Extended Forecast Discussion NWS Weather Prediction Center College Park MD 110 AM EDT Tue Apr 07 2020 Valid 12Z Fri Apr 10 2020 - 12Z Tue Apr 14 2020 ...Overview and Guidance/Predictability Assessment... A broad upper-level ridge across the northeast Pacific is forecast to expand northward into Alaska then eastward into Canada 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 to the east and interacts with the northern stream remains a point of contention among the guidance, although spread among the guidance has reduced substantially over the past 24 hours. The GFS has slowed the system down substantially over the past couple runs, now much closer to the ECMWF, albeit still toward the faster side of the spread. Meanwhile, the ECMWF has been much more consistent, and continues to appear quite well-centered within the guidance spread with respect to that feature, perhaps even slowing down the system slightly in recent runs. Starting blend of the ECMWF/Canadian and ensembles resulted in the development of a wave of low pressure across the western Gulf Coast states on Sat, lifting northward across the Tennessee Valley on Sun, and reaching the Northeast on Mon. Spread becomes greater through time, and the details of the synoptic evolution by Sun-Mon become a bit more uncertain, due to uncertainties/inconsistencies surrounding how 1) the feature interacts with the northern stream wave and 2) the higher uncertainty surrounding that feature. In the northern stream, model consensus remains relatively good on a vigorous shortwave expected to exit the Northeast on Fri, with a quite deep low pressure system expected to pull away from New England as it moves into the Canadian Maritime provinces. With some of the models forecasting a central pressure near 970 mb, this could challenge monthly low sea level pressure records over eastern Maine into Nova Scotia. Guidance spread remains a bit higher with the next northern stream shortwave, forecast to dive southeastward across western Canada Fri/Fri night, and into the U.S. northern tier on Sat. There remains some spread in the sharpness of this shortwave, with the 12Z ECMWF digging this feature farther south across the western U.S. through the weekend compared to the 12Z/18Z GFS (which were more progressive). Both were within the multi-center ensemble envelope of solutions. 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 focused between the Rockies and the Appalachians. Given the described considerations, the WPC forecast was based on a blend of the 12Z ECMWF/Canadian/ECMWF ensemble mean, and the 18Z GEFS mean. More weight was placed on deterministic solutions during days 3-4 (Fri-Sat), with a gradual trend to majority ensemble means later in the forecast period. ...Weather/Hazard Highlights... Light rain/snow and gusty winds could linger across portions of northern New England into Fri as the deepening low pressure system pulls away from the region. Farther south, showers and thunderstorms are expected to become increasingly widespread across the Gulf Coast, lower Mississippi Valley, and the Southeast Sat-Sun in association with a stationary frontal boundary 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 in the lower Mississippi Valley. Models/ensembles show a signal for some degree of heavy rainfall potential, especially across portions of the Southeast as the low deepens across the Tennessee Valley on Sun. As the low pressure system continues to move north along/near the Eastern Seaboard late Sun into Mon, rain should also overspread the Mid-Atlantic and Northeast. Depending on the exact track and intensity of the surface low, there is some possibility for gusty winds along the New England coast, and snowfall across interior/high elevation areas of northern New England - but forecast confidence is not sufficiently high at this time to place too much confidence in these aspects of the system. 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 snow across portions of the northern and central Rockies by the weekend, with perhaps some snowfall potential even extending into the adjacent High Plains. Potential exists for significant snowfall but uncertainty exists in the strength of the forcing. A cooler air mass in the wake of the front (highs 5 to 15 deg F below average) is forecast to overspread much of the central and eastern U.S. Fri-Sat. Across the Southwest, highs are still forecast to be 5 to 15 deg below average on Fri as the upper low affects the region. Temperatures across the Southwest should begin to gradually moderate on Sat, with this trend continuing into the weekend when highs may reach close to seasonal norms. To the north, a polar air mass is expected to push southward into the Rockies and central U.S. by the weekend and into early next week, with high temperatures as 15 to 25 deg below average possible. Ryan/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