Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM 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 late this week and for the Central U.S. this weekend/early next week... ...Pattern Overview... During the medium range period, an omega style blocking pattern is expected to form over the continental U.S. and slowly transition out of the blocking pattern by early next week. Two closed upper lows will initially be located over the West Coast and Northeast on Friday with a ridge strengthening over the Central U.S. and southern Canada. The Low over the Northeast will support an impactful coastal storm while the low over the West Coast supports another frontal system moving east across much of the West. The northeastern low will exit into the Atlantic and the western now will move into the Central U.S. over the weekend, pushing the frontal system across the Rockies into the Plains. Another upper low will move down the West Coast to the Southwest early next week, which may bring another frontal system across the southern tier of the nation. ...Guidance/Predictability Assessment... Model guidance is in good agreement on the upper level pattern with an expected amount of uncertainty and model spread for this time period. The highest amount of uncertainty will be surrounding the evolution of the western system as it moves into the Central U.S. over the weekend into early next week. The evolution will ultimately depend on interactions of upper level features (how high pressure over southern Canada affects the strength and location of the upper low) and complex frontogenesis transitions. This system will likely have implications for fronts, precipitation, and cloud cover, which could affect viewing conditions for the total solar eclipse on Monday. The WPC forecast was mainly derived from a multi-model blend of the 06 UTC GFS and 00 UTC ECMWF/CMC/UKMET for maximum system detail consistent with above normal predictability Friday into Sunday. Opted to add the 06 UTC GEFS and 00 UTC ECMWF ensemble means for Monday and Tuesday along with the deterministic guidance to reduce individual model differences but maintain some depth of systems. Slightly higher weight was given to the ECMWF compared to the CMC and GFS because is aligned better with the ensemble mean solutions. The latest 12 UTC model and ensemble suite remains mainly in line. ...Weather/Hazards Highlights... The risk of excessive rainfall leading to flash flooding is less then 5 percent for Days 4 and 5 (Friday and Saturday). The Marginal Risk area that was previously in the Day 5 outlook for parts of Montana, Wyoming, and South Dakota was removed because rainfall rates will be limited due to a lack of instability and precipitation should switch over to snow and mixed precipitation during the overnight hours. There may be flash flooding concerns early next week in the south-Central U.S. Potent closed upper low rotation will support a major Nor'easter offshore New England, with impacts lingering into Friday. This system will support 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 will likely linger in into Friday for the Northeast, and westerly flow could create upslope flow and snow across the Appalachians Thursday into Friday and also across the Lower Great Lakes where there could be some lake effect enhancement. Temperatures are forecast to be cooler than average for the Eastern Seaboard behind this system, gradually warming up early next week. In the West, the strongly amplifying upper trough/closed low and associated surface frontal system will act to cool temperatures. This may risk record cold daily maximum temperatures in southern California. Precipitation chances will spread across the West late this week into the weekend, with precipitation falling as rain at low elevations and mixed precipitation and snow at higher elevations. 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 with moist return flow from the Gulf of Mexico ahead of the system. This will initiate and spread organized precipitation across a broad region with cyclogenesis/frontogenesis. Snow will also be possible in the northern Rockies and nearby vicinity with rain and thunderstorms farther south and east. Behind the system, high winds and critical fire danger will be possible in the southern and central Plains as humidity drops and winds increase. Renewed light to moderate precipitation is possible along the West Coast early next week as a new Pacific upper trough digs down towards the Southwest, then the focus for precipitation will shift into the south-Central U.S. on Tuesday as the system progresses. Heavy rainfall may lead to flash flooding concerns for the southern Plains and lower Mississippi Valley Monday night through Tuesday. Dolan/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