Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 PM EDT Sun Mar 14 2021 Valid 12Z Wed Mar 17 2021 - 12Z Sun Mar 21 2021 ...Heavy rainfall possible in the Southeast Wednesday... 19Z Update: The 12Z deterministic model suite is in above average model agreement on the synoptic scale pattern through the medium range period. For the closed low crossing the east-central U.S. late in the week, the operational GFS is slightly faster than the parallel GFS, with the latter closer in line with the model consensus. By next weekend, the ECMWF/CMC solutions are slightly west with the upper ridge axis across the High Plains compared to the GFS solutions. The EC mean is likely not amplified enough with the building trough over the western U.S. by the end of the forecast period, and given the agreement among the deterministic guidance here, a fair amount of the ECMWF/GFS/GFS parallel was incorporated through the entire forecast with a slight increase of the ensemble means by Sunday. The previous discussion is appended below for reference. /Hamrick ...Overview... Upper ridging in the Bering Sea will promote troughing into the Gulf of Alaska and northeastern Pacific Ocean, offshore Washington state, later this week. An upper low in the Plains will move eastward, carrying a surface cold front to the East Coast by late Thursday, possibly producing areas of heavier rain. High pressure will build in behind it from Canada with cooler and drier air. By next weekend, the Pacific system will likely move ashore with mild temperatures from the Rockies to the Upper Midwest. ...Model Guidance/Predictability Assessment... Through the 12Z/18Z guidance, a deterministic/consensus blend sufficed for the Wed-Thu period as an area of low pressure lifts out of the Plains and into the mid-Mississippi and Ohio River valleys. The Canadian and GFS were on the slower and faster side of the spread, respectively, and were gradually removed from the blend. The 18Z parallel GFS (V16) was more aligned with the ECMWF and ensemble consensus. By next weekend, some solutions showed a lingering upper trough over the Southeast, perhaps spurring cyclogenesis along the frontal boundary to varying degrees. Depicted a rather meager area of low pressure that slowly nudges eastward rather than wraps ups near the coast given the lead time and dearth of near-shore ensemble members (especially in the GEFS compared to the ECMWF EPS, which was stronger aloft over the Southeast). Over the West, ongoing preference toward the slower end of the solutions (generally non-GFS) has served well but would still allow a lead weaker front and then more defined front to come ashore late Thu and early Sat, respectively. GEFS mean and ECMWF ensemble mean were in good enough agreement to use as a blend (with larger differences in the Southeast). ...Weather/Threats Highlights... Rainfall will increase in areal coverage out of the Plains/mid-Mississippi valley Wednesday into the Southeast along the warm front. Locally heavier rain is possible along with some thunderstorms from the ArkLaMiss into the Tennessee valley. Areas north of the frontal boundary may see some light snow from the Corn Belt into the higher terrain of the Northeast as the system pushes offshore. Rainfall may turn to light snow as it ends over the central Appalachians as colder air filters in from the northwest. Assuming the front stays offshore next weekend, the East will be largely precipitation-free. Approaching Pacific systems will spread low elevation rain and higher elevation snow to the West Coast (NorCal and northern Sierra into coastal OR/WA) Wednesday-Thursday, with higher amounts possible from the system next weekend as the upper trough moves in as well. Light snow could spread into the central and northern Rockies as the front moves through the Great Basin and over the Divide. Mild temperatures over the Southeast Wednesday (widepsread 70s to near 80 with record highs possible over Florida) will give way to 60s behind the cold front. Temperatures will be about 5 to 10 degrees colder than normal for areas in the East Fri-Sat. With ridging expected to build out of the Rockies, well above normal temperatures are expected for the High Plains next weekend (perhaps 15-20 degrees above normal). Fracasso/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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml