and likely beyond. Considering that the pattern right now has been locked into place for months and is still solidly in place, it will take time for this pattern to begin falling apart. My expectations have not changed since my winter outlook, as I continue to expect the current pattern to hold in place with a few minor changes at times until at least the middle of January. There could be a few snow events until then, but otherwise above average temperatures are expected to continue, with the best risk of snowstorms staying in the Northeast. Towards the second half of the winter, the potential is there for more cold and snow, although it does not appear that any strong cold and heavy snow pattern will set up like the last two winters. More information will be posted on the long range on occasions throughout the rest of the month, with an updated winter outlook to be posted on January 1st.