Extended Forecast Discussion NWS Weather Prediction Center College Park MD 102 AM EDT Mon Jul 27 2020 Valid 12Z Thu Jul 30 2020 - 12Z Mon Aug 03 2020 ...Late week heavy to excessive rainfall threat from the Central Plains to the lower Ohio Valley... ...Tropical system threat for the Caribbean then possibly the Bahamas/Southeast U.S. in a week... ...Overview and Guidance/Predictability Assessment... The medium range period features renewed troughing across the Northeast/Midwestern states, as well as troughing becoming established off the Northwest coast as trough energy/height falls work their way inland. In between, upper ridging should build across the Intermountain West/Rockies/High Plains as a subtropical high settles over the Southwest. Overall, the model guidance continues to show better than average clustering on the large scale pattern lending to a majority deterministic model blend through day 5. As usual, some variance continues with the smaller scale details, especially towards the end of the period, but leaning towards the ensemble means helps mitigate these differences. The NHC is also monitoring a risk for tropical system development over the Caribbean later this week, which models and ensembles continue to show a possible threat for the Bahamas/Southeast U.S. by early next week. There remains considerable uncertainty in the details and track of this potential system, so forecast confidence at this point is low, although it bears monitoring over the next week. ...Weather/Hazard Highlights... A wavy frontal boundary settling from the Mid-Atlantic to the mid-south and Central Plains will become a focus for numerous showers and thunderstorms through much of the period. Amplifying upper level energy rounding the top of the southern U.S. ridge should bring more organized moderate to heavy rainfall from the Central Plains/Mid-Mississippi Valley to the Tennessee and lower Ohio Valleys. Models continue to show considerable precipitable water values upwards of 2" and enhanced convergence/instability with frontal wave development within this region. This may result in an excessive rainfall/runoff threat along with possible strong to severe thunderstorms. This activity may eventually lift through the Ohio Valley this weekend/early next week as the flow amplifies across the region, though guidance remains uncertain on the exact details at the longer time frame. Meanwhile, upper level high pressure over the Southwest U.S. and building ridging northward should support moderately anomalous summer heat across the Intermountain West/Rockies/High Plains. From the Southern Plains to the Southeast, daytime highs should be near normal but the combined summertime heat and humidity will result in triple digit heat indices. Farther north, temperatures could be 5 to 10 degrees below normal into next weekend across the Central Plains within a region of substantial cloud cover and precipitation. Santorelli Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml