Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 AM EDT Tue Apr 20 2021 Valid 12Z Fri Apr 23 2021 - 12Z Tue Apr 27 2021 ...Heavy Rainfall/Flooding Threat across the South Friday into this Weekend... ...Overview... Blocky upper ridging forecast over Alaska and Greenland at high latitudes and across the Greater Antilles at lower latitudes favors upper troughing over much of the CONUS during the medium range. One system will push into the West Coast this weekend as another moves through the Southeast and turns the corner up along the coast as a nor'easter into Monday. Ridging will build in between these two systems next week across the Plains ahead of the advancing western system. The pattern is rather chilly for late April with precipitation chances focused near each frontal system in the West, South, and East. ...Guidance/Predictability Assessment and Preferences... Through the 12Z/18Z model cycle, a blended deterministic solution sufficed for the first few days of the forecast. Finally the models have mostly converged on the system timing out of the West/Southwest, mostly in line toward the ECMWF-thinking. Trend has been a bit quicker through the lower Mississippi Valley, but was hesitant to jump as quick as the ECMWF at this point. A middle-ground solution fit well. As the system lifts toward the coastal Mid-Atlantic/Northeast, 18Z GFS and 12Z ECMWF formed a reasonable pairing near their ensemble means and each other (Canadian too weak and farther south), wrapping up another occluded system near Maine. Back to the West, trend has been only a bit quicker, perhaps still within the noise, with much better ensemble overlap than in the previous several days. Gave extra weighting to the ECMWF ensemble mean as it has been most stalwart in this wintry/blocky flow pattern, though the GEFS mean depicted a plausible solution as well. Utilized the 00Z/01Z NBM for most of the gridded fields with some added ECMWF ensemble mean influence. ...Weather/Hazards Highlights... Rainfall will expand along and ahead of a warm front across the lower Mississippi Valley on Friday, aided by Gulf of Mexico moisture and instability. Locally heavy rainfall is likely into the Southeast through late Saturday as the area of low pressure moves eastward. Runoff and flooding issues are quite possible, especially in areas that have received much above normal rainfall this month. The system is then forecast to lift up the East Coast as trailing northern stream energy from the Great Lakes merges into the coastal system. Enhanced rainfall is possible into New England with windy conditions region-wide as the low deepens. In the West, an amplifying Pacific upper trough/low and surface front will work into the West Coast over the weekend into early next week, eventually bringing a lower elevation rain/mountain snow focus into CA (mainly the Sierra, but even down into the higher elevations of Southern California). Rainfall does not appear to be more than light to perhaps modest along the coast. Once the system moves to the Plains by Tuesday, potential will increase for convectively-driven rainfall. A chilly air mass Friday over the East (but especially eastern Montana) will moderate this weekend. An area of above normal temperatures over parts of the West/Great Basin ahead of the upper trough/low will shift eastward across the Rockies and into the Plains by next week. Cooler than normal temperatures will spread into the West Mon-Tue as readings may be 5-15 degrees colder than normal. Fracasso 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