Meteorological Summer, the quarter of the year with the warmest average temperatures, officially begins June 1 but for the Northeast it will start off feeling more like spring than summer. After a warm Friday, a stubborn blocking pattern will develop this weekend that will persist through at least the first week of June. Air masses from both interior and Atlantic Canada will keep temperatures cool, but brief periods of seasonal temperatures are expected.
Back-door cold fronts interrupting pleasant summery weather with clouds and unseasonably cool weather have become a common pattern in the month of May. Unfortunately, there is no indication this pattern will cease any time soon. The ridge of high pressure centered over Northern Mexico and southern Texas bringing record heat to the central U.S. will continue to dominate the upper-level pattern for the duration of the extended period. Its intensity will prevent the progression of troughs over the West Coast of the U.S. from progressing further eastward into North America. Consequentially, the series of upper-level lows that approach the west coast of the U.S. will weaken and be forced to ride over the ridge into Canada before reintensifying as they are directed towards the Northeast, bringing cool Canadian air with them.
Most long-range models are in consensus that this persistent pattern of a heat dome over the central U.S. with adjacent troughs of cooler air on either coast will continue throughout the extended period. One common metric used to confirm this pattern is the North Atlantic Oscillation Index. The NAO Index is defined as either the sea level pressure difference between the Icelandic Low and the Aleutian High or the 500-mb height difference between these same systems. The higher the pressure difference, the more straight the flow of the jet stream. The lower the pressure difference, the more susceptible the jet stream is to developing troughs and ridges.
The following graphic from NOAA’s Climate Prediction center displays the mean NAO index forecast from the MRF ensemble for the next 7-14 days. There is general agreement among its members that the NAO Index will become negative in early June, suggesting that cool air from Canada will be able to penetrate into the Eastern U.S. The NAO was positive throughout the month of May, hence the northeast, especially away from the coast, experienced prolonged periods of warmer than normal temperatures.
Below are the 500mb heights and height anomalies in decameters and 2 meter temperature anomalies in degrees Fahrenheit from the 06 UTC May 29 GFS, courtesy of Tropical Tidbits for the afternoon of Monday, June 4. Lower than normal heights of the 500mb surface are associated with colder than normal temperatures. The most anomalous cool temperatures for next Monday are evident over the Northern Mid-Atlantic southward through the Carolinas, where the GFS depicts temperatures of up to 16 degrees below normal. Normal high temperatures range from the mid-70s in central Pennsylvania to the mid-80s in the Carolinas.
We are not expecting every day over the extended period to remain this cool. There will be brief breaks of normal, to slightly above normal temperatures as subtle changes in the upper-level flow bring southerly winds to the region.
In addition to cool temperatures, this upcoming pattern also favors locally unsettled conditions wherever cold frontal boundaries become established. Clouds, showers and thunderstorms will develop with the passage of cold fronts and will persist wherever these boundaries set up as waves of low pressure ride along them. Away from frontal boundaries, pleasant conditions will ensue. We will keep you updated about where and when precipitation is expected and when summer makes a more persistent appearance as the pattern progresses.