Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1257 PM EDT Mon Jul 27 2020 Valid 12Z Thu Jul 30 2020 - 12Z Mon Aug 3 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... 17Z Update: There appears to be above average model agreement during the first half of the forecast period, with the greatest model spread generally north of the Canadian border. The 00Z UKMET became quite amplified over the central Plains with the building trough and was on the edge of the ensemble spread by Saturday, so it was incorporated through Friday in the model blend. The 00Z GFS appeared too weak with the upper trough across the Great Lakes and Ontario/Quebec by next Monday, but the 6Z and 12Z runs indicate a solution that is more in line with the ensemble means. In terms of sensible weather, a corridor of unsettled weather appears likely from the central Plains to the southern Appalachians in the vicinity of a meandering frontal boundary with multiple rounds of MCS activity along it. Heat will continue to build across the Desert Southwest as a strong upper level ridge becomes anchored over that region. Also, the Tropics will also bear watching by the weekend and into early next week as a potential tropical disturbance comes closer to the southeast U.S., and future forecasts will continue to provide better clarity on potential track and intensity. /Hamrick ...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