Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 342 AM EDT Tue Jun 29 2021 Valid 12Z Tue Jun 29 2021 - 12Z Thu Jul 01 2021 ...Historic heat wave continues across interior sections of the Northwest and Northern Rockies... ...A stagnant airmass over the central United States will lead to additional chances for flash flooding from the Southern High Plains to the Middle Mississippi Valley today and Wednesday... ...Oppressive heat found across the Northeast and Mid-Atlantic to end the month of June before a cold front approaches the region on Wednesday, accompanied by the threat of severe weather... It has been a historic stretch of heat throughout the Northwest over the last few days, with numerous all-time temperature records falling throughout Washington and Oregon. The unprecedented heat will continue through the rest of this week for the Northern Great Basin and Northern Rockies, while areas east of the Cascade Mountains enjoy temperatures closer to normal starting today. The upper-level high pressure system responsible for the record-shattering heat is forecast to slide eastward along the U.S.-Canada border near northern Idaho today and eventually weaken by Thursday. As a result, the core of the extreme heat will be found across interior Washington and Oregon through midweek, as well as northern Idaho and northwest Montana. High temperatures are expected to soar into the 100s and 110s across this region (20-30+ degrees above average). Cities such as Spokane, WA and Boise, ID could threaten monthly temperature records. In fact, the forecast high temperature of 110 degrees today for Spokane, WA would also break their all-time temperature record (108 degrees set in August of 1961). Warm overnight low temperatures in the 70s are forecast as well, which can make conditions even more dangerous. Excessive Heat Warnings are in effect throughout the Northern Great Basin and Northern Rockies, as well as parts of western Nevada and interior California where high temperatures into the triple digits can be expected. Residents are advised to stay in air conditioned buildings, drink plenty of water, and avoid strenuous outdoor activities. For the central U.S., a lingering stationary front is expected to continue producing scattered showers and thunderstorms capable of triggering instances of flash flooding. This boundary currently bisects the country between the Great Lakes and Southern High Plains. Additionally, much of the central/southern Plains and Middle Mississippi Valley has experienced a very active and wet week, which can lead to compromised soil and a more vulnerable flash flooding risk. A Slight Risk (level 2/4) of Excessive Rainfall has been issued for today across southeast New Mexico, and both today and Wednesday for parts of southeast Kansas and southwest Missouri. Flash Flood Watches have also been issued and parallel the aforementioned stationary boundary between northern Illinois and West Texas. Oppressive heat will be the main weather story today across the Mid-Atlantic and Northeast. Above average temperatures and heat indices into the triple digits will make for a great day to cool off indoors, by the pool, or at the beach. High temperatures are forecast to reach the mid-to-upper 90s today and Wednesday, with a few daily high temperature records possible between Philadelphia, PA and Boston, MA. Excessive Heat Warnings have been issued for parts of New Jersey, northern Delaware, and southeast Pennsylvania, with Heat Advisories stretching into Upstate New York and southern Maine. A cold front approaching the region is expected to bring relief from the heat by Wednesday evening, but will also bring the potential for damaging wind gusts. The Storm Prediction Center has issued a Slight Risk (level 2/5) of severe thunderstorms for much of the Northeast and New England between eastern Maine and central Pennsylvania. Snell Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php