The winter of 2013-14 has had two recurring themes; brief yet very strong cold outbreaks, and short range surprises with storm systems. The latter yet again appears to be the case, as a low pressure on Tuesday that was previously expected to remain mostly offshore is now anticipated to track close enough to produce a widespread moderate snowfall across parts of the region, including the area. A more prolonged round of arctic cold then returns, as except for next weekend, highs will remain in the 10s and 20s for the next 8-10 days.
Today – Tuesday: Moderate Snow, Wind, Cold Expected
After a temporary pattern relaxation over the last 1-2 weeks as the frigid temperatures in Canada retreated northward, a wintry pattern is once again setting up as strong ridging persists over the northeast Pacific and western US, while a strong upper level low shifts southeast into the US/Canada border, dragging another cold air mass with it into the region. As it does so, a shortwave will round the base of the trough, forcing the development of a low pressure near the lower Mid Atlantic, which then intensifies and tracks with a more northerly component into Nova Scotia as the trough axis gains a more neutral and later on negative tilt. As of yesterday, this was forecast to remain mostly offshore, with scattered snow showers the main impact. Over the last 24 hours, however, several key changes have occurred with the model guidance that will lead to a more significant snowstorm affecting the region.
Model Analysis: Throughout the last set of model runs, several relatively small-scale changes have occurred in the upper levels which have resulted in more significant changes near the surface. A common theme this winter has been for models to underestimate shortwaves entering the west coast of North America, and this has been gradually corrected with time over the recent runs. Continuing onward as the shortwave reaches the central US, the trend has been for a stronger and sharper shortwave and vorticity max, which supports a further west and stronger surface low pressure and leads to more precipitation over the region. Each model run has trended slightly northwest of the previous run, continuing to support more snow; at this time, the CMC is the wettest model run with a little over 0.60″ of precipitation, while the ECM is among the drier models, although it has occasionally displayed a dry bias this winter and has taken longer than the other models to catch up with the trend, and is currently considered a weak and dry outlier.
Considering that the heaviest precipitation is expected to remain offshore and that the flow remains progressive, the system is generally expected to be a fast mover, with at least a 12-hour window for snow, although at least moderate snow rates are expected during this time period, possibly heavy at times southeast of the I-95 corridor. Snow to liquid ratios are expected to be higher than the standard 10:1, given a relatively thick dentritic growth zone (DGZ) along a saturated column extending up to 600mb and strong lifting with high upward vertical velocity peaking around 700mb, although wind is expected to be a limiting factor for some, especially near the coast where a NNE wind of 15-25 mph is expected, which is likely to support ratios near 12:1 to 15:1. Given the factors above, I am currently siding with precipitation totals ranging from 0.35″ in northwestern areas to 0.55″ near NYC and 0.75″ in eastern locations, which would support snow accumulations of at least 3-6″ inland, 5-9″ in the immediate NYC area, and 7-11″ in eastern parts of the area.
Forecast for NYC Area: Increasing clouds are expected tonight with Tuesday’s highs peaking at midnight in the mid 20s inland and upper 20s-low 30s elsewhere, with daytime highs ranging from the mid-upper 10s inland to the low-mid 20s near the coast. Snow showers are expected to begin developing after 11am-1pm mainly west of NYC, with steadier snow developing after 4-6pm. Widespread moderate snow is expected to cover the area during the evening and early overnight hours, possibly falling heavily at times southeast of NYC, before winding down between 2-5am for most locations except for eastern Long Island/CT, where snow may persist until 6-7am. As the low will steadily deepen as it tracks near the 40N/70W benchmark, a tightening pressure gradient will lead to strengthening northerly winds throughout the storm, reaching 10-15 mph for most locations and up to 20-25 mph near the coast; the winds will also help to drain cold temperatures from northern New England into the area, with temperatures steady in the mid-upper 10s for most of the storm before falling into the single digits inland and low 10s near NYC and the coast by the end, leading to wind chills in the 0 to -10 degree range by Wednesday morning.
At this time, snow totals of at least 3-6 inches in northwest NJ/SE NY, 5-9 inches in the immediate NYC area and southern CT, and 7-11 inches in Long Island are forecast. As the model guidance continues to slightly change around with the specific details, and considering that yesterday morning there were little to no indications even supporting a widespread moderate snowfall, there is some uncertainty in the forecast regarding the inland extent of the moderate snowfall and how heavy precipitation falls for coastal locations, and the outlook is subject to some changes. Some of the latest runs have indicated that the heaviest precipitation sets up near the immediate NYC area as opposed to Long Island, which would lead to increased snow totals in NYC, which will continue to be monitored as a possibility. Another forecast update will be posted this evening, and frequent storm updates are currently scheduled to be posted beginning 3pm on Tuesday.
Wednesday – Friday: Cold Week Ahead
The pattern relaxation over the last 1-2 weeks with temperatures returning into the 40s and 50s will end up just as a temporary break from the winter pattern, as the strong upper level low shifting into the US/Canada border along with Tuesday’s snowstorm will help to pull another cold air mass into the region; 850mb temperatures will not be as cold as previously anticipated due to Tuesday’s storm, although they will remain colder than average, reaching the -16C to -20C range for Wednesday and Thursday. Wednesday will be the coldest day, with a persistent breezy northwest wind at 10-20 mph behind Tuesday’s low pressure, partly sunny skies and highs ranging from the mid 10s inland to the upper 10s-low 20s near NYC and the coast. Wind chill values especially in the morning are expected to fall into the 0 to -10 degree range.
As the elongated upper level low swings through on Thursday into Thursday night, a weakening frontal boundary will move through the area, paving the way for the remainder of the cold air mass to spill into the rest of the region overnight into Friday as a strong high pressure builds into the north central US. Mostly cloudy skies are expected for Thursday with isolated snow showers possible and highs ranging from the upper 10s inland to the mid 20s near NYC and the coast. Following the passage of the weakening front, overnight lows are expected to fall into the single digits again north and west of NYC, with NYC and the coast reaching the low to mid 10s. Partly sunny skies and lighter winds are expected for Friday with highs reaching the low to mid 20s for most of the area.
Saturday – Next Week: Warmer Weekend, Then Another Arctic Cold Outbreak
This winter’s tendency for brief yet strong cold outbreak continues into the medium range, as indications point to another cold outbreak early next week. Ridging in the northeast Pacific and western US, which has been persistent over the last week, will build further north as a train of storms affects the western Aleutian islands in Alaska, and for a short period of time will cut off from the main flow as an upper level low reaches California, acting as a rex block. Along with this development, a piece of the polar vortex currently north of Canada will gradually shift southward into Canada before reaching southeast Canada by the late weekend into early next week, with 500mb heights near 480dm likely. This will drag frigid temperatures in Canada once again close to the US/Canada border, with some of the extreme cold likely to enter the northern US. Several aspects make this different from the early January cold outbreak, however; in that outbreak, a weak block was present over central Canada, which along with a low pressure developing over the central US aided in a significant southward displacement of the core of the cold air mass into the Midwest and Great Lakes, where 850mb temperatures fell to near -30C to -35C. In this case, there is no blocking or ridging to be found over eastern or central Canada, which is likely to keep the core of the cold air mass north of the US in this case, preventing the cold from reaching the significant southward extent that early January featured. While there remains some uncertainty with the exact setup and southward extent of the frigid temperatures, confidence is increasing for another round of well below average temperatures.
As a low pressure develops and rapidly deepens near southern Canada on Saturday associated with the approaching upper level low, a strengthening southwest wind is expected for the area, which will gradually remove the low level cold air with highs expected to reach the upper 20s to low 30s, along with a risk of scattered snow showers. The timing of the upper level low will determine how warm Sunday gets; at this time, I am siding with a slower push of cold with highs on Sunday able to reach the mid 30s-low 40s, although this could significantly change should the cold air mass enter faster than currently expected. As previously noted, there remains some uncertainty with the southward extent of the cold air, although at this time the type of air mass appears conductive to support sub-zero lows inland and single digits elsewhere, with highs in the 10s to possibly low 20s likely. As this is still a week away, the outlook is subject to change for this time frame. The cold is then expected to moderate heading into the middle of next week when the next chance for snow and/or rain may exist.
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Sunday, January 19 Observations:
A colder air mass returned for Sunday, with partial sunshine and high temperatures cooling down into the low to mid 30s, returning to near average for the first time since January 10 for most of the area. Temperatures generally peaked late in the day, however, with increasing cloud cover overnight.
The highest temperature was 39 degrees in Westhampton Beach, NY, and the coolest high temperature was 31 degrees in Sussex and Andover, NJ.