Multiple rounds of locally heavy rain with the risk for flash flooding,as well as severe thunderstorms, expected in the north-central U.S....Late-season wet snow for the northern Rockies beginning on Monday....A heat wave will quickly spread from the Plains Sunday into the GreatLakes/Upper Ohio Valley on MondayA plume of tropical moisture will bring the threat for heavy downpoursand flash flooding to the central Gulf Coast...Some severe storms will  be possible Saturday evening ahead of the cold front over the northern High Plains where the Storm Prediction Center has outlined a Slight Risk (level 2/5) for the threat of very large hail, significant damaging winds, & a few tornadoes. The front will progress southeastward through the day Sunday before stalling out Sunday evening, with the next round of storms expected along an axis from the Upper Mississippi Valley southwestward into the Upper Missouri Valley. A Slight Risk of Excessive Rainfall is in effect as clustering of storms with repetitive motions along the stalling front with abundant moisture available will lead to heavy rainfall rates/totals & the risk for flash flooding. A Slight Risk of severe weather is also in place for the threat of some large hail. Lee cyclogenesis over the central High Plains ahead of another upper-level wave over the West will bring strengthening southerly flow, with the stalled front expected to begin slowly lifting back to the north as a warm front through the Upper Mississippi Valley on Monday. A Moderate Risk of Excessive Rainfall (level 3/4) has been introduced as clustering/slow-moving storms along the boundary as well as still abundant moisture & impressive upper-level dynamics will promote more widespread, intense downpours & potentially numerous, significant incidents of flash flooding. This risk will continue into Monday evening/night, just beyond the current forecast period. A Slight Risk for the threat of some large hail & damaging winds is also in place.   Another impact from this unseasonably strong series of upper-level systems for mid-June will be much colder air sweeping southward to the west of the cold front, particularly into the northern Rockies. High temperatures will be mostly in the 60s Sunday from the Pacific Northwest east through the northern Great Basin & into the northern Rockies. These cooler temperatures will spread further southward in the Great Basin & into the northern Plains by Monday. Highs for the mountain valleys of the northern Rockies will only be in the 40s & 50s. Precipitation chances will also pick up here Monday, with a Winter Storm Watch in effect for the potential of several inches of snow at higher mountain elevations. While a mostly cold rain is expected at lower elevations, a few of the higher mountain valleys may see some snow mix in.   To the south, an impressive flow of moisture from the Caribbean on the western side of the ridge will stream into the central Gulf Coast over the next couple of days.