Extended Forecast Discussion NWS Weather Prediction Center College Park MD 122 AM EDT Thu Apr 18 2024 Valid 12Z Sun Apr 21 2024 - 12Z Thu Apr 25 2024 ...Overview... A generally progressive flow pattern is expected during the medium range period with not much expected in terms of significant weather hazards. Troughing across the Great Lakes into the Northeast will be periodically renewed as shortwaves traverse through the base of the trough, bringing some generally light precipitation from parts of the Upper Midwest to the East. Out West, upper ridging early in the week will slide eastward as the next trough and possible closed low reach the coast by Wednesday and slowly moves inland. ...Guidance/Predictability Assessment... The latest guidance shows fairly good agreement across the CONUS through much of the period (Sunday-Thursday). There remain some timing differences with shortwave troughing through the Southeast Sunday-Monday and again with the next shortwave through the Great Lakes/Northeast Tuesday-Wednesday. These details create some uncertainty in the eventual (generally light) precipitation pattern across the Southeast and Northeast early next week, but a general deterministic model compromise for the most part gave a good starting point. Energy dropping south out of a large closed low over Hudson Bay early to mid next week also presents a forecast challenge and how it may or may not interact with the shortwave into the Northeast at the same time. The 17/12z run of the ECMWF was much stronger with this, bringing a deep closed low through southeast Canada and into the Northeast late period, while the GFS and CMC favored just a weak shortwave. Was not ready to be as aggressive as the ECMWF so favored the GFS (and CMC), along with the ensemble means for this system. There is also a lot of uncertainty late period with the evolution of a deep closed low over the Gulf of Alaska which may split into two factions -- the southern part likely headed towards the West Coast by next Wednesday. There is agreement in possible closed low development off the West Coast but significant differences in the north-south placement of this low. The CMC at this point seemed an outlier as it then splits the energy again bringing a closed low into the Southwest and another one into British Columbia. Favored the ensemble means pretty heavily for this system, along with smaller contributions from the GFS and ECMWF. ...Weather/Hazards Highlights... Combined shortwave energy across the Tennessee Valley/Southeast will interact with a front dropping slowly through the Southeast supporting moderate to locally heavy rainfall from the Central Gulf Coast into the Southeast/Carolinas on Sunday. Though QPF amounts are not particularly high, there is some anomalous moisture and instability present which would support higher rain rates and possible flash flooding. A marginal risk is in place across this region for the Day 4 Excessive Rainfall Outlook. The next shortwave through the Northern Plains-Great Lakes-Northeast will be accompanied by some generally light to moderate rain next week along the attendant cold front through the Upper Midwest, Ohio Valley, and Northeast. A flash flood threat is not anticipated with this system in the absence of any appreciable moisture anomalies or instability. The next round of light rain/mountain snow should arrive in the West by next Tuesday ahead of the amplified trough, with some precipitation farther inland across the Northern Rockies as well. The mean upper trough over the East much of the period should support generally below normal temperatures for most areas east of the Rockies except Florida Sunday-Monday. Some warm up expected from the Southern Plains into the Mid-Atlantic around next Tuesday but the next shortwave into the northern tier should keep temperatures chilly from the Rockies to the Great Lakes and eventually into the Northeast Tuesday-Thursday. Initial ridging out west should support above normal temperatures, but temperatures look to drop back below normal along much of the West Coast and inland by next Wednesday-Thursday. 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 (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