Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EDT Sun Apr 12 2020 Valid 12Z Wed Apr 15 2020 - 12Z Sun Apr 19 2020 ...Southeast U.S. heavy rain threat through midweek... ...Overview and Guidance/Predictability Assessment... Longwave pattern remains mostly stable through the first half of the extended period as one shortwave slides across the Great Lakes and another drops into the Northwest and crosses the Rockies with broad troughing across the Central and Eastern states. Meanwhile, across the West and Southwest, upper level ridging should hold through at least Thursday and build back again by the weekend in the Northwest. Some minor timing and intensity differences in the guidance, but a mostly deterministic blend between the latest runs of the GFS, ECMWF, and UKMET provided a good starting point for days 3-5. The biggest forecast question in the extended range continues to be with handling of shortwave energy undercutting the Eastern Pacific Ridge. By Thursday or Friday, models mostly agree this energy may close off into an upper low and slide southward along or just offshore California. Deterministic models continue to show high variability both with respect to timing and intensity of this feature, but at this point, all do show at least a closed system. However, there remains a fair amount of spread as shown by the more washed out ensemble means (which feature just an amplified trough at this point). A blend of the GFS, ECMWF, and UKMET with the ensemble means worked well for days 3-5. Beyond this, the forecast gets increasingly chaotic as the energy ejects into the Southwest with the ECMWF/CMC on the faster side of the guidance envelope and the GFS a bit slower but more in line with the ensemble means. However, run to run continuity in the deterministic models is very poor so itā€s hard to side with any particular solution at this point. For this reason, leaned heavily on the ensemble means at the end of the period, with smaller contributions from the GFS and ECMWF just for a little trough definition. ...Weather/Hazard Highlights... Main weather threat in the extended range is heavy rainfall potential along a wavy trailing frontal boundary which settles over the Southeast U.S. and acts to focus deeper moisture along it on Wednesday into Thursday. To the West, additional shortwave energy digging southward through the Intermountain West will again support snow in parts of the northern/central Rockies on Wednesday. As the Western U.S. closed low/trough enters the Southwest/Four Corners region next weekend, it should begin to tap Gulf of Mexico moisture bringing showers and thunderstorms back into the forecast across portions of the South. For temperatures, cold Canadian air will spread down into much of the Central and Eastern U.S. Wednesday and Thursday with daytime highs 10 to 20 degrees below normal from the Plains to the Midwest and parts of the Mid-Atlantic. Temperatures east of the Rockies should begin to moderate back closer to normal during the middle to latter parts of next week, while temperatures should trend warmer across parts of the Northwest as upper ridging builds aloft. Santorelli 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