Extended Forecast Discussion NWS Weather Prediction Center College Park MD 146 PM EDT Thu Apr 08 2021 Valid 12Z Sun Apr 11 2021 - 12Z Thu Apr 15 2021 ...Overview and Guidance Evaluation/Predictability Assessment... Forecast confidence remains slightly below average across the CONUS with some significant uncertainties in the latest model guidance that result in considerable sensible weather differences, particularly by days 5-7. A blocky upper level pattern is expected with ridging amplifying over the eastern Pacific/western U.S. through the period. Multiple closed lows and general troughing over the northern Plains and Great Lakes will meander around and gradually elongate as another closed low asserts itself over the Intermountain West. The compact wave coming from the Central Plains to Great Lakes early in the forecast (Sunday) now has better agreement with some minor timing issues (the 00Z/06Z GFS was a fast outlier but the 12Z now appears to be in better agreement). The next shortwave that eventually closes off over the Northern Plains early next week shows some fair agreement with perhaps the GFS a bit aggressive/amplified with the deepening 500 mb low. The biggest model differences lie with the following western U.S. shortwave that may carve out a longwave trough or closed low over the Intermountain West toward the end of the period. All of the deterministic guidance now shows some version of this solution but with fairly wide latitudinal spread. Finally, blockiness over the Canadian Maritimes will keep a closed low more or less in place through the period. The GFS/UKMET is the slowest to eject this out (compared to the more zonal/progressive ECMWF) but again, there is more than normal model spread west to east with the position of this feature. All told, the WPC blend this cycle leaned on the 00Z ECMWF as its primary component with lesser weight given to the 00Z/06Z GFS given some of the early on timing issues. The CMC/UKMET appeared useful in the day 3-4 timeframe but by days 5-7, the blend was heavily composed of the ECENS mean and some GEFS mean. ...Sensible Weather and Potential Hazards... Moisture and instability along and ahead of the cold front crossing the East may produce areas of locally heavy rainfall over parts of Florida while elsewhere from the Great Lakes to Northeast, light to moderate amounts are forecast. As the front stalls/washes out across portions of TX and the Gulf Coast, returning Gulf moisture overrunning that boundary may be the focus for scattered to numerous showers and thunderstorms. The repeating/multiple days of unsettled weather could lead to some instances of heavy rain and flooding, particularly over the more saturated/sensitive areas that have seen much above normal precipitation the last few weeks. The area of greatest concern is eastern LA, much of MS and AL. Strong Canadian high pressure building southward through the Plains and inverted troughing over the CO Rockies should favor an upslope precipitation event early to mid next week. The best potential for meaningful snowfall is over portions of southern WY and the central/northern CO Rockies. Much of the CONUS including the Mid-Atlantic and Northeast will see above normal temperatures on Sunday. The exception is for the northern Rockies where readings 10-15F below normal are expected. As troughing sets up over the northern Rockies and northern Plains later in the period, those below normal temperatures are forecast to remain in place and gradually spread southward further into the Plains. Many areas will see readings of 10F to as much as 20F below normal for highs. Nighttime lows will be mostly near normal for the CONUS, aside for the eastern U.S. where readings of 10-20F above normal are forecast Sunday and Monday. Taylor 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