Temperature-departure from average (F) expected Wednesday from the GFS ensemble mean (Penn State)

Just yesterday, mid-afternoon temperatures were only in the low 50s in Boston with showers and a northeast wind off the cold Atlantic gusting to 30 mph. It’s not hard to imagine June weather worse than that. Though it gets a little better as the week moves on, forecasts from the GFS ensemble-mean, as shown above for midweek, still keep high temperatures in Washington, D.C. only in the 70s through Thursday.

Part of the reason why it’s so cool in the East is because one of the main branches of the jet stream over the Western Hemisphere has recently contorted itself into a tangled mess of high-altitude winds.

The large north-south excursions these jet stream winds are currently taking (shown by the yellow arrow in the picture below on the left) are intimately tied to the displacement of huge bubbles of warm air (red shading) to Polar latitudes and exceptionally cool air (blue shading) over the eastern U.S.

Upper level winds and temperature anomalies valid today (left) and next week (right). (Penn State)

As is often the case this time of year, this pattern will support an enhanced risk for organized convection (thunderstorm complexes) underneath the southwesterly jet stream over the United States. In this region (hatched in the image above on the right), strong wind shear and an abundant supply of potential energy will be available for thunderstorms to tap. Severe weather may come into play in this part of the country on multiple occasions during the next couple of weeks.

Cooler air will likely slide southward underneath the jet stream and temporarily interrupt the summer-like weather in the Northern Tier of the United States from time to time once the new pattern arrives. Wiggles in the upper flow will occasionally allow brief, glancing blows of modified Canadian air to penetrate into the Lower 48.

Still, the overall setup suggests warmer-than-average conditions will be the dominant theme east of the Rockies for the final two-thirds of the month, while cooler than average weather will settle in to the West Coast. This would mean that places in a large swath from Kansas City to DC will once again start racking up some 90-degree days (for reference, Kansas City had sixty two 90-degree days last year with a maximum of 109°F, while DCA had fifty with a max of 104°F).

In the global picture, changes in tropical weather that will accompany this regime shift are going to relocate the risk for tropical-cyclone gen­esis (development).

Right now, the best place for hurricane development in the Northern Hemisphere is in the west Pacific. During the last couple of days, typhoon Mawar –currently a category 1 storm with 80 mph winds- emerged from a large area of stormy weather in this part of the world (area encircled in the image below).

Infrared satellite picture from late yesterday. (UW-CIMSS)

In association with the upcoming MJO movement, areas shaded in red are expected to have less thunderstorm activity than normal, while areas in blue likewise more. (CPC)