Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 AM EDT Tue Apr 2 2024 Valid 12Z Fri Apr 05 2024 - 12Z Tue Apr 09 2024 ...Major New England coastal storm with maritime hazards and high wind/coastal flooding along with heavy snow to linger into Friday... ...Widespread precipitation/cooling pattern for the West into late week to work over the Central U.S. this weekend/early next week... ...Overview and Guidance/Predictability Assessment... Model guidance seems reasonably agreeable with the overall pattern through this forecast period as highlighted by formation and subsequent slow transition of a massive Omega style blocking pattern over the lower 48 and vicinity starting by later this week. 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 next Sunday-Tuesday. Forecast spread and uncertainty are generally below normal overall, but does increase into early next week with the ejection of system energies from the West to the Plains along with complex associated cyclo/fronto genesis transitions. Forecast 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 was mainly derived from a multi-model blend of the 18 UTC GFS and 12 UTC ECMWF/Canadian/UKMET for maximum system detail consistent with above normal predictability Friday into Sunday. Opted to use a composite of the 18 UTC GEFS and 12 UTC ECMWF ensemble means later Sunday into next Tuesday along with best clustered model guidance of the 12 UTC ECMWF/Canadian to both reduce individual model differences, but maintain some depth of systems. The latest 00 UTC model and ensemble suite remains mainly in line. ...Weather/Hazards Highlights... Potent closed upper low rotation will support a major Nor'easter offshore New England, lingering into Friday. This system will suport significant wrap-back early spring heavy snows over the Northeast as depicted 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 leading into this forecast period. 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 precipitation across the broad region with cyclogenesis/frontogenesis. A WPC Marginal Risk Excessive Rainfall area has been introduced for Day 5/Saturday for locally focusing rainfall up across the northern High Plains given an anomalous moisture plume feed and upslope/terrain lift and susceptability. Snow may be possible in the northern Rockies and vicinity 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. 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