Extended Forecast Discussion NWS Weather Prediction Center College Park MD 228 PM EDT Thu Apr 18 2024 Valid 12Z Sun Apr 21 2024 - 12Z Thu Apr 25 2024 ...Overview... A reasonably progressive flow pattern is expected during the medium range period, with not much expected in terms of significant organized 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 amplified upper trough/low reach the coast in a week. ...Guidance/Predictability Assessment... Recent 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 guidance 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. Opted for another guidance composite. 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 with the southern part likely headed towards the West Coast in about a week. There is some agreement in possible closed low development off the West Coast, but significant differences in the north-south placement of this low. Favored the ensemble means along with smaller contributions from the deterministic models. ...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 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 area remains in place across this region for the WPC Day 4/Sunday Excessive Rainfall Outlook, but have reduced the areal coverage/inland extent given coastal position of the nearby/wavy front as per latest WPC/guidance QPF and ensemble heavier QPF probabilities. 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 Thursday. Santorelli/Schichtel 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