Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 AM EDT Sun Mar 29 2020 Valid 12Z Wed Apr 01 2020 - 12Z Sun Apr 05 2020 ...Overview... Main players near North America later this week will be an incredibly strong upper high moving east from the North Slope of Alaska through the Canadian archipelago. With 500mb heights likely above +3.5 sigma (and sea level pressure perhaps into the low 1060s mb which would be higher than records for all of April), this teleconnects to modestly zonal flow over the CONUS but mean/averaged troughing near both coasts. Second area of above average heights will be near the Aleutians, which also supports downstream troughing into the Pac NW/British Columbia. Overall pattern will be drier and relatively cool for areas between the Rockies and Appalachians but mild for California and Great Basin into the Southwest. ...Guidance/Predictability Assessment... Model/ensemble guidance continued to struggle with the evolution of the quasi-zonal/quasi-blocky flow with a fairly active subtropical jet and a very wavy/splitting northern stream along the US/Canada border. End result will be some progressive and some slower systems depending on interactions of smaller-scale features that keep changing (expectedly) from run-to-run. Thus, have found it useful to rely mostly on the ensembles when some consensus and continuity exists. This was best represents by the 18Z GFS/12Z ECMWF and their ensemble means for the start of the period, transitioning to nearly all ensemble weighting by next weekend. Central system Wed-Thu is forecast to move eastward much slower than previously forecast (though many ensemble members were quicker) but the ensemble trend was telling. That front should only slowly move toward the East Coast by next Sun per the consensus as ridging from the north (northern Quebec/Baffin Island) attempts to bridge southward toward rising heights in the wake of the departing lead system Wed into Thu off the East Coast. Confidence remained no better than average given all the changes in the guidance recently. ...Weather/Hazard Highlights... Precipitation will expand ahead of the Plains system on Wed with higher rainfall chances closer to the Gulf of Mexico (S Texas) with precipitable water values near/over 1 inch (+1 to 2 sigma). This will steadily spread eastward as the front approaches the East by Sat into Sun. Pac NW will see some light rain/snow as systems just off the coast lift into British Columbia. Area of cold temperatures (10-20 deg below normal) will move out of Montana and moderate after Thu behind the cold front. Slightly cooler than normal temperatures will spread through the Plains Fri-Sat. Milder temperatures are expected over California into the Southwest with low 90s in the lower deserts by next Sun. Fracasso Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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