Extended Forecast Discussion NWS Weather Prediction Center College Park MD 236 AM EDT Fri Apr 23 2021 Valid 12Z Mon Apr 26 2021 - 12Z Fri Apr 30 2021 ...Heavy rain threat from parts of the Southern Plains/Lower Mississippi Valley into the Midwest next week... ...Guidance/Predictability Assessment and Preferences... The dominant feature of the period will be shortwave energy which enters the Northwest U.S. Monday/day 3 and should amplify as it makes its way eastward into the Upper Midwest/East Coast by late next week. The models and ensembles show good agreement on the overall large scale pattern in the medium range, but continue to differ in timing and details, particularly later in the period. The GFS continues to be the fastest to move the trough eastward, but has trended slower its past few runs, which brings it closer in line to the slower ECMWF/CMC/UKMET. The presence of a cut off southern stream low over northern Mexico (now shown by the ECMWF along with the CMC and UKMET) could be the difference in timing. The northern stream energy shows good timing even as far out as day 7, although the GFS is still an outlier with a much stronger/closed low solution. One other feature of note is with a closed low exiting the Northeast on Monday, with the UKMET the quickest to lift it northward and the CMC continuing to be the oddball in pulling the upper low well south/east of the remaining guidance. WPCs blend for this cycle favors a majority deterministic model blend (leaning more towards the slower solutions for the main trough) the first half of the period, gradually increasing the ensemble mean contributions late period to account for normal differences in the details. This approach fits well with previous shift continuity. ...Weather/Hazards Highlights... The system affecting the Northeast early in the period will wrap around showers (some snow in higher elevations) into Monday across northern New England. Windy conditions are also expected across much of the Northeast as the low deepens and tracks into the Canadian Maritimes. The main system tracking into the West on Monday will spread lower elevation rain/mountain snow through parts of the Great Basin and Rockies Monday into Tuesday. Some leading precipitation may also stream across extreme northern tier into the Upper Great Lakes. Once the system reaches the Plains by midweek, leading moisture flow out of the Gulf of Mexico and plenty of warmth will increase potential for severe weather (see SPC for details on the severe threat) and areas of heavy rainfall. Currently, the best signal for highest rainfall totals exists from eastern TX/Lower Mississippi Valley northward into the Midwest/Ohio Valley Tuesday and Wednesday. Some moderate to locally heavy rainfall should also spread into the East and Southern states later in the period ahead of the cold front. Ahead of this trough, an area of above normal temperatures will progress from the central-southern Plains into the East over the course of the period. Warmest anomalies in the plus 10-20F range should extend from the southern two-thirds of the High Plains through the Midwest, Great Lakes/Ohio Valley, and Northeast. Some areas may approach or reach their warmest readings so far this season. Cooler air behind the western cold front will likely bring high temperatures down to 10-20F below normal over parts of the southern half of the West early next week. In modified form (with decreasing coverage of double-digit anomalies) the cool air will reach the Plains by midweek or so. The West Coast into western Great Basin should rebound to above normal by next Thursday as an upper ridge builds in. Expect parts of the Northern Plains and vicinity to be on the chilly side into Tuesday before warming to near or above normal by Wednesday or Thursday. Santorelli/Rausch 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