30
Day

Early stages of a blocky slow to change pattern across the U.S. developing late last present weekend prompted substantial model shifts in May forecasts.  Forecast changes begin late this workweek (May 8-9) in both the Northeast (cooler) and West (hotter) after a pair wet and cool closed lows over the Ohio Valley and Southwest exit off the East Coast and weaken over the Southeast respectively. Instead of expanding a summerlike ridge currently sandwiched in between over the northern Plains and Upper Midwest eastward into the Northeast as most models forecast last week, all latest runs retrograde the core of unseasonably strong ridging westward generating several consecutive days of the hottest temperatures of the year (mid 100s) across the Southwest (11°-15° above average) into the middle of the 2^nd full week of May. In response to increased pattern amplitude  models converged to colder downstream solutions as an unseasonably cold Canadian closed low settles directly into the Northeast cooling  temperatures 10°-15° below average into next weekend (May 10-11), while widespread rain-cooled conditions focus across he Southeast .  Longer range forecasts valid the 11-15 day period are less aligned but deterministic models which recently performed best limit magnitude and duration of seasonably warm air reaching the Northeast early in the period ahead of yet another multiday cold air outbreak going into the 3^rd full week of the month (May 18-19). Repetitive cool Canadian air reinforcement into the Northeast through late spring along with wetter forecasts across the Southeast  set the stage for a relatively mild start to June across the majority of the Midwest and East (seasonal – slightly above average). In contrast expansive drought currently stretching from the Southwest to the northern-central Plains is unlikely to fully erode adding confidence to a hot start to summer across most of the western half of the U.S.