Extended Forecast Discussion NWS Weather Prediction Center College Park MD 126 AM EDT Wed Apr 08 2020 Valid 12Z Sat Apr 11 2020 - 12Z Wed Apr 15 2020 ...Heavy rainfall/runoff/winds threat for the South then Appalachians/East Coast Sat-Mon... ...Heavy snow threat/cold winds to spread southward over the Rockies/High Plains Sat-Tue... ...Overview and Guidance/Predictability Assessment... A broad upper-level ridge across the northeast Pacific is forecast to expand northward across Alaska then eastward into western Canada during the extended forecast period. This will promote a broad region of downstream cyclonic flow across much of the CONUS. An upper-level low initially across the Southwest U.S. is forecast to slowly move eastward, eventually interacting with a broad northern stream trough forecast to develop across the central/eastern U.S. The speed with which the upper low moves east and interacts with the northern stream remains a point of some contention among the guidance, but spread continues to decrease through time. The GFS continues to trend slower, but remains faster than all other guidance (which has also slowed down slightly). The ECMWF and UKMET have been much more consistent with this system over recent days, and these solutions were preferred for handling of this feature during days 3-4. These solutions indicate quick deepening of a surface low across the Southern Plains on Sun as the upper-level system moves east, with energy quickly transferring to development of another surface low farther southeast, across eastern Texas. This new surface low is then forecast to move rapidly northeastward across the Tennessee/Ohio Valleys later Sun and Sun night as it deepens, reaching the Great Lakes Mon morning. Spread by Mon (day 5) starts to increase, but the ECMWF/UKMET remained reasonably well-centered with the ensemble spread. By Tue, guidance suggests development of another low east of New England, which then quickly moves into Canada. Farther west, additional shortwave energy appears likely to drop southward through western Canada and into the northwestern U.S. by late Tue or Wed, further carving out the broad region of lower heights from the Rockies east across the central U.S. Model showed quite a bit of spread on the timing and nature of this wave, with more disbursed ensembles showing a feature that is likely too weak. Nonetheless, increasing spread by days 6-7 warranted increasing use of ensemble means in the forecast. The WPC medium range forecast was based heavily on a blend of the 12Z ECMWF/UKMET during days 3-4 (Sat-Sun). The component of ECENS/GEFS ensemble means was gradually increased over time, comprising a majority of the forecast by days 6-7 (Tue-Wed). ...Weather/Hazard Highlights... Showers and thunderstorms are expected to become increasingly widespread from the Gulf Coast and lower Mississippi Valley to the Southeast over the weekend as an initially stationary front in the Gulf of Mexico moves north as a warm front ahead of the developing Southern Plains/western Gulf Coast low pressure system. Models/ensembles show an increasingly strong signal for potential heavy rain/flooding concerns from the lower Mississippi Valley to the Southeast and Southern Appalachians Sat-Sun. As low pressure moves northeast across the lower Mississippi and Tennessee Valleys and quickly deepens, gusty winds may also pose a threat. Additionally, while uncertainty is relatively high, severe weather may also pose a threat across portions of the Southern Plains into the Arklatex on Sat, spreading east into portions of the Gulf Coast and Southeast on Sun. As the system continues to move north and east on Mon, expect widespread precipitation across the northeastern states, with potential coastal low development leading to a possible band of enhanced precipitation across New England. With the parent low pressure system farther west a bit stronger in the latest guidance, the setup appears warmer than previous model cycles across the Northeast, and any threat for winter weather risk on Mon appears low, and confined to interior/high elevation locations in northern New England. Across the Rockies and High Plains, a strong cold front is expected to spread a strikingly cold air mass for April southward from Canada into the U.S. over the weekend and persisting into next week. High temperatures 20 to 30 deg F below average are forecast across the northern Rockies and adjacent High Plains on Sat, spreading south across much of the Rockies and central U.S. Sun-Mon. Additionally, the system is also expected to produce a threat for heavy snow, working its way southward across the northern/central Rockies and portions of the adjacent High Plains Sat-Mon. Ryan 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