JUNE HEAT WILL CONTINUE TO SIZZLE IN THE NORTHWEST

June is the first month of meteorological summer. This means more of the summer heat will emerge across at least portions of the country. Almost all the cooler spring air has retreated northward. June features the longest daylight lengths of the year. Daylight lengths range from 13 hours across the South to 14 to 16 hours in the North. The peak in the severe weather season begins its decline in June as the jet stream retreats even further north. Organized severe weather events transition to smaller scale complexes for the most part, especially as we progress into late June. Average highs are generally in the 70s and 80s in the North. Average highs climb into the 80s and 90s in the South with some 100s in the Desert Southwest.

The pattern during the first half of May was largely driven by a ridge over the Pacific Northwest/western Canada and a trough stationed over central/eastern Canada that would often extend into the eastern half of the US. This provided above to well above normal temperatures to much of the West during the first two weeks of May, with the warmest anomalies favoring the Pacific Northwest during the first week before sliding across the Interior West by week two. Meanwhile, outside of a brief warmup or two, temperatures leaned below normal across much of the East and into the Midwest/South Central US these first two weeks of May. This cooler airmass struggled to impact Florida due to a stubborn ridge near the state. A more changeable pattern emerged during the second half of May. Warmth exited the West during week three and shifted across the Central/Eastern US, with cooler air filling in across the West in its wake. By the start of the fourth week of May, the pattern reverted to being warmer in the West and a bit cooler in the Northeast/Central US. May would end with a round of anomalous warmth developing across the Upper Midwest, along with a more modest warmup in the East. A trough brought cooler readings to the Southwest over the final few days of May. Drought expansion across the South remained a concern during May, especially through the first 20 days or so. A wetter, more active pattern that did begin to form in April and continued into early May across the Southern Plains did eventually spread through the Southeast, mainly over the final third of the month. This wet end to May did eat into some of the rainfall deficits impacting the South, but for most areas it was not enough to erase them. With the more active conditions in the South, below normal precipitation was prevalent across the West and the North Central US this May. The Northeast was more of a mixed bag, with much of the Northeast Urban Corridor seeing slightly below normal rainfall in May but pockets of the Northeast near the Canadian border did see a slightly wetter than normal May.

Tropical forcing should move across the Pacific Ocean towards the Americas during the first week of June. This should allow for ridging to take hold across western Canada and into the western US, leading to above normal temperatures overhead that will extend into the Midwest. A trough will initially keep a cooler airmass in place in the East, but as tropical forcing shifts into the Americas, the cooler pattern will give way to above normal temperatures in the Northeast as week one progresses. This overall pattern should linger through week two, with the most anomalous warmth impacting the Interior Northwest through the Northern High Plains. Texas through the Southeast will experience near to slightly below normal temperatures during the first half of June, aided by an increased coverage of clouds and rain/storms. With an El Nino base state becoming more firmly established, tropical forcing during the second half of June should be most prominent over the Pacific Ocean. This would favor the ridge being maintained across western Canada and towards the Interior West. Summer heat would continue underneath this ridge, especially across the Interior Northwest and the Northern Rockies. However, a trough will attempt to undercut this ridge near Baja California. While warm anomalies are favored in the Desert Southwest, this should allow the region to avoid the most oppressive heat. Temperatures should generally remain above normal across the Midwest/Northeast during the third week of June before this warmth should trend closer to seasonable overall late in the month. The seasonable start to June across the South should carry over into the third week of June. There are signs that the Southeast ridge may begin to pop up over the final 7-10 days, and this would provide Texas and the Southeast with the best opportunity for seeing an increase in summer heat this month.

The persistent MJO-related tropical forcing over the Pacific Ocean should be supportive of a wet and active pattern during the first 2-3 weeks of June across the Central/Southern High Plains towards the Southeast. The highest threat for severe storms will generally favor the Central/Southern Plains from western Nebraska to western Texas. While strong storms cannot be ruled out in the Southeast during this period, it will be more common for these to be more garden-variety storms. For the South, locally heavy rainfall leading to flash flooding is more likely to be the bigger concern. With the storm track suppressed to the south, this should lead to below normal precipitation to be favored from the Northwest through the Upper Mississippi Valley and the Great Lakes over the first 20 days of June. These drier than normal conditions will generally extend through the Northeast, though moisture from the Southeast may be driven northward into the region at times and a few areas seeing regionally wet conditions will be possible. By the final ten days of June, the storm track should begin to shift northward if the Southeast ridge develops as anticipated. The favored storm track would be across the Central Plains through the Ohio Valley, but the Midwest and the Northeast should see a modest uptick in rainfall chances as well. The pattern should trend drier from Central/Eastern Texas through the Southeast as this occurs. The summer monsoon should keep an increased coverage of showers and storms across the Four Corners late in June, and some of this activity will seep into the Central/Southern High Plains on occasion.

There is a small window during the first five days of June where some there is some risk for tropical development in the eastern Gulf. Tropical forcing will be more favorable across the Americas, which could amplify moisture associated with the Central American Gyre to begin June, and this may coincide with a slight decrease in wind shear over the Gulf. There will still be enough shear where if anything forms, it is likely to remain weak. The main threat would likely be heavy rainfall near Florida and portions of the Southeast Coast. Tropical risks should decrease late in week one and into week two as shear increases over the Gulf once again and tropical forcing mostly focuses over the Pacific basin. Another window that could carry the potential for tropical development is during the third week of June, as some tropical forcing should return to the Americas and wind shear over the Gulf may ease. It is possible a system drifts away from the Central American Gyre towards the eastern Gulf during this time frame. Similar to the first week of June, should anything form in the eastern Gulf it is likely to be weak due to some lingering wind shear and should primarily be a rainfall concern. Tropical risks should fall slightly below normal to end the month.