January 2, 2017 Forecast Analysis

Blog posts will resume as of today. While not on a daily basis, posts will mostly serve to provide a meteorological analysis of the forecast, while the Forecast Summary page features a short, impact-oriented synopsis of the forecast.

 

2017 started off similarly to where 2016 ended, with warmer than average temperatures and little snow across the Northeast. Changes are on the way for the next week, albeit temporary, as an anomalously deep trough extends across the vast majority of the US, potentially accompanied by a snowstorm next weekend in parts of the East Coast, followed by a return to the warmer than average pattern into the second week of December.

 


Tonight – Wednesday: Warmth & Rain Return

The current upper-tropospheric pattern is largely dominated by a high-latitude block over the Northeast Pacific, with geopotential height anomalies nearly +3 standard deviations above normal. This ridge has been in place over the last several days, largely a result of major ridge building downstream of a rapidly deepening extratropical cyclone over the Northwest Pacific on 28 December. This ridge largely remained positively tilted (implying a southwest to northeast axis), with a deep trough digging into the northwest United States, a pattern configuration which is unfavorable for cold over the northeast United States.

 

500 hPa geopotential height anomalies valid at 00 UTC 2 January (last night). Red shadings denote higher than average geopotential heights, as is the case over the Northeast Pacific with the high-latitude ridge. Image from Alicia Bentley’s Maps.

A shortwave trough rounding the base of the trough, visible over northern Mexico in the image above, progressed into the Southern Plains today, which along with a very warm and moist air mass in place to provide instability and a southwesterly mid-level jet streak to provide strong vertical wind shear necessary for the development and maintenance of supercells, aided in facilitating a severe weather outbreak, with widespread damaging wind and several tornado reports across the Deep South.

This shortwave trough will continue to track northeast overnight into Tuesday, with its associated surface cyclone tracking into Ohio. Meanwhile, a strong 1036-hPa anticyclone will remain stationed over southeast Canada, resulting in cold air damming across the region, a scenario in which low-level northeasterly flow in a stable air mass is unable to move up over the higher terrain in the Appalachians, and is instead forced to accelerate southward, building a ridge of high pressure down the eastern spine of the mountains. With low-level northeasterly flow and cold air in place across much of the Northeast, a secondary surface low pressure will develop off the coast along the thermal gradient, further enhancing the northeasterly flow over the tri-state area and keeping daytime high temperatures mostly in the upper 30s to low 40s.

12z NMM model valid on Tuesday afternoon (4pm EST), depicting widespread moderate to heavy rain across the tri-state area associated with the secondary coastal low, situated near eastern Delaware on the image above.

Much of the showers over the area today in association with an antecedent coastal low pressure will taper off tonight, with dry, cloudy conditions likely through early Tuesday afternoon. Heavier rain and possible thunder associated with weak elevated instability is expected later on Tuesday afternoon, followed by drier conditions overnight as the system departs. Cold air advection in the wake of this system will lag behind by several hours, with a sufficiently warm air mass lingering on Wednesday in conjunction with a northwesterly downsloping wind to allow temperatures to warm up into the upper 40s to low 50s for most locations.


Thursday – Next Weekend: Coastal Low Uncertainty

Over the next few days, the Pacific block will retrograde westward as the original ridge collapses and a secondary one develops farther west, enabling the resumption of a northwesterly flow across Canada. The current trough over the Northwest US will split from the main flow and linger over the Northeast Pacific, while a piece of vorticity breaks off and interacts with the Northeast US low, resulting in a rapidly deepening cyclone over eastern Canada. A strong northwesterly flow behind this system will advect a cold air mass equatorward into much of the United States, with temperatures struggling to rise above freezing on Thursday.

How the upper-level flow evolves afterwards remains uncertain, and subtle shifts in the model guidance may have significant downstream implications. Much of the uncertainty originates within the next 36-48 hours, especially regarding the the extent to which the Northwest US cutoff upper level low (ULL) separates from the main flow.

18z GFS valid at Thursday morning. Key features to follow are the upper level low (ULL) over Oregon, and the western extent of the trough over Montana. Image from Tropical Tidbits.

The GFS depicts more separation, with the ULL slowly re-emerging over Oregon on Wednesday night. This larger separation between the cutoff low and the main flow allows for a faster progression of the latter, with a cyclone quickly deepening well offshore on Friday as a shortwave trough rounds the base of the longwave trough. The ULL then progresses eastward, amplifying in response to ridge building over the West Coast and with sufficient spacing between it and the Northeast trough, which along with a favorable right-entrance jet streak quadrant aids in facilitating surface cyclogenesis over the northern Gulf of Mexico. Along with the anomalously cold air mass in place, this scenario would produce snow unusually far south into Arkansas through North and South Carolina, bypassing the NYC area to the south.

12z ECM valid on Thursday morning. Note that the ULL over the Northwest US is less separated from the trough over Montana than in the GFS.

The ECMWF, on the other hand, depicts less separation of the two features, which results in less ridging downstream of the ULL, leading to less amplification of the trough, less cyclonic vorticity advection downstream of the trough as a result of the smaller trough amplitude, and weaker forcing for surface cyclone development. The resulting scenario is possible light snow on Friday associated with the initial shortwave trough, while the latter storm remains weak, strung out, and quickly moves offshore without developing into a major cyclone.

In cases like these, it is important to recognize that with high uncertainty, deterministic model outputs (e.g., a single GFS or ECM run) represent only one possible scenario of what might occur, and determistic forecasts (i.e., providing a forecast for exact snowfall and rainfall amounts with no range of error) have a large bust potential. Ensemble guidance can be useful, especially when interpreted as the range of scenarios that might occur and how differences in the evolution of the upper-level flow might affect the downstream result.

12z GFS ensemble member forecasts for the potential storm next weekend. While there is agreement regarding the existence of a low pressure, solutions range from a near-blizzard in New York City to no precipitation north of Florida.

In this case, the model ensembles show a wide range of solutions, which is a good indicator of high uncertainty. Almost all of the ECMWF ensemble members fail to depict any solution resembling the GFS. Even a non-negligible amount of the GFS ensembles disagree with the operational run. It is noteworthy that both a portion of the GFS and ECM ensemble members depict more snow in the tri-state area on Friday with the initial shortwave trough, as a slower trough would support cyclogenesis occurring farther west, closer to the coast, with precipitation closer to affecting the tri-state area.

While model uncertainty is too large to draw a solid conclusion regarding the forecast, given the GFS’ disagreement with the majority of the model guidance and even its own ensemble members, along with its trend towards less separation of the trough over the last several runs, it appears at this time that a possible trend over the next day or two is towards less separation of the ULL and the main flow, resulting in a weaker storm over the southern US over the weekend. In the event that the trend represented on the GFS does not continue, or the ECMWF scenario is incorrect, a likely outcome would be a significant snowstorm somewhere between at least North Carolina and New Jersey, which will have to be monitored closely.

At the same time, Friday will need to be watched for any trends with the coastal low pressure; current odds appear slightly more likely for a west trend than an east trend, although whether this is enough to produce a light-moderate snow event over the tri-state area appears uncertain.


In any event, this cold pattern is unlikely to persist for much longer. As previously noted, the Pacific ridge ultimately retrogrades west, followed by ridge building over the Western US by the late week downstream of a deep trough over the Northeast Pacific. This trough will eventually progress east into the West Coast, resulting in a broad ridge extending throughout much of the East Coast by the second week of January, bringing about at least a temporary resumption of the warmer than normal pattern.

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