Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 PM 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 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 significant point of contention among the guidance, but through the 00Z/06Z guidance there continues to be a more narrowly-clustered consensus. However, closed lows are notoriously difficult to time, and their consensus in the medium range can sometimes be fool's gold. 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. Even a large minority of GEFS ensemble members were slower than the GFS. 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 into 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 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 northeastward into the Canadian Maritime provinces. With some of the models forecasting a central pressure near 970mb, this could challenge monthly low sea level pressure records over eastern Maine into Nova Scotia. Guidance spread remains 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 as the 00Z ECMWF dug this feature farther south across the western U.S. through the weekend compared to the 00Z/06Z 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 00Z ECMWF/Canadian/ECMWF ensemble, the 06Z GEFS mean, and the previous WPC forecast. Despite some differences in track/timing of embedded features, this starting point maintained some definition through the period without committing too strongly toward the ECMWF. ...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, with the potential for localized heavy rain as well. 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 (some part of the event is now captures in the short term winter weather desk). 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. Potential exists for significant snowfall but uncertainty exists in the strength of the forcing. Above average temperatures are forecast 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 (with some record highs the central/northeast Gulf Coast into Florida especially Thursday). 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 through. 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. To the north, a polar air mass is expected to push southward 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. Fracasso/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