During the next couple of days, a bubble of very warm air aloft (marked by the white “H” in the image on the right) will replace the cooler, showery upper trough currently over the East (identified by the blue “L” on the left).

Upper-level winds now (left) and forecasted for Saturday (right) from the GFS ensemble mean.  White “H” marks the core of the warm air (ridging) aloft.  Blue “L” marks the showery upper low.  Area hatched outlines thunderstorm-prone region in this pattern.  Blue arrow on the right outlines a strong upper trough.  (NOAA)

The unsettled weather in place right now from the Carolinas to New England will turn much warmer and more humid in time for the Memorial Day weekend.  Highs will blast into the 90s in places such as Kansas City, Chicago, Cincinnati, eventually the D.C. area and even New England.

There is more uncertainty regarding the duration of the warming in the Northeast and Mid-Atlantic, and along the northern boundary of the ridge (hatched area in image above).  In this zone, cooler air to the north may leak southward at times as a “backdoor front” and interrupt summer’s visit.  Big thunderstorms will most likely form in this corridor, owing to its proximity to fast winds aloft and very warm air underneath the ridge.  In general, a region like this along the edge of a hot upper-level ridge is known as the “ring of fire” for its propensity to support long-lived, severe storm clusters, known as “MCCs”, or mesoscale convective complexes.

If that weren’t enough, tropical development in the Eastern Pacific and Caribbean basins will continue to be supported by this pattern, even though former tropical storms Aletta (in the East Pacific last week) and Alberto (off the Carolina Coast now) have effectively disintegrated or will do so very soon.

All of these changes can be linked to global-scale shifts in the tropical circulation.  As discussed in our last post, a large area of disturbed weather is slowly moving over the eastern Pacific and western Caribbean (encircled area below).

Infrared satellite imagery from last night.  Area encircled outlines disturbed weather capable of supporting widespread thunderstorm activity.  (Cooperative Institute for Meteorological Satellite Studies)

And though this large, thunderstorm-rich environment had little to do with Alberto directly, it intimately supported Aletta last week in the east Pacific and continues to nurture newly formed Tropical Storm Bud in roughly the same area.

Bud is now a 40 mph storm.  The experts at the National Hurricane Center (NHC) expect it to become a 90 mph hurricane in a couple of days and move on a course toward the Mexican coast.

Expected track of Tropical Storm Bud. (National Hurricane Center)

A powerful midlatitude trough (shown in the forecast map for Saturday by the blue arrow) will dive into western North America late this week and effectively block Bud from getting anywhere close to the United States.

As we move forward through the last third of the month, the relatively stormy weather in this sector of the tropics will likely slide slowly eastward.  As it does, the chances for development in the Caribbean may increase as well.  And though the odds of tropical cyclone formation there will exceed average for this time of year, this setup by no means guarantees a storm.