A cold front quickly moved through the area today, producing up to 0.15″ of rain in what was the 5th largest rain event this fall in NYC. Behind this front, colder temperatures return with highs in the low-mid 50s on Friday and Saturday. A brief warm up into the upper 50s is expected on Sunday, only to be followed by colder temperatures for the middle of next week with highs struggling to warm past the low 40s.
Tonight – Monday: Slowly Cooling Down
The current pattern consists of a fast, progressive flow aloft, with a broad mid-upper level trough currently over the region expected to quickly move through, bringing a cold air mass into the region behind the cold front with 850 millibar temperatures falling to near -6C on Friday. As another shortwave trough quickly races east through the northern Great Lakes with the progressive flow, heights aloft are expected to rise on Saturday and Sunday with a southwest flow bringing a slightly warmer air mass into the region. A dry cold front will move through on Sunday associated with a low pressure over southern Canada, but this cooler air mass will be fast moving as well, already out of the region by Monday with a resumed southwest flow.
Regarding expected conditions in the NYC area, mostly sunny skies are expected to return for Friday and the weekend with a breezy NW wind at 10-15 mph and highs reaching the upper 40s inland and the upper 40s-low 50s elsewhere. After cold overnight lows, falling into the mid 20s-low 30s away from NYC and the coast, where lows will fall into the mid 30s, temperatures will remain chilly on Saturday with mostly sunny skies, lighter winds and highs in the upper 40s-low 50s for most. A weak low pressure will stay well to the north on Saturday night and Sunday, with a dry cold front moving through on Sunday. This will bring high temperatures into the low 50s inland and mid-upper 50s elsewhere with a breezy west wind at 10-20 mph. Temperatures will then slightly cool down for Monday as a cooler air mass briefly moves in, reaching the low to mid 50s for most.
Tuesday – Friday: Much Colder; Highs In Low 40s
Pattern Analysis: While the pattern becomes more complex by the late week, the origins of the pattern change can be traced to current events in the West Pacific basin, which remains very active as typhoon Haiyan, one of the most powerful tropical cyclones in recorded history with 195 mph sustained winds, struck the Philippines today as a category 5-equivalent typhoon. A strong trough will dig south over northern Japan, aiding in the development of anomalously strong north Pacific ridging, with 500mb heights up to 588-594 decameters. As this ridge amplifies, a cutoff low will develop off the western US coast, which will allow the ridging currently in the NE Pacific to shift over the cutoff low and into western North America, where a strong arctic air mass will be displaced, quickly shifting south through Canada and reaching the northern US border by Sunday. This air mass will then spread into the northeastern US by Monday night into Tuesday with the passage of a cold front that may produce isolated snow showers over parts of the region, perhaps including the NYC area.
The next area of uncertainty is regarding Wednesday and Thursday, as some of the recent model guidance has indicated a storm potential. The last two years have proved that with the right setup, an early season storm can still be capable of producing accumulating snow all the way to the coast, and in this case there is plenty of cold air available with 850mb temperatures near -10C. The main question, however, is regarding whether the setup will be favorable for a storm to form, as any snow potential would depend on the amplification/intensify and location of the potential low pressure. Out of today’s models, the ECM was the most amplified, showing a major rain/snow storm, while the GFS started with a rain/snow event and gradually trended further east and more progressive. While typically, the ECM is considered a reliable model, it has not performed very well in the medium range this fall with a recurring bias to overamplify potential storms, and in some cases showing nor’easters that in fact never developed. The trend this fall has generally been for more progressive and weaker storms in the area, and in this case the pattern does not appear to be much slower, as with a progressive flow aloft and a lack of blocking features downstream to slow down the digging shortwave trough as the NAO and AO remain positive, it quickly tracks towards the coast, with the pattern indicating the trough becomes negatively tilted too late and too far east to support a major storm over the region. This scenario would support some scattered rain/snow showers as opposed to a significant storm.
At this time, given recent trends and the overall pattern, I am siding towards a progressive setup where Tuesday’s front continues to track offshore, with any low pressure sparing the region of significant impacts other than isolated rain/snow showers. As this is in the medium range, this is subject to some changes, especially if the ECM scenario of a a cutoff low forming verifies, in which case probabilities of a more significant storm would be enhanced.