Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 341 AM EDT Sun Sep 08 2013 Valid 12Z Sun Sep 08 2013 - 12Z Tue Sep 10 2013 ...Concerns for flash flooding and severe weather continue across the Northern High Plains and Upper Midwest the next couple of days... ...Monsoonal moisture will promote additional shower and thunderstorm activity over the Desert Southwest and into the Rockies... ...A new surge of very cool air arrives across the Great Lakes into the Northeast while heat lingers across the South... A slow-moving upper low over the Pacific Northwest is expected to gradually weaken and shift eastward toward the Northern Plains and Upper Midwest over the next couple of days. Already this is focusing numerous showers and thunderstorms across especially portions of the Northern Rockies and into the Dakotas, and this will be the theme for the remainder of the weekend and into the start of the new week with the expectation of multiple clusters of heavy showers and thunderstorms impacting the Northern Plains and gradually advancing across the Upper Midwest Monday and Tuesday as low pressure lifts toward the region. Localized threats of severe weather are expected, and sufficiently heavy rainfall is possible that there could be some areas of flash flooding given the relatively slow movement of the system. Farther southwest back through the Central Great Basin and down into the Desert Southwest, monsoonal moisture has been on the increase again, and this moisture will be rather well entrenched for the next few days. Consequently, expect there to be scattered areas of additional showers and thunderstorms on a daily basis, with the more concentrated activity expected to be over the the higher terrain of the Southwest. Given the rather anomalous extent of moisture, there will be locally heavy rainfall amounts and at least a localized threat for flash flooding. The highest risk for excessive amounts is expected to extend from Arizona northeastward up into southeastern Utah and western Colorado. Meanwhile, the expansive upper ridge of high pressure which has been persistently centered over the Central U.S. over the last week, will begin to weaken somewhat over the next few days, and this will allow for some break in the heat across the South, but overall conditions should remain very warm to hot, along with a continuation of dry conditions. Elsewhere, a cold front will sweep through the Ohio Valley, Mid-Atlantic and Northeast on Sunday and allow a reinforcing shot of very cool air to settle southeastward from Canada. A few showers and thunderstorms will be possible with the front, but most areas should stay dry. Cool high pressure should remain across the Northeast until Monday when it will then push off the East Coast. That will then allow for a warming trend again, initially across the Midwest and Ohio Valley on Monday, but then into the Mid-Atlantic and Northeast on Tuesday. Orrison Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_wbg.php