Sunshine has become a rare commodity across much of the Northeast. Extended periods of wet, cloudy, foggy, and overall dreary days have dominated our weather pattern across the region since mid-April. Fortunately, the doom and gloom cannot last forever. Signs point to the relentless weather pattern that’s been producing these miserable conditions to let up in time for Memorial Day.
Coastal areas of New York and New England have been under the tightest grasp of spring gloom. Islip, NY on Long Island set a daunting record last week for the longest stretch of rainy days since records began in 1963. A remarkable 18 consecutive days of measurable precipitation (0.01″ or more) was observed at Islip’s MacArthur Airport from April 20 – May 7, 2019. That streak may be extended to the 26 days starting April 12 if trace amounts of precipitation are included. Since the start of April, only April 1, April 4, April 10-11, and May 8-9 were completely free of precipitation. All that rainfall accumulated to 5.95″ in April and 1.47″ in May, through May 11. Similar dismal weather has been observed in the coastal areas of New England.
Curiously, Spring has been relatively pleasant for areas south of New York City and for areas along and west of the Appalachian Mountains. Consider Scranton, PA, where the longest rainy streak of measurable precipitation since April 1 has only been six days, and, where temperatures for the month of May (through May 11) were running +5.8° above normal. This is a drastic difference compared to Islip’s meager +0.1° departure. Further south, the weather has been even more pleasant in Atlantic City, NJ, where the longest rainy streak since April 1 was a mere 2 days, and, where temperatures through May 11 were running +3.3 °F above normal. The city only experienced 16 days of measurable precipitation in the same period during which Islip, just 120 mi to the northeast, experienced 31 days.
Why is that?
The weather pattern across the country over the course of the last month has been dominated by a sub-tropical ridge of high pressure centered over the southern US, extending northward into southern Canada. It has baked the South, the lower Mid-Atlantic, and parts of the Central US with gradually intensifying summer heat, while also pumping copious amounts of moisture from the Gulf of Mexico into the Central US. Since it hasn’t budged has meant wet, low-pressure systems being forced to continuously track around it: first through the Mississippi Valley, then across the Ohio Valley before heading through western New York into Canada.
A ridge of high pressure in the East is generally associated with warm and dry conditions for coastal areas of the Northeast. So why has it been so dreary? As discussed in our Spring Outlook, the contrast between the summer-like heat south of the Mason-Dixon line and the winter-chilled waters of the North Atlantic has made the Northeast susceptible to “back-door” cold fronts originating over the ocean as opposed to land. Blocking high pressure near Greenland and the Canadian maritime has made the region especially susceptible to this cold maritime air mass, and has resulted in nearly perpetual confluence (merging) of the subtropical and polar jet streams over Maine and New Brunswick. Where air converges aloft, it sinks below, creating an enhanced region of high pressure off the coasts of New England and the Canadian maritime.
The stubborn surface high pressure has blocked the northeastward intrusion of subtropical air that would otherwise make frequent appearances under an eastern ridge. The clash of the two air masses created a quasi-stationary frontal boundary, along and north of which warm air upglide has created perpetual clouds over the cool oceanic air. This boundary has also served as an ideal breeding ground for low pressure to develop, hence the frequent rain. This boundary has commonly oscillated back and forth between the Mason-Dixon Line and Southern New England. Therefore, areas further south have spent more time in the warm, subtropical air mass.
The doom and gloom is finally on its way out. The blocking pattern responsible for the confluence of the polar and subtropical jet streams over Maine, the Atlantic maritimes, and their coastal waters is best characterized by the negative phase of the North Atlantic Oscillation (NAO). Its negative phase describes a weak pressure difference anomaly between the Icelandic low and the Azores high. Under such a regime, the polar jet stream is more susceptible to the development of troughs and ridges, which “bend” the polar jet stream in a wave-like fashion.
The graphic below from the Climate Prediction Center plots CFS weekly 500mb height anomalies aggregated for the weeks May 12 – 18 and May 19-25 with initial conditions from May 11. The positive anomalies (warm colors) represent high pressure/upper-level ridging while the negative anomalies represent low pressure/upper-level toughing. Notice that the CFS is forecasting for reduced ridging in the vicinity of Greenland throughout the week of May 19 compared to this upcoming week of May 12. This suggests a gradual increase in the NAO Index.
The probability of expecting warmer than normal conditions as projected by the CFS for the same time periods follows in suit with 60-75% chances in the Northeast through parts of the South-Central US:
The CFS is just one model, but it does have support from Canada’s long-range ensemble mean and roughly half of the members of the GEFS. A time series of the GEFS NAO forecast shows significant spread in forecasts for May 25, which is the end of its range. As a climate model, the CFS is well-calibrated for seasonal and sub-seasonal timescales whereas the GEFS offers limited skill in identifying the pattern beyond Day 8. It is expected that the GEFS mean will gradually trend toward increasing the value of the NAO index as May 25 approaches. Note that a full breakdown of the NAO is not expected until at least the first week of June.
The damp, dark, cloudy, miserable days that much of the Northeast has grown accustomed to over the last moth will gradually dwindle in frequency over the coming weeks. With a subtropical ridge over the southern US expected to continue to be a major player in the weather pattern, an early start to summer may still be in store.