Residents of the Northeast awaiting the arrival of consistent, dry summer weather and residents of the Northern Plains who are already awaiting its departure can finally rejoice. The stubborn synoptic pattern that has resulted in weeks of heavy rain in parts of the Mid-Atlantic, cool weather in New-England, and record heat in the Plains will finally undergo changes later this month.
The pattern in the East will be driven by a return to the positive regime of the North Atlantic Oscillation (NAO). A positive NAO index represents a positive anomalous surface pressure or 500 hPa height surface difference between Iceland and the Azores. Under this regime, the jet stream shifts further north, favoring the westward shift and expansion of the Bermuda High, which will act to pump seasonal and some above-normal temperatures into the Northeast. With an active northern jet-stream continuing further north, New Englanders still need to look out for back-door cold fronts that can send temperatures below-normal territory for a couple of days. But overall, seasonal temperatures in the upper 70s to mid 80s are expected, with the possibility for a few days in the low 90s east of the Appalachians.
In the South, a similar pattern that resulted in several weeks of widespread afternoon downpours in May during the previous positive phase of the NAO will return. The heat dome over the Southern Plains and Northern Mexico is expected to weaken but not dissipate. The converging flows of these adjacent systems of high pressure could result in the formation of another upper-level low pressure system over the Gulf of Mexico, which would provide a consistent source of tropical moisture into the South for widespread afternoon showers and thunderstorms. These conditions could potentially result in the formation of another Alberto-like tropical or sub-tropical cyclone in the Gulf of Mexico.
Meanwhile, the Southwest and Southern Plains will continue to confront the heat, but the consecutive days of scorching temperatures above 100 degrees will finally subside. As the dome of high pressure situated over the region since early May is subdued, its center will not be as stationary as it has been. There will still be some days at or above 100 degrees, but most days will be in the low to upper 90s, with the core of the heat focused over smaller areas.
The Upper Midwest and the Northern and Central Plains will enjoy a remission from the heat with the exception of a few days when the ridge over the Southern Plains briefly expands northward. A series of troughs from Canada and the Northern Pacific will ensure that heat that reaches up into the north will not last. High temperatures will generally range from the low 70s to low 80s in the Upper Midwest and Northern Plains, and from the mid 70s to upper 80s in the Central Plains. But while these troughs will keep this region cool, the presence of the heat dome to the south will ensure that the reinforcing shots of cold air will bring chances for thunderstorms with them.
As the heat dome shifts around, the Rocky Mountains and Pacific Northwest may experience warmer than normal temperatures at times throughout the second half of June. With the exception of the Northern Rockies, which will be caught in the parade of troughs of low pressure, the second half of June should be mainly dry. Even the rain forests of northwestern Washington could catch a break from the rain. Troughs will be more transient, allowing the Pacific Northwest to bask in warmth and sunshine in between systems. This is especially true east of the Cascades in Washington and Oregon, where more frequent days of downlope flow could permit temperatures to warm into the 90s.
California may not be as lucky as other areas in the West. The transient nature of the jet stream in the West may result in a cut-off low pressure system developing over California, potentially resulting in days of fog and drizzle between San Francisco and Los Angeles. This will accentuate the typical pattern of “June Gloom” if it were to develop. In coastal California, June Gloom refers to the fog and low stratus that persists through the early afternoon due to the pressure gradient between the cool Pacific and hot slopes of the mountains. Cold, moist air rushes eastward and cools the hot air sitting over land, causing moisture to condense into fog and clouds.