Extended Forecast Discussion NWS Weather Prediction Center College Park MD 128 AM EDT Tue Jul 28 2020 Valid 12Z Fri Jul 31 2020 - 12Z Tue Aug 04 2020 ...Late week heavy to excessive rainfall threat from the Mid-Mississippi Valley to Ohio Valley... ...Tropical system threat for the Caribbean then possibly the Bahamas/Southeast U.S. this weekend/early next 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. In between, upper ridging should build across the Intermountain West/Rockies/High Plains as a subtropical high settles over the Southwest. This general trough-ridge-trough pattern looks to maintain itself through much of the period. Overall, the model guidance continues to show better than average clustering on the large scale pattern lending to a complete deterministic model blend through day 4. As usual, some variance continues with the smaller scale details by day 5 and beyond, but incorporation of the ensemble means helps mitigate these differences. The largest forecast concern for the medium range period continues to be with potential tropical development over the Caribbean later this week, currently being monitored by the NHC. By late weekend/early next week, models and ensembles show support for a possible threat to the Bahamas/Southeast U.S. coast, highly dependent on influence from subtropical Atlantic high pressure. This scenario is supported by deterministic models GFS/CMC (and associated ensembles), while the ECMWF and its mean/ensembles tend to pull something westward towards the Gulf of Mexico. At this point, WPC stayed close with continuity, but there remains considerable uncertainty in the details, exact track, and whether or not a meaningful system even develops. As such, forecast confidence on any solution 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 south-central Plains will become a focus for numerous showers and thunderstorms through the weekend. Amplifying upper level energy dropping into the Midwest, along with surface low development along the front, poses a threat for moderate to heavy or excessive rainfall from portions of the middle Mississippi Valley to the Ohio Valley. This activity may eventually lift north with the surface low towards the lower Great Lakes, with rainfall also likely south and east along the attendant cold front into the Appalachians and Mid-Atlantic regions. Rainfall from any potential tropical system may begin affecting the Bahamas as early as Friday, with moisture ahead of the system bringing rainfall towards the Southeast coast late weekend/early next week. Meanwhile, upper level high pressure anchored over the Southwest U.S. and building ridging northward should support moderately anomalous summer heat across the Intermountain West/Rockies/High Plains and the Southwest. Daytime highs near 100 degrees are possible (110 to 120 across the desert Southwest) which are roughly 5 to 10 degrees above average. From the Gulf Coast to the Southeast, daytime highs should be near normal but the combined heat and humidity will result in triple digit heat indices approaching 110 in some places. Farther north, temperatures could be 10 to 15 degrees below normal into next weekend across the Central Plains/mid-Mississippi Valley 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