Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 343 AM EDT Mon Jun 28 2021 Valid 12Z Mon Jun 28 2021 - 12Z Wed Jun 30 2021 ...Record-shattering heat forecast across the Northwest with cooler temperatures finally entering areas west of the Cascades by Tuesday... ...Additional rounds of heavy rain and flash flooding likely over the Southern High Plains today... ...Flash flooding threat continues from the Midwest to the Southern Plains through Tuesday, including parts of southwest Louisiana and the Upper Texas Coast... ...Above average temperatures and oppressive heat found across the Northeast early this week, while well below average temperatures continue across much of the Southern Rockies/Plains... The hottest temperatures of the ongoing historic heat wave are expected across the Pacific Northwest today, before the core of the unprecedented heat shifts into interior sections of the region on Tuesday and Wednesday. High temperatures are forecast to soar 30+ degrees above average throughout Washington and north-central Oregon to start the workweek, with temperatures peaking in the 100s and 110s. Numerous daily, monthly, and all-time temperature records are forecast. To put it in perspective, today will likely go down in history as the hottest day ever recorded for places such as Seattle, WA and Portland, OR (records date back to the 1940s here). This level of heat is extremely dangerous and can be deadly if proper heat safety is not followed. Low temperatures are expected to remain very warm and in the mid-70s, which can make it even more difficult to stay cool. Excessive Heat Warnings have been issued throughout most of the Northwest, Northern Great Basin, as well as parts of western Nevada and interior California. Residents are urged to stay in air-conditioned buildings, avoid strenuous outdoor activities, drink plenty of water, and check on family members/neighbors. Cooler temperatures associated with a cold front and onshore winds are forecast to enter areas west of the Cascades by Tuesday, but extreme heat will remain and build farther inland across the Northern Great Basin this week. Additional high temperature records are a guarantee for interior sections of Washington and Oregon over the next few days, extending into most of Idaho and western Montana on Tuesday. For the central U.S., showers and thunderstorms will pose a threat of flash flooding once again today and linger through midweek. A stationary boundary bisecting the country from the Lower Great Lakes to West Texas will be the focus for thunderstorm activity and intense rainfall rates. The Southern High Plains in particular are forecast to have the greatest chances of significant flash flooding. A Moderate Risk (level 3/4) of Excessive Rainfall has been issued through Tuesday morning across southeast New Mexico and parts of West Texas, where widespread rainfall amounts over 2 inches are possible over previously saturated land. Farther east, slow-moving thunderstorms may also lead to flash flooding between central Oklahoma and northern Illinois. Much of this region has dealt with numerous instances of heavy rain and flooding over the last week; therefore, any additional downpours could lead to flash flooding. Not to be outdone, tropical showers with efficient rainfall rates are also possible throughout the Upper Texas Coast and Southwest Louisiana today and Tuesday. Thunderstorms here will have the capability of producing 2 to 4 inches of rain in a very short amount of time, which would likely lead to ponding on roadways and swollen streams/rivers. Meanwhile, an area of low pressure approaching the Southeast has been highlighted by the National Hurricane Center as having a medium chance of tropical cyclone formation. Regardless, this system is expected to swing onshore by tonight and produce isolated pockets of heavy rain over southeast Georgia and southern South Carolina. The northwest corner of the country isn't the only place dealing with oppressive heat this week. While not nearly as extreme, parts of New England and the Northeast are expected to experience hot and steamy weather through Wednesday as a Bermuda high pressure system ushers consistent southerly winds up the East Coast. High temperatures are forecast to reach the low-to-mid 90s from Virginia to Maine, with heat indices into the triple digits. Heat Advisories have been issued between Delaware and Maine, including several major cities. The opposite is true for the Southern Rockies and Southern Plains, where much below average temperatures are expected to remain through at least Wednesday. Cloud cover and showers are forecast to keep high temperatures in the 70s, which equates to 20-30 degrees below average. Snell Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php