Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1239 PM EDT Sat Apr 17 2021 Valid 12Z Tue Apr 20 2021 - 12Z Sat Apr 24 2021 ...Heavy Rainfall for parts of Florida Tuesday... 17Z Update: Similar to the overnight forecast, the WPC medium range forecast was favored more in line with the ECMWF/CMC/EC mean given better clustering among these models for most of the forecast period. The exception is that the 00Z CMC is out of phase near the Hudson Bay region with a secondary closed low aloft. Regarding the GFS/GEFS mean, they are quicker to eject the southwestern U.S. shortwave across the Plains by the end of next week, and also displaced well to the east with the eastern Pacific upper ridge axis as part of this quicker progression. The 00Z UKMET was an outlier solution over California with a much stronger upper low dropping southward compared to all of the other ensemble members, so this solution was ruled out for this forecast cycle. The WPC forecast included more of the EC mean by Friday and Saturday, and also incorporated some of the previous WPC forecast with fronts/pressures to maintain good overall continuity. The previous discussion follows below for reference. /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. This favors troughing over most of the CONUS next week and cooler than normal temperatures to all but the West. A wavy frontal boundary will bring a chance of heavy rain over Florida from the short range into the medium range early in the week before dissipating. A lead boundary over the Plains to Midwest on Tuesday will bring light rain and snow into the Great Lakes as it winds up over southeastern Canada. A western system may tap the Gulf for moisture later next week and bring widespread rainfall to the lower Mississippi Valley by next weekend. ...Guidance/Predictability Assessment and Preferences... The GFS/GEFS have struggled over the past week or so with the amplified/cutoff flow from the Pacific across North America, though over many areas they seemed fairly adequate. However, overall preferred the ECMWF-led consensus with the 12Z Canadian and the ECMWF ensembles which were generally more amplified than the GFS/GEFS. There was a notable trend toward a slower progression across Canada and the Northeast next Thursday with a farther west upper low near the Canadian archipelago, allowing the Northeast system to close off and wrap up over Maine. With the upstream upper low still forecast to drop down into southern Canada Friday, this should be enough of a kicker overall to nudge the Maine system farther to the northeast. By later in the week, the GFS may be too slow over Texas/Mexico with an upper/sfc system (related to a slower progression of the incoming Pacific system toward WA/OR) but the breakdown/relaxation of the amplified pattern suggests low confidence in evolution. Opted to remain closer to the ECMWF ensemble mean which typically handles amplified flow the best, but even it can be too slow once the pattern opens up (which can favor the GFS/GEFS). ...Weather/Hazards Highlights... Northern and central Florida may see a continuation out of the short range (Mon) into the medium range (Tue) with rainfall along a front that will slowly push southward. Several inches are possible in some areas during the period. Generally light precipitation is expected near a frontal system moving into the Great Lakes and Northeast Tue-Wed with some snow on its northwest side. 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. A modest signal exists for at least some heavy rainfall over the lower Mississippi Valley next Fri-Sat. 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 next week. The central states will see the coldest readings relative to normal for late April, perhaps 10-25 degrees below normal on Tuesday east of the Rockies behind the frontal boundary. That cold air mass will push eastward but moderate though parts of the East may see at least one day with temperatures 5-15 degrees below normal (Thu along the I-81 corridor). 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