Extended Forecast Discussion NWS Weather Prediction Center College Park MD 409 PM 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... Two moderately progressive upper troughs will affect the lower 48, between a blocky high-latitude regime featuring Alaska/Greenland ridges plus a broad upper low over central-western Canada, and a Greater Antilles ridge that eventually builds into southern Mexico. The leading surface system will track from the southern High Plains through the mid-South before turning the corner near the coast as a nor'easter Sunday-Monday. Upper ridging will build in behind this system as the upstream Pacific system moves into and through the West during the first part of next week with low pressure reaching the Plains by Tuesday. Aside from an area of warmth ahead of the Pacific/western system, the pattern will be rather chilly for late April with heaviest precipitation focused over the South/East with the leading system and the West with the feature emerging from the Pacific. ...Guidance/Predictability Assessment and Preferences... The system tracking out of the southern High Plains and expected to develop into a nor'easter still exhibits fine-scale shortwave sensitivity that may temper confidence in specifics for a time. There is improving consensus toward recent ECMWF/ECMWF mean runs that have depicted formation of an upper low over New England and/or the Canadian Maritimes by early next week. However spread persists for the open shortwave that crosses the central/southern U.S. and separate northern tier U.S./southern Canada energy that will likely provide the greater input for the closed low. Precise timing of closed low formation and the track will affect precipitation coverage and amounts over New England. Overall prefer a compromise approach with more ECMWF/GFS weighting relative to the strong/inland 00Z UKMET and farther offshore 00Z CMC. The 00Z ECMWF mean fit in with this idea a lot better than the weak and suppressed 06Z GEFS mean. The 00Z GEFS mean was closer to consensus so that run provided the minority GEFS input for the forecast. 12Z model runs thus far maintain general support for aforementioned preferences. Models and ensembles have maintained better than average clustering for the Pacific upper trough that reaches the West early in the week and brings a fairly strong low pressure system into the Plains by next Tuesday. The main trend over the past 24-36 hours has been for the GFS to trend a little faster with the upper trough, though a slower trend in the new 12Z ECMWF shows there may still be some shuffling of specifics even though the overall feature exhibits better than average confidence. A blend of operational guidance followed by some ensemble input along with the 06 GFS/00Z ECMWF/00Z CMC late in the period represented consensus well for this system. ...Weather/Hazards Highlights... Rainfall will expand and intensify across the South from Friday into Saturday as low pressure tracks out of the southern High Plains. The leading warm front will interact with Gulf of Mexico moisture and instability, likely producing areas of heavy rainfall. Runoff and flooding issues are quite possible, especially in areas that have received much above normal rainfall this month. Currently expect the axis of highest rainfall to extend from southern Arkansas/northern Louisiana into South Carolina. There will be potential for severe weather as well so check Storm Prediction Center outlooks for the latest information. The system should then lift up the East Coast as trailing northern stream energy from the Great Lakes region merges into the coastal system and possibly closes an upper low. Enhanced rainfall is possible from parts of the Mid-Atlantic into New England with windy conditions region-wide as the low deepens. A little snow at highest elevations could be possible depending on exactly how the system evolves. In the West, an amplifying Pacific upper trough/low and surface front will work into the West during the weekend into early next week. This system will spread lower elevation rain/mountain snow focus from the Pacific Northwest and northern Rockies southward into California and the Great Basin/central Rockies. The highest totals with this system will likely be from the Pacific Northwest into the Sierra Nevada. The majority of precipitation should be light to moderate though parts of the central West Coast and Sierra Nevada could see a period of locally enhanced activity. Once the system moves to the Plains by Tuesday, potential will increase for convectively-driven rainfall over parts of the Plains and Mississippi Valley. A chilly air mass Friday over the East and especially northern High Plains (where highs will be 10-20F below normal) will moderate some this weekend but there will still be decent coverage of below normal highs over the eastern half of the country. An area of above normal temperatures over parts of the West/Great Basin ahead of the Pacific upper trough/low will shift eastward across the Rockies and into the Plains by next week. Best potential for one or more days with temperatures 10F or more above normal will extend from near the Great Basin through the central-southern Rockies into the Plains/Midwest. Trailing the front that crosses the West, temperatures over the region will decline to 5-15F colder than normal by next Monday-Tuesday. Rausch/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 Hazards: - Heavy precipitation across portions of the Central Great Basin, California, and the Southwest, Sun-Mon, Apr 25-Apr 26. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Southern Plains, Fri-Sat, Apr 23-Apr 24. - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Northern Plains, the Upper/Middle Mississippi Valley, and the Southern Plains, Tue, Apr 27. - Severe weather across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Fri, Apr 23. - Severe weather across portions of the Southeast and the Lower Mississippi Valley, Sat, Apr 24. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Great Lakes, the Mid-Atlantic, the Northeast, the Central Appalachians, and the Ohio Valley, Fri, Apr 23. - High winds across portions of the Northeast, Mon, Apr 26. 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