The pitter-patter of rain drops on gutters across the Northeast Sunday afternoon signaled the end of winter across the majority of the East Coast. A surge of mild air spreading northeastward will bring temperatures of at least 50°F as far north as New York City Monday. Temperatures are expected to be slightly cooler Tuesday and Wednesday, but they will only be a few degrees cooler than average ahead of a more substantial warmup late week that . This pattern of frequent temperature swings is quintessential of spring and is expected to continue indefinitely. But as promised near the end of February, snow and extreme cold are not expected to be associated with the intermittent temperature downswings.
There is abundant support suggesting winter’s demise besides the increasing sun angle. First and foremost, the Arctic Oscillation (AO), the climate mode that describes the intensity of the circulation in the arctic, is in its positive phase. This means that the polar vortex and the circulation about it is stronger than average. A strong polar vortex can deflect most perturbations approaching it, inhibiting the southward penetration of cold air. It is expected to remain in its positive phase for at least the next several weeks.
The positive AO generally requires any new cold outbreaks to originate entirely in the mid-latitudes. With an abundant snow pack in western North America and Canada, a strong northerly or northwesterly flow could seamlessly generate an East Coast-bound cold airmass before the increasing sun angle and growing warmth melts the snow. This will prove difficult to realize considering the positive North Atlantic Oscillation (NAO) and the continuously active subtropical Pacific jet stream.
The NAO is the climate mode that describes the mean sea level pressure difference between the polar Icelandic low and the subtropical Azores High. The “Icelandic Low” is related to the Polar Vortex and the Azores are an archipelago centered in the subtropical Atlantic roughly 1000 miles from Portugal. A close cousin of the AO, the NAO describes the storm track of the North Atlantic Ocean. In its positive phase, the pressure difference between the Icelandic Low and Azores High are greater than average. This is evident in the 500 mb GEFS anomaly plot below for 12 UTC March 22 courtesy of PivotalWeather, in which there is anomalously strong ridging near the Iberian Peninsula and anomalously deep troughing near Iceland forecast for the week of March 20. This implies a west-to-east polar jet stream and does not support a blocking ridge of high pressure near Greenland.
A Greenland block would result in a meridional northwest-to-southeast flow that could intensify cold air reservoirs in Canada. This pooling of cold air can spread across the Northeast and result in storm tracks that favor East Coast snowstorms. But in its absence, the storm track will be dominated by the active subtropical Pacific jet stream, which will tend to carry storms and airmasses from the warm Gulf of Mexico to the Great Lakes before cutting east upon confluence with the east-west polar jet stream.
That spring has arrived and snow chances are low does not necessarily imply that a period of persistent warmth is imminent. Ridging related to El-Nino’s increased thunderstorm activity over the central Pacific will spread to the Western United States this month as the Pacific/North American teleconnection pattern (PNA) enters its positive phase this weak, peaking round March 20 before subsiding. This favors above normal temperatures and below normal precipitation in the Western US from strong ridging and below normal temperatures in the Eastern US due to persistent or frequent troughs.
The positive regime of the PNA implies there may be frequent troughs over the Eastern US but actually says little about the intensity or availability of cold air. The 00 UTC March 10 CFS, courtesy of PivotalWeather, demonstrates a more detailed pattern less smoothed out by ensemble members than the GEFS for 12 UTC March 22. As depicted below, it is quite plausible for there to be a strong ridge in the western US without an accompanying trough feeding on air of arctic origin over the East Coast. The positive phase of the NAO as forecast will help counteract the downstream impacts from the PNA ridge. Temperatures are expected to be below normal to be sure, but they likely will tumble short of the caliber of the arctic blasts that resulted in accumulating snow along the I-95 corridor near Easter 2018.
As previously stated, the stronger than average polar vortex defined by the AO’s positive phase and the zonal west-to-east flow defined by the NAO’s positive phase restricts the southward intrusion of polar air into North America. Therefore the height anomalies of the rather dampened troughs projected by the CFS are quite meager. The progressive nature of the pacific jet stream and lack of Greenland blocking will seldom permit intensification of troughs sufficient for intense snow and cold in coastal areas. These passing troughs may bring snow to the Midwest and interior portions of the Northeast but will likely fall short of extending the snow to coastal areas or the interior South. Additionally, the cold induced by the snowpack in Western North America will generally be off-limits to the East Coast as the PNA ridge will inhibit storm progression across them.
There are signals that the PNA ridge will eventually weaken in late March as the Madden Julien Oscillation (MJO), a complex of thunderstorms circumnavigating the tropics, reintensifies over the Indian Ocean. The feedback of the MJO when centered over the Indian Ocean is correlated with warmer than normal conditions over the Eastern US.
Spring has finally arrived for the East Coast of the US but at least one more period of below-normal temperatures is expected before April. A series of troughs may bring a chance for snow for interior portions of the Northeast and Midwest for roughly one week after March 20. But coastal areas may have to wait for the 2019-2020 winter season for the next accumulating snow event.