Extended Forecast Discussion NWS Weather Prediction Center College Park MD 235 PM EDT Fri Jul 07 2023 Valid 12Z Mon Jul 10 2023 - 12Z Fri Jul 14 2023 ...Heat expected to rebuild from the Southwest to the Gulf Coast next week... ...Heavy rain concerns likely to focus over portions of the Gulf Coast and the Northeast on Monday, and back into the Midwest and Mid-Mississippi Valley by mid-week... ...Overview... Little change is anticipated in the upper level pattern next week, which will feature a deep upper level low anchored near the Hudson Bay and an upper level ridge over the Southwest. Multiple waves of energy will rotate around the low across the Midwest and Northeast and drive rounds of showers and storms that could result heavy rainfall and flooding. Underneath the ridge, hazardous heat will build over the Southwest and Texas and expand eastwards to the Gulf Coast region through next week. Hazardous heat will likely persist into the week two timeframe as well per the latest forecast from the Climate Prediction Center. ...Guidance Evaluation/Predictability Assessment... Models show very good agreement on the overall large scale pattern across the CONUS through next week. As is typical for these time frames, there remains plenty of uncertainties in the details regarding a couple of shortwaves moving through mean troughing from the Midwest into the Northeast, which has implications for surface features and sensible weather. This is related to rather significant uncertainties in the placement of the parent upper low near Hudson Bay later next week. The UKMET is the furthest north with the low, the CMC is the furthest south, and the GFS and ECMWF lie in between the other two. The ensemble means support a solution close to the GFS and ECMWF, so they were favored in the model blend for this afternoon's forecast. A blend of deterministic solutions was used for the first half of the forecast period, and ensemble means were included in the blend for the second half of the period. ...Weather/Hazards Highlights... 18Z Update: The Day 4 Excessive Rainfall Outlook remains mostly unchanged in the South and Central U.S., but an additional Slight Risk area was introduced in the Northeast over northeastern New York and western New England. A slow moving low pressure system will produce a prolonged period of rainfall over this area this weekend into early next week, and repeated rounds of heavy rainfall are forecast each day. The highest rainfall amounts are forecast over the terrain where up-sloping will contribute to heavy rainfall that could result in scattered instances of flash flooding. The Day 5 Excessive Rainfall Outlook remains very similar to the previous forecast with only slight southward and eastward adjustments to the Marginal Risk areas over the Northeast and North-Central U.S. to account for trends in model guidance. Looking ahead at late next week, the heaviest rainfall will likely focus along a frontal boundary meandering over the Mid-Mississippi Valley. A surface front draped across the Southern U.S. should help focus areas of heavy rainfall early in the week, particularly near the central Gulf Coast, where a slight risk is highlighted on the Day 4/Monday Excessive Rainfall Outlook along with a Marginal Risk on Day 5/Tuesday. Guidance continues to trend south with the heaviest rainfall and this is reflected in this latest update. There are also signals for a possible band of convection extending back northwest into the central Plains within an axis of favorable moisture and instability and to the north of the western edge of the front which should lift back north as a warm front. Maintained the marginal risk for this region on Day 4 as there is still a fair bit of uncertainty in the specifics of where the best heavy rainfall will set up. A weak surface low lifting along the Northeast Coast should focus rainfall to the north of it across northern New England/Maine and a marginal risk was maintained on Monday's outlook and continued into the Tuesday as well. Elsewhere, a cold front sagging south across the Northern U.S./Midwest should also focus showers and storms Tuesday and beyond. The new day 5/Tuesday Excessive Rainfall Outlook highlights a marginal risk along this boundary, with potential for an upgrade to a slight eventually especially if heavy rain falls within a region from southeast Nebraska into Illinois which has seen above normal precipitation as of late. Enough uncertainty still to preclude that inclusion for now. Hazardous heat is expected to return and expand across the southern tier through at least next week (and likely extending beyond per latest CPC forecasts) as the upper ridge strengthens over the Southwest. The greatest high temperature anomalies (of 10-15F above average) are forecast under the center of the upper high over West Texas and New Mexico, with a broadening area of above normal temperatures elsewhere. Increasing humidity will result in oppressive heat indices possibly exceeding 110F from the eastern half of Texas to the Gulf Coast, and hazardously high heat indices could expand to the Florida peninsula as well late next week. Humidity will be naturally lower back into the Southwest, though air temperatures could still reach 110-115F in some places. Little relief is expected in these areas overnight with anomalously warm nighttime lows in the forecast. Above normal temperatures will also expand into the West Coast states late next week. Elsewhere, a cold front dropping south from Canada should bring temperatures down to 5-10F below average over the northern to central Plains next Wednesday-Friday. The Southeast and up the East Coast may see slightly cooler than average temperatures on Monday, but temperatures will moderate back to near normal through the rest of the week. Dolan/Santorelli 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#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