Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Mon Jul 15 2024 Valid 12Z Thu Jul 18 2024 - 12Z Mon Jul 22 2024 ...Dangerous heat and wildfire threats will build over much of the West this week and into next week... ...Monsoon conditions continue for the Four Corners States; heavy rain will develop in the South, Southeast, and Southern Mid- Atlantic... ...Overview... An amplified upper pattern will result in a strong ridge across the western U.S. and trough across the eastern U.S. through most of the medium range period. An upper trough will attempt to move into the Pacific Northwest late in the period, but progress will be slow due to the strong ridge over much of the West. Dangerous heat will develop underneath the ridge, and temperatures may reach near record values over the weekend. In addition to dangerous heat, hot temperatures, low humidity/dry fuels, and breezy winds will result in an increased threat of wildfires in portions of the Pacific Northwest on Thursday. Better moisture will be located to the south across the Four Corners region and southern Rockies where monsoon conditions will likely continue through this week into next week. Underneath the trough in the East, a slow moving frontal boundary will sag into the south-central U.S., Southeast, and Mid-Atlantic. This boundary is forecast to stall and create a prolonged period of heavy rain that could result in flooding concerns. Areas north of the boundary will finally see a break in the heat as cooler air moves in from Canada. ...Guidance/Predictability Assessment... Models are in good agreement on the large scale upper level pattern, and there is an expected amount of uncertainty in the finer scale details for the medium range period. The main model differences concern how the models are handling energy in the upper level troughs in the Pacific Northwest and eastern Canada, which will affect location and timing of surface fronts and sensible weather. The WPC blend was comprised of a general blend of deterministic guidance from the GFS/ECMWF/CMC/UKMET for the first half of the period, and ensemble means from the GEFS/CMCE were added to the blend for the second half of the period to iron out some model differences. The afternoon forecast put slightly less weight on the 00Z UKMET and ECMWF since these deterministic solutions strayed farther away from the other solutions and ensemble means. ...Weather/Hazards Highlights... A big weather story will be the building and persistence of a major heatwave over much of the West this week and into next week. The NWS HeatRisk product is showing widespread Moderate to locally Major to Extreme HeatRisk values developing mid-week and strengthening this weekend, and record-setting temperatures will be possible. This heat wave is forecast to continue into the week two period, per the Climate Prediction Center outlook, and hot temperatures could result in a prolonged period of enhanced wildfire danger as well. Meanwhile, monsoonal moisture will linger over the Four Corners region, which will allow for daily rounds of precipitation that could result in flash flooding, especially near steep terrain and burn scars. A Marginal Risk remains in the Excessive Rainfall Outlook (ERO) for Day 4/Thursday and Day 5/Friday from the Four Corners region to the south-central Rockies/High Plains with favorable upper diffluence, instability, and moisture. Favorable upper level support and a well defined/wavy surface front will shift southward across the central and eastern U.S., leading to multiple rounds of heavy showers/thunderstorms. With ample moisture and instability in place, locally heavy rainfall will be possible and may lead to instances of flash flooding. The Day 4/Thursday and Day 5/Friday EROs show a slowly southward shifting Marginal Risk area from east Texas through the South/Southeast and southern Mid-Atlantic. An embedded Slight Risk area now focuses the threat across eastern North Carolina, far southeastern Virginia, and far northeastern South Carolina given latest guidance trends and convection bias in a pattern with highly favorable upper jet right entrance region support. The wavy boundary will eventually stall over the region into the weekend, providing focus for daily summertime convection and local heavy rain for the South/Southeast and Mid-Atlantic. The frontal system moving south across the central and eastern U.S. will finally bring relief from the heat that has been plaguing the eastern two-thirds of the nation since early July. This front will usher in cooler air from Canada, and temperatures will drop to near or below average mid-week through the weekend. Dolan/Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw