Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 PM EDT Mon Apr 1 2024 Valid 12Z Thu Apr 04 2024 - 12Z Mon Apr 08 2024 ...Major New England coastal storm with maritime hazards and high wind/coastal flooding along with Northeast heavy snow into Thursday... ...Widespread precipitation/cooling pattern for the West into late week to work into the Central U.S. through next weekend... ...Overview and Guidance/Predictability Assessment... Model guidance seemed reasonably agreeable with the overall pattern through the period. The large scale features consist of an upper trough/closed low over the Northeast finally ejecting into the Atlantic early next week, another upper low tracking from the eastern Pacific into the West later this week pivoting into the north-central Plains early next week with ridging ahead of it, and additional energy digging into the West by Sunday-Monday. For the late week Northeast system, the 00Z and now 12Z CMC runs split energy between two closed lows as some energy pivots southwest around the main low. Other guidance shows this shortwave but keeps it within the main upper low. Upstream, the upper low/trough shows good agreement moving through the West, but a little more spread into the weekend/early next week as it ejects into the High Plains along with a surface low. Generally the ECMWF runs are on the western side, with GFS runs on the southeastern side. Spread is certainly within typical levels for the Days 6-7 period but has implications for frontal positions (and cloud cover for the eclipse in the central to east-central U.S.). The WPC forecast used a multi-model blend of deterministic guidance favoring the GFS and ECMWF through the early part of the period. Included and gradually increased the proportion of the GEFS and EC ensemble means Days 5-7, reaching 40 percent means to reduce individual model differences but maintain some depth of systems. Regarding QPF, had to increase wraparound northeastern U.S. precipitation late this week because models were generally higher than the default NBM. ...Weather/Hazards Highlights... Potent closed upper low rotation will support a major nor'easter across New England lasting into Thursday. This system will lead to significant wrap-back early spring heavy snow as highlighted in the WPC Winter Weather Outlook. While snow will be the primary precipitation type inland, protracted fetch off the Atlantic with this deep low may keep near coastal areas warm enough near the ground for sleet and rain. The heaviest of the wet snow is expected to fall across the higher elevations of the interior Northeast. Additionally, the deep surface low should produce high winds, that for a time should be onshore and could lead to coastal flooding. Lighter snow likely lingers into Friday, while westerly flow could create upslope flow across the Appalachians Thursday-Friday for snow there and also across the Lower Great Lakes for some enhancement there. Temperatures are forecast to be cooler than average for the Eastern Seaboard behind this system, gradually warming up into early next week. Models and ensembles meanwhile show a highly amplified upper pattern also upstream across the U.S. later week, with a ridge over the central U.S. and potent upper troughing digging down the West Coast. This strongly amplifying upper trough/closed low and leading and wavy cold front/secondary Canadian cold surge digging into the West will act to cool temperatures. This may risk record cold maximum temperatures through southern California. The spread of enhanced precipitation and windy/unsettled flow will include Pacific Northwest through California rains closer to coastal areas, along with a prolonged period favorable for terrain/mountain snows increasingly inland across much of the West later week into the weekend as main upper trough energy and height falls shift robustly inland. No excessive rainfall risks are in place, though the risk may be nonzero in southern California which has seen a wet pattern lately. The energy will eject northeastward over the central U.S. Sunday into Monday along with an associated lead return conduit of moisture and clouds. This will initiate and spread organized moderate precipitation across the broad region with cyclogenesis/frontogenesis. Snow may be possible in the northern High Plains with rain and thunderstorms farther south and east. Then behind the system in the southern/central High Plains, high winds and critical fire danger are possible. Renewed light to moderate precipitation is possible along the West Coast early next week as new new Pacific upper trough system energies again dig back down there. Tate/Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw