Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 333 AM EDT Wed Jun 30 2021 Valid 12Z Wed Jun 30 2021 - 12Z Fri Jul 02 2021 ...Dangerous heat continues across interior sections of the Northwest and Northern Rockies, while extending into the Northern High Plains... ...One more day of oppressive heat throughout the Mid-Atlantic and Northeast before a cold front produces rain and severe weather chances by this evening... ...Flash flood threat found between the Ohio Valley and Central Plains today ahead of an advancing cold front, shifting into the Mid-Atlantic and Tennessee Valley by Thursday... The ongoing extreme heat wave over the Northwest has likely peaked, but dangerous temperatures are still forecast across the region through the end of this week. The upper-level pattern responsible for the scorching temperatures is expected to gradually weaken and allow for highs to settle around 10 to 20 degrees above average over the Northern Great Basin and Northern Rockies on Thursday and Friday, versus 30+ degrees above average expected today. Record-shattering high temperatures will still be found this afternoon throughout interior Washington, Oregon, and Idaho. High temperatures into the 100s and 110s will once again challenge monthly and even all-time temperature records. Low temperatures only dipping into the upper-70s will make it even harder too cool off after a full day of blazing heat. Well above average temperatures are also expected to extend into much of Montana and the Northern High Plains by Thursday. High temperatures here are forecast to reach the upper-90s and triple digits. This has prompted Heat Advisories and Excessive Heat Watches to be issued. Meanwhile, Excessive Heat Warnings remain in effect throughout the Northern Great Basin and Northern Idaho, as well as portions of northwest Nevada and northern California. Incomparable to the historic heat found across the Northwest, much of the Northeast will also be dealing with oppressive heat today. High temperatures are forecast to reach the mid-to-upper 90s throughout the Mid-Atlantic and Northeast, with heat indices into the triple digits. Excessive Heat Warnings have been issued for central/northern New Jersey, northern Delaware, and southeast Pennsylvania, while Heat Advisories stretch from southern Maine to eastern Pennsylvania. A cold front will bring relief from the heat by this evening, but not without the chances for severe thunderstorms. As the frontal boundary collides with the warm and humid airmass located over the Northeast, thunderstorms are expected to develop and rapidly strengthen. The Storm Prediction Center has issued an Enhanced Risk (level 3/5) for severe weather across southern New England, with a surrounding Slight Risk (level 2/5) extending from eastern Maine to central Pennsylvania. The main threat associated with the vigorous thunderstorms are expected to be from damaging wind gusts, although large hail and isolated tornadoes cannot be ruled out over northern New England. The aforementioned cold front may also produce a flash flooding threat between the Ohio Valley and Central Plains today, before gradually pushing southeastward and into the Tennessee Valley, central/southern Appalachians, and Mid-Atlantic by Thursday. The combination of high atmospheric moisture and a relatively slow-moving cold front may produce showers and thunderstorms that remain over similar areas for extended amounts of time. A Slight Risk (level 2/4) of Excessive Rainfall has been issued today between western Pennsylvania and northeast Oklahoma. This means scattered flash flooding is possible, especially throughout urban and low-lying areas. By Thursday, the frontal boundary is expected to shift the heavy rainfall threat south and east. Much of the Tennessee Valley and Mid-Atlantic could see 1 to 3 inches of rain by Friday morning, with locally higher amounts. A Slight Risk of Excessive Rainfall has been issued for Thursday and extends from Long Island to Middle Tennessee. Elsewhere, scattered showers and thunderstorms can be expected across much of the southern half of the country, besides central Texas. Thunderstorms are also possible throughout the Southwest and Central Great Basin, which could lead to isolated instances of flash flooding. Snell Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php