Extended Forecast Discussion NWS Weather Prediction Center College Park MD 124 AM EDT Mon Apr 06 2020 Valid 12Z Thu Apr 09 2020 - 12Z Mon Apr 13 2020 ...Overview and Guidance/Predictability Assessment... A broad upper-level ridge across the northeast Pacific is forecast to expand during the extended forecast period, promoting 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. by the weekend. The speed with which the upper low moves east and interacts with the northern stream remains a significant point of contention among the guidance, but there is evidence of trends toward somewhat more consensus. The GFS has consistently been the fastest piece of guidance here, and also also continues to separate the feature into two components, one that races eastward more quickly and another that lingers in the southern stream for much longer. Meanwhile, the ECMWF has been much more consistent, and continues to appear quite well-centered within the guidance spread with respect to that feature. The ECMWF remains favored, and would result in the potential development of a wave of low pressure across the western Gulf Coast states on Sat, lifting north across the Tennessee Valley on Sun, and reaching the Northeast Sun night-Mon. Spread becomes greater through time, and the details of the synoptic evolution by Sun-Mon become a bit more uncertain, due to uncertainties surrounding how the feature interacts with the northern stream wave, and the higher uncertainty surrounding that feature. In the northern stream, model consensus remains relatively good on a vigorous shortwave expected to cross the Northeast Wed-Thu, with a deepening low pressure system expected to cross eastern Canada, and a surface low expected to develop off the coast of New England on Thu before quickly moving northeast into the Canadian Maritime provinces. Guidance spread remains higher with the next northern stream shortwave, forecast to dive southeast across western Canada Fri/Fri night, and into the U.S. northern tier on Sat. The ECMWF now digs this feature farther south across the western U.S. through the weekend, while the GFS keeps things more progressive and takes the shortwave east across the Midwest/Great Lakes. As with last night, GEFS/ECENS ensemble means generally reflect similar ideas to their respective deterministic solutions, albeit a bit more moderated. Guidance does continue to generally agree, however, that a fairly broad region of cyclonic upper-level flow will develop across the central/eastern U.S. by the weekend, carved out by consecutive northern stream shortwaves. This, along with the northeast Pacific ridge, will favor southward transport of polar air into much of the CONUS. Given the described considerations, the WPC forecast was initially based heavily on the ECMWF during days 3-4 (Thu-Fri). During days 5-7 (Sat-Mon) more weight was placed on ECENS/GEFS ensemble means through time to account for increasing spread. A small minority of deterministic ECMWF was used through the end of the forecast period. ...Weather/Hazard Highlights... The trailing cold front south of the deepening eastern Canada surface low is expected to focus showers and thunderstorms across portions of the Gulf Coast, lower Mississippi Valley, and Southeast Thu-Fri. The front is forecast to stall across the Gulf of Mexico on Sat, eventually moving north as a warm front again on Sun ahead of a developing wave of low pressure across the lower Mississippi Valley. This scenario will keep scattered to numerous showers and thunderstorms in the picture from the Gulf Coast to the Southeast through the weekend. Farther north, low pressure forecast to develop along the New England coast on Sun is expected to bring a zone of enhanced precipitation across portions of New England. Sufficient cold air should be in place to support accumulating snows for portions of interior northern New England, especially across western and northern Maine, where several inches of snow are likely. The second northern stream shortwave expected to dive into the northwestern/north central U.S. late in the week will be accompanied by a cold front and a polar air mass attempting to nose southward across, and in the lee of the Rockies. This system will increase chances for mountain snow/valley rain across portions of the northern and central Rockies by the weekend. Above average temperatures will be possible across portions of the Southeast and southern Mid-Atlantic on Thu ahead of the cold front, with highs 5 to 15 deg F above average forecast. A cooler air mass in the wake of the front (highs 5 to 15 deg below average) is forecast to overspread much of the central and eastern U.S. Thu through the weekend. Farther west, highs are forecast to be 10 to 20 deg below average across southern California and much of the Southwest Thu as the upper low passes. Temperatures across these areas should begin to gradually moderate on Fri, with this trend continuing into the weekend, when highs may reach close to seasonal norms. Elsewhere, a polar air mass is expected to push south into the Rockies and central U.S. by the weekend into early next week, with high temperatures as much as 10 to 20 deg below average possible. 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