Unseasonably cold air in place across the majority of the Eastern half of the U.S. this past weekend will briefly moderate closer to seasonal temperatures after a very cold start to the day Monday featuring overnight frost conditions deep into the mid South and Southeast. However, a final surge of cold Canadian air forecast to focus more directly into the Midwest and East midweek has models shifting markedly colder (15°-20° below average) for a 1-2 day period Wednesday-Thursday along with some mountain snow in the Northeast. Late week pattern change is certain to effectively end cold air transport into the U.S. while directing less intensely cool Pacific storms back through the West. This will end the extended period of well above average warmth underway west of the Rockies into midweek, but periodic rain-cooled temperatures (mainly 7°-10° below average daytime highs) starting during the latter half of this week (Apr 25-27) will be far from extreme. As the West cools the warmest air yet this year will flood most of the Midwest and East generating near 15° above average temperatures more typical of late May or early June for several consecutive days during the 6-10 day period. Models are highly uncertain with timing, track, and strength of the series of Pacific storms once they eject east of the Rockies. However, rain-cooled Eastern conditions are near certain to remain weak with the 11-15 day period marking the 1^st opportunity for multiple days of temperatures closer to seasonal levels, in an otherwise mild late spring pattern likely to be warmest (relative to average) across the northeast quadrant of the U.S. during most of May.