Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Sun Apr 18 2021 Valid 12Z Wed Apr 21 2021 - 12Z Sun Apr 25 2021 ...Record cold likely for parts of the central U.S. Wednesday and Thursday morning... 19Z Update: The 12Z deterministic guidance is in above average agreement for the middle to end of next week regarding the overall synoptic scale pattern. One of the first differences to emerge is a stronger upper low/trough in southern stream flow across the southwestern U.S. by Friday with the GFS, and this amplified solution continues through next weekend as the trough crosses the Gulf Coast region. There has been an improvement in the model clustering over the eastern Pacific by Friday and into Saturday with the next storm system approaching the West Coast, with the 12Z CMC much closer to the consensus compared to its 00Z run, which was not used in the front/pressures forecast owing to the greater differences here. The WPC forecast primarily utilized the UKMET/GFS/ECMWF/13Z NBM as a starting point in the forecast process, with a little less of the GFS owing to the greater amplification of the southern tier trough. The overnight discussion follows below. /Hamrick ...Overview... Strong upper ridging is forecast to drift west-northwestward out of northwestern Canada and through Alaska as an upper low in the northeastern Pacific wobbles its way toward the Pacific Northwest next weekend. This favors troughing over most of the CONUS next week and generally cooler than normal temperatures to all but the West. A system moving into the Northeast on Wednesday will bring light rain and snow into the eastern Great Lakes region into the Northeast/New England as it winds up over southeastern Canada. Another system exiting the Southwest by Friday may tap the Gulf for moisture and bring widespread rainfall to the lower Mississippi Valley into the Southeast (perhaps locally heavy) next Fri-Sun. ...Guidance/Predictability Assessment and Preferences... The amplified/closed pattern so typical of April continues to challenge the guidance. Ensembles generally agree on the evolution but the ECMWF ensembles continued to be favored due to ongoing better performance over the past week or so (from the Pacific across North America). However, the Canadian ensembles, typically more amplified like the ECMWF ensembles, were notably closer to the GEFS ensembles. Thus, incorporated a minority weighting of the GFS/Canadian to start the forecast but a majority of the ECMWF. By next Fri-Sun, ECMWF became rather suspect out of the Pacific (much quicker than its ensemble mean) but was still a plausible solution, as was its depiction of an East Coast system. Favored the ECMWF and GEFS mean by next weekend amid lowering certainty. It is a pattern more reminiscent of winter than spring over the CONUS. Way farther upstream, Super Typhoon Surigae will recurve east of the Philippines and lift into the westerlies, but may enter on the back side of a trough off Japan late in the week with little direct influence on the CONUS at that point. ...Weather/Hazards Highlights... Generally light precipitation is expected along a frontal system and on the north/northwest side of the surface low as it moves into the Northeast Wednesday. As the system deepens through the Northeast, rain/snow could increase over New England where temperatures may be cold enough for snow especially at higher elevations and overnight. The next system of interest would be out of the southern Rockies around Fri and through the Plains. Rainfall should expand over Texas and points eastward with at least some heavy rainfall quite probable over the lower Mississippi Valley next Fri-Sat and into the Southeast by Sunday. Warmer than normal temperatures are expected for the West Coast closer to or underneath upper ridging, with much milder air just ahead of the Pacific system late in the period. The region between the Rockies and Appalachians will see the coldest readings relative to normal for late April on Wednesday, perhaps 10-25 degrees below normal. This will lead to record cold low temperatures from Texas northward toward the Corn Belt. That cold air mass will push eastward but moderate Thu-Fri into the Northeast/Mid-Atlantic. Another cooler shot of air may arrive from Canada through the northern Plains/Upper Midwest Fri-Sat. 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