Extended Forecast Discussion NWS Weather Prediction Center College Park MD 354 PM EDT Tue Apr 11 2023 Valid 12Z Fri Apr 14 2023 - 12Z Tue Apr 18 2023 ...Overview with Guidance/Predictability Assessment... As the medium range period begins Friday, models continue to show good agreement for a low pressure system to lift across the southeastern U.S. with the heavy rain moving just off the Carolina coasts. Meanwhile, the wintry weather associated with an upper trough moving into the northern Plains should begin tapering off over the northern Rockies on Friday. But as the front advances farther south, the wintry precipitation will likely shift south into the central Rockies/High Plains Friday night. Meanwhile, a developing low pressure wave on the front will begin focusing another area of wintry precipitation most likely from eastern North Dakota to northern Minnesota Friday night into Saturday. Model guidance this morning has slowed down the northeastward progression of this system, favoring a solution similar to yesterday's forecasts. The relatively slow lifting of the low center into Canada would keep a higher chance of wintry precipitation across the upper Great Lakes Sunday night, and may linger into Tuesday morning as lake-effect snow showers even into the lower Great Lakes. The slower system motion would also keep a higher chance of a wintry mix over interior New England into Wednesday morning. Over the Pacific Northwest, precipitation associated with the next Pacific system of concern should reach the coast Friday night. Models and ensembles agree that a gradual amplification of the associated upper trough will occur as the trough pushes slowly inland by late this weekend and through early next week, with expected differences in shortwave timings. Given the above considerations, the WPC forecast begins with an even blend of the 00Z ECMWF/EC mean, the 06Z GFS/GEFS, and a smaller contribution from the 00Z CMC/CMC mean, transitioning to mainly a blend of the ensemble means by Day 7. ...Weather/Hazards Highlights... By Friday the low pressure system in the Southeast should be lifting northeastward with some rain possible in the Carolinas while the heaviest rains are forecast to be just offshore. More scattered and lighter rain/thunderstorms should spread farther north into the northern Mid-Atlantic region on Saturday as the upper system opens up. Snow will be possible in higher elevations of the central Rockies by Friday behind surface low pressure in the central High Plains and with the support of a trough aloft. Precipitation is likely across the central U.S. late week as well. Some enhanced convection is possible in the central U.S. on Friday with a likely focus over the central Plains/Middle Mississippi Valley. There may be a nonzero flash flooding threat with this thunderstorm activity, but no excessive rainfall areas are depicted in the outlook for now given ample uncertainty in the placement of heavy rain and whether or not amounts would be heavy enough. Additionally, there remains some chance for springtime snow to extend into parts of the northern Plains and the Upper Midwest/Upper Great Lakes for Friday into the weekend and possibly lingering into early next week. Meanwhile expect rainfall to expand in coverage to the south and east late week into the weekend, with moist flow ahead of the cold front possibly helping to increase rainfall chances across the western/central Gulf Coast region once again. The best agreement in models and ensembles for the placement of heavy rain is across southeastern Louisiana on day 5/Saturday, where there is a Marginal Risk of excessive rainfall in place. Model guidance has generally trended the heaviest rain farther south and off the Louisiana coast since yesterday. After a dry period for the West into Friday, precipitation chances will increase for the Pacific Northwest on Saturday as a weak atmospheric river channels into the region on the southern side of the northeastern Pacific upper low. Precipitation in the form of lower elevation rain and higher elevation snow is likely to expand east into the northern Rockies and perhaps south into northern California early next week. The most anomalous temperatures as the period begins Friday should extend through the Upper Midwest/Great Lakes to the Northeast with readings exceeding normal by at least 15-25F. High temperatures as high as the low 80s over snow covered grounds across the northern tier states would promote snowmelt. These temperatures should gradually moderate closer to normal through early next week after an initial backdoor front and then the main cold front comes through. Meanwhile the trough pushing through the western and central U.S. and its attendant cold front will help produce cooler than normal temperatures especially in terms of highs across the West and portions of the central U.S. by the weekend. The West and then the central U.S. are expected to flip back above average early next week as upper ridging comes in behind the first trough. Kong/Tate 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 Hazards: - Heavy rain across portions of the Lower Mississippi Valley, Sat, Apr 15. - Severe weather across portions of the Central Plains and the Southern Plains, Fri, Apr 14. - Flooding possible across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding occurring or imminent across portions of the Southeast, the Great Lakes, the Southern Plains, and the Upper Mississippi Valley. - Flooding likely across portions of the Southeast, the Mid-Atlantic, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Much above normal temperatures across portions of the Northeast, the Central Appalachians, the Mid-Atlantic, the Upper Mississippi Valley, the Great Lakes, and the Ohio Valley, Fri, Apr 14. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#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