Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Sat Mar 16 2024 Valid 12Z Tue Mar 19 2024 - 12Z Sat Mar 23 2024 ...General Overview... An amplified upper level pattern is expected to be in place across the continental U.S. for the beginning of next week, featuring a deep upper trough across the Eastern U.S., and a Rex block over the Western U.S. with a closed upper high over the Pacific Northwest and a loitering upper low over the Desert Southwest. The trough over the East slowly lifts out some by midweek with more broad cyclonic flow aloft. The southwest upper low evolves into an open wave and then ejects eastward across the Southern Plains going into Thursday, and reaching the Southeast U.S. by next Saturday with a potential nor'easter developing. The upper ridge breaking down across the northwestern U.S. will allow for a Pacific trough to move in with a return to rain and mountain snow by Friday and into Saturday. ...Guidance/Predictability Assessment... The 00Z guidance suite remains in reasonable agreement with the flow pattern into early next week, with some uncertainty in the details and timing of individual shortwaves across the Eastern U.S. within the larger scale upper trough. Greater model uncertainties are introduced when the Rex block breaks down across the Western U.S. and a trough from the eastern Pacific eventually develops near the West Coast. This results in below average confidence across the northern tier of the U.S. as shortwave perturbations track eastward in northern stream flow. Confidence is better across the southern tier of the U.S. with the upper trough ejecting from the Desert Southwest across the Southern Plains, even though there are some timing differences with the ECMWF depicting a faster solution, and the CMC slower, and the GFS between these solutions. The WPC forecast incorporated a general deterministic model blend for Tuesday and Wednesday, with more weighting applied to the GFS and ECMWF. After that, there was a gradual increase in the use of the ensemble means to about 50-60 percent of the blend by next Saturday, along with some of the GFS and ECMWF and some previous WPC continuity through Friday. ...Weather/Hazards Highlights... After multiple days of heavy rainfall across the Gulf Coast region this weekend, there will be a welcomed abatement in both rainfall coverage and intensity going into early-mid next week. There will not be much in the way of organized rainfall on Tuesday across the continental U.S., except for some areas of light rain across portions of Texas and the Four Corners region. Going into Wednesday, there may be enough return flow and instability from the western Gulf to fuel a greater coverage of showers and thunderstorms across southern Texas, but currently below the threshold of needing a Marginal Risk area at this time. To the north, a secondary cold front combined with an upper level disturbance crossing the Great Lakes/Northeast should bring some generally light precipitation across the region. Sufficient cold air and low level moisture may combine to produce some accumulating snows for portions of the central Appalachians and downstream of Lake Erie and Lake Ontario on Tuesday. Looking ahead to the end of the week, showers and thunderstorms are expected to increase in coverage across the Gulf Coast region as Gulf moisture advects northward ahead of a developing surface low, with the greatest rainfall likely remaining offshore over the northern Gulf based on latest model trends. There is also increasing model signal for a potential nor'easter to develop off the coast of North Carolina by early Saturday as the Gulf low crosses Florida, and if this comes to pass, heavy rain and gusty winds could develop for coastal portions of the Carolinas. Snow will likely make a return to the northern Rockies late in the week as well. Temperatures across the East Coast are forecast to be up to 10 degrees below average on Tuesday as a Canadian surface high will be in place. There will likely be some frost/freeze concerns for any areas where spring green-up has started as temperatures dip well into the 30s to near freezing, especially Tuesday morning across much of the Southeast states. Meanwhile, it will be pleasantly mild from the Pacific Northwest to the Central Plains through midweek, followed by a gradual cooling trend to close out the work week. It will remain cold across North Dakota and into northern Minnesota with highs in the 20s and low 30s for the end of the week. Hamrick 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