Extended Forecast Discussion NWS Weather Prediction Center College Park MD 404 PM EDT Fri Apr 16 2021 Valid 12Z Mon Apr 19 2021 - 12Z Fri Apr 23 2021 ...Multi-day Heavy Rainfall Threat for Florida... 19Z Update: There was good overall consensus among the guidance for this forecast cycle. Given some differences noted with the ECMWF over the Northeast U.S., in particular regarding the stronger upper level low around next Wednesday that was depicted in the 00Z run, it was given slightly less weighting in the forecast blend and slightly more of the CMC and GFS. The UKMET was among the most amplified with a western U.S. shortwave for early-mid next week when compared to the ensemble plots, so it was not favored as much for that reason. The use of the ensemble means was gradually increased for the Wednesday through Friday time period to account for the increasing model spread by the end of the week. The previous forecast discussion is appended below for reference. /Hamrick ...Overview... Strong upper ridging over northwestern Canada (500mb height anomalies around +3.5 sigma) as well as over the Bermuda Triangle will favor renewed troughing over the central CONUS next week and at times in the Northeast and Southwest via split flow. This will bring cooler than normal temperatures to most of the lower 48 and push most of the precipitation chances to the south; namely, Florida. There, a wavy stationary boundary will promote periodic rainfall which may be heavy at times. ...Guidance/Predictability Assessment and Preferences... With the ECMWF and GEFS ensembles in relatively good agreement, favored the deterministic models that fell within their overlap the best during the period. This was mostly near the 18Z GFS and next nearest to the 12Z ECMWF. Uncertainty in the northern stream flow out of Canada later next week as well as the breakdown of the upper ridge in the Pacific led to large differences in the upper pattern over much of the CONUS and, as such, utilized a mostly ensemble mean blend to mitigate future large changes. Ensemble trends were toward stronger closed centers in the higher latitudes suggesting punctuated zonal flow to its south (over the CONUS) and limited predictability. ...Weather/Hazards Highlights... A frontal boundary stalling over the Gulf of Mexico and Florida will be the focus for several rounds of potentially heavy rainfall as the parallel upper flow from the southwest bring in episodes of moisture and rainfall. The greatest risk will be over northern and central Florida where several inches are likely in some areas during the period. ECMWF EPS EFI highlights the area along and north of I-4 for anomalous rainfall early in the week, then sinking southward. This may be a significant multi-day event leading to flooding issues. Generally light precipitation will be possible near another frontal system dropping south and eastward through the central and eastern U.S. for the first half of next week. However, the central Rockies/western High Plains could see modest snow on Monday as a cold front stalls in the area, yielding upslope snow potential. Temperatures will be much colder than normal early in the week with sub-freezing highs (15-30 degrees below normal). The cold airmass will spill onto the southern Plains Tuesday where highs may only reach the mid 50s over northern TX into OK, which may challenge record cold highs. Farther west, warmer than normal temperatures are expected for the West Coast underneath ridging, though that may be tempered a bit as a Pacific frontal system digs southward over the region. Precipitation should be minimal and mainly over the mountains. The East will see a couple chances of light rain and maybe some wet snow in far northern areas, with temperatures near to slightly below normal. 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 Hazards: - Heavy rain across portions of the Southeast, Mon-Tue, Apr 19-Apr 20. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Fri, Apr 23. - Floodin possible across portions of the Southeast and the Lower Mississippi Valley. - Flooding occurring or imminent across portions of the Southeast and the Lower Mississippi Valley. - Much below normal temperatures across portions of the Plains, Tue-Wed, Apr 20-Apr 21. - Much below normal temperatures across portions of the Central/Southern Plains and the Middle/Lower Mississippi Valley, Wed, Apr 21. 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