Extended Forecast Discussion NWS Weather Prediction Center College Park MD 348 PM EDT Sun Mar 31 2024 Valid 12Z Wed Apr 03 2024 - 12Z Sun Apr 07 2024 ...Major New England Coastal Storm into Wednesday/Thursday with maritime hazards and high wind/coastal flooding/rainfall threats along with an inland heavy snow threat... ...Widespread precipitation/cooling pattern for the West midweek- late week to expand into the Plains next weekend... ...Guidance/Predictability Assessment... The medium-range period will begin on Wednesday likely with cyclogenesis well under way over the eastern U.S. Models have generally shown a slight slow down of the forward speed of a triple-point low that is forecast to develop rapidly over the Mid- Atlantic states Wednesday morning. Similar slow down is noted regarding the low center lingering near the lower Great Lakes as it weakens and dissipates with time. These are in response to a trend toward more amplified interaction between a cold upper trough digging down from central Canada with another upper trough lifting northeast toward the Ohio Valley. Thursday will likely be the day when New England will be impacted by a significant Nor'easter at peak intensity. Given the trend for a more amplified upper level pattern, models continue to indicate some tendency to slow the core of the low pressure system near the New England coast, with the GFS and ECMWF showing the possibility of looping/meandering motion of the low center near the coast as it interacts with the upper low. Nevertheless, the Canadian model (CMC) as well as the ensemble mean solutions from GFS, EC, and CMC mean are much less prominent with respect to the looping motions. By next weekend, models indicate a tendency to keep the deep low from exiting too quickly into the Atlantic. In the meantime, models continue to trend toward a more amplified upper trough to dig farther south into the West Coast during the latter half of the week. The CMC was the least-amplified guidance regarding this upper trough, which did not agree well with the GFS, ECMWF and their ensemble means. Therefore, only 10% of the 00Z CMC/CMC mean was included in today's WPC medium-range forecast blend from Day 4 onward, with the rest based on the consensus of 00Z EC/EC mean and the 06Z GFS/GEFS. This blend tends to provide detail while reasonably mitigating these differences consistent with individual predictability. WPC product continuity is well maintained with this plan. ...Pattern Overview and Weather/Hazards Highlights... A major Nor'easter will present heightened threat for coastal flooding/heavy rains and inland heavy snows Wednesday into Thursday for New England 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 heavy rain. Given wet ambient soils, an area of marginal threat of excessive rainfall has been kept for Day4/Wednesday across eastern/southeastern New England. Flooding concerns in this area are expected to be a combination of rain changing to snow with uncertainty on the timings of their durations, along with strengthening winds and sea-water flooding near the coast. Wednesday into Thursday morning will likely be the period when the heaviest of the wet snow is expected to fall across the higher elevations of interior New England. Additional heavy snow is possible into Thursday farther northeast across interior Maine. The snow could linger into Friday and even Saturday across New England but with a progressively lighter and more scattered nature. Depending on how close the center lingers near the coast, the snow could mix with and/or change over to rain closer toward the coast if the low center edges closer to land or makes landfall. Toward the end of next week, models show good agreement on a highly-amplified upper pattern across the U.S. with a ridge building over the central U.S. as the deep low slowly pulls out of the Northeast and the next upper trough digs just off the West Coast. This strongly amplifying upper trough/closed low and leading and wavy cold front/secondary Canadian cold surge digging into the West should act to cool temperatures and spread enhanced precipitation and windy/unsettled flow to include coastal rains down through California along with a prolonged period with interior and terrain/mountain snows increasingly inland across much of the West midweek into next weekend. Depicted areas of precipitation are currently on the modest to moderate side given track/moisture, but ample upper support and favored terrain lift may lead to locally heavier focus and activity to monitor. The WPC Winter Weather Outlook shows the greatest snow probabilities over Northwest to northern Intermountain/Rockies terrain/mountains. Activity will spread into the Plains later next weekend with cyclogenesis and frontogenesis over the central High Plains being blocked by a highly amplified omega blocking pattern that is forecast to set up across the U.S. from coast to coast. Kong/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