AN EARLY START TO SUMMER THIS MAY IN THE WEST

May is usually the month when spring is in full bloom and signs of summer begin to emerge. The beginning of the peak of severe weather season starts in May. Even though most of the colder air has retreated, there is still a risk that strong systems that can generate severe storms as the jet stream has not finished its northward retreat. The length of daylight is nearing its peak for the year in May. Daylight lengths range from 13-15 hours per day, longest in the north. Average highs surge into the 60s and 70s across the North with 80s and some 90s taking hold in the South.

Above normal temperatures were prominent across much of the Central/Eastern US this April, aided by multiple rounds of anomalous warmth occurring across the regions. The initial warm burst occurred across the South and East over the first 3-4 days of April, while the Upper Midwest missed out on this warmup. After a brief cooler interlude, the next warm shot impacted the Central US over the April 10-17 time frame before reaching the East by April 12-18. Temperatures reached as much as 12-25 degrees above normal during its peak, and New York City set a daily record high of 88 degrees on April 15. One more quick shot of modest cool anomalies worked across the Midwest/East entering the final third of April, with a trough along the East Coast keeping temperatures near to slightly below normal through much of the final third of the month. Warmth would return to much of the Central US late in April, particularly across the South Central US. Oscillating temperatures were seen across the West much of April, but readings generally leaned above normal for April due to a warmer overall first ten days. Precipitation in April featured a sharp divide in haves and have-nots. Drought conditions worsened further in April across the Mid-Atlantic and the Southeast as well below normal rainfall fell. This contrasted with a much wetter pattern this April across the Great Lakes/Upper MS Valley towards eastern Nebraska and the Southern Plains. In the West, the storm track for April favored increased rain and mountain snow from Northern/Central California through the Northern Rockies, with a dry slot extending from the Four Corners towards the Northern Plains. After an active first three days of April, severe weather risks became more limited for the first third of April. The pattern turned much more active between April 13-17, with multiple tornado outbreaks during that period across the Central US. Additional rounds of severe weather impacted the Central/Southern Plains through the Ohio Valley over April 23-28.

A ridge near Greenland will keep a trough trapped across much of eastern North America during the first ten days of May. This will produce cooler than normal temperatures across most of the Central/Eastern US over the first 10-12 days of the month, interrupted only by a brief 2-3 day warmup working across the Central/Southern Plains and the East in the middle of this period. A ridge will allow for above normal temperatures to be prominent across the Northwest during the first week of May. This ridge should expand across the remainder of the West moving into week two, causing the core of the warmest anomalies to slide away from the Pacific Northwest and settle across the Interior West. The -NAO ridge near Greenland should break down over the course of week two, allowing for temperatures to moderate across the Plains and the South. The cooler airmass will be slower to fizzle out across the Upper Midwest and the Northeast, particularly near the Great Lakes. This general pattern should persist into the third week of May, with the warmest anomalies impacting the Interior West, modest cool anomalies favoring the Upper Mississippi Valley through the Interior Northeast, and more seasonable temperatures across the Plains and the South. MJO-related tropical forcing should become more amplified during the final 7-10 days of May over the Pacific Ocean. This should help amplify ridging across the West and troughing in the East. This may result in an early start to summer heat across the West, while cool risks will increase once again across the Midwest and the East that may extend all the way through the Gulf Coast.

Rain and storm chances over the first 2-3 days of May will favor Texas and the Gulf Coast. A more organized cold front during the latter half of week one will bring additional rain and storm chances from the Arklatex Region through the OH/TN Valleys and into the East. On the cooler side of this system, some light snow could mix in with the rain across portions of the Great Lakes and higher terrain of the Northeast. Activity should focus back across Texas and the Gulf Coast by the start of week two as the trough in the East strengthens behind this front, while not much more than light and spotty showery activity is likely across the Midwest and the Northeast. Aside from areas near the Gulf Coast, severe weather risks will be below normal during this period. The storm track should shift a bit further north during the latter half of week two and into week three, with a system or two expected to sweep across the Central Plains towards the Mid-Atlantic. In addition to increased rainfall, this may allow for the severe weather threat to increase from the Central Plains and North Texas towards the Mid-Atlantic. The Gulf Coast should see a reduction in rain/storms in mid-May. Rain chances should remain limited across the Midwest during this mid-May period. As a trough should become more amplified during the final third of May, the favored storm track should shift back towards Texas and the Southeast, with a particular focus near the Gulf Coast, before sliding up the East Coast. Below average rainfall is expected across the Northern/Central Plains through the Upper Midwest to end May. Below normal precipitation is likely across the Northwest during the first half of May and should generally continue into the second half of the month. Systems could undercut the ridge near the Northwest during the first week of May, which could provide a few opportunities for rain and mountain snow across California and the Great Basin. Chances for these systems impacting the Southwest should gradually decrease moving into the middle and later stages of the month.

Risks for tropical development should be slightly below normal for May. There will be periods of reduced wind shear over the Gulf during the second half of May, along with generally above normal sea surface temperatures. However, tropical forcing should tend to be more unfavorable during the second half of the month across the Atlantic Basin. If there is a brief window where forcing could be slightly more favorable, it would be centered around the third week of May. Due to this, if a weak system were to form, sometime during this third week of May in the Gulf or off the coast of Florida would be the best opportunity. Even during this time frame, it is more likely that a tropical system does not form. The most likely outcome is that the first storm of the season holds off from forming until sometime during the first half of June.