Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1242 PM EST Mon Mar 02 2020 Valid 12Z Thu Mar 05 2020 - 12Z Mon Mar 09 2020 1745 UTC Update... Overall, model spread is a bit below average at the larger scales through the medium range, although some significant differences remain when it comes to the details of specific systems. A blend of the 00Z ECMWF/06Z GFS was used as a starting point for this forecast update during days 3-4 (Thu-Fri). In general, guidance has trended slower with a southern stream shortwave and low pressure system crossing the lower Mississippi Valley and Southeast Thu-Fri. This feature is forecast to phase with a stronger northern stream wave on day 5 (Fri), with a slower southern stream wave tending to favor a stronger phasing interaction, and an eventual low pressure system off the Eastern Seaboard that is stronger and tracks farther west. A look at the totality of model/ensemble guidance over the past few cycles confirms this trend for a surface low that tracks a bit closer to the Northeast U.S. coastline. Still the question remains as to just how much influence this system will have on the weather across the Northeast late this week. Given the trends, boosted probabilities a bit on the Winter Weather Outlook for Fri/Fri night across much of the interior Northeast. Elsewhere, models show consensus that a leading shortwave should move onshore along the West Coast on Sat, with another northern stream wave diving into the Pacific Northwest by Sun. The GFS/ECMWF showed timing differences with the leading wave - with the GFS on the fast side (and outside consensus with little support among ensemble members). Spread was a bit less with the second wave across the Northwest, and primarily confined to amplitude differences. Given these differences, opted to go with the ECMWF along with increased ECENS/GEFS ensemble means during the day 5-7 (Sat-Mon) time period. Ryan Previous Discussion (issued at 0659 UTC)... ...Overview and Guidance/Predictability Assessment... The opening shortwave responsible for heavy rainfall at the end of the short range period, will be moving across the Southeast to start the medium range period (Thursday). This should eventually interact/merge with northern stream energy dropping into the north-central U.S., eventually leading to rapid surface low development off the East Coast. Days 3 and 4, there is fairly good model agreement with evolution of these systems, with spread increasing beyond. By day 5, the upper low should be somewhere off the Northeast coast, with the GFS the fastest/farthest east and the ECMWF farthest west. The UKMET and CMC lie in the middle, and are supported better by the ensemble means. Although, run to run continuity remains poor with the location of the eventual deep surface low so confidence in any particular solution is low at this point. Out west, an initial Gulf of Alaska/Northeast Pacific upper trough will steadily amplify mid to late week and then upstream energy will drop southward, most likely closing off an upper low well off the West Coast next weekend. The primary forecast consideration continues to be that the latest GFS runs are more aggressive than most other guidance to bring leading eastern Pacific trough energy through the West and into the Plains Friday and into the weekend. The ECMWF/CMC are about 12 hours slower, and better supported by the ensemble means (which may be a hair faster). Better than average model agreement to start led to a general deterministic model blend for days 3 and 4 for the WPC fronts/pressures progs. Beyond that, as spread increases both with the Eastern and Western U.S. systems, favored a blend of the ensemble means, though continued with a fair amount of deterministic ECMWF/CMC even into day 7 to maintain some definition of features. This fit well with previous WPC continuity. ...Weather/Hazard Highlights... Heavy rainfall across the Southeast at the end of the short range period should be mostly winding down by the beginning of the medium range period. Heavy rain potential along the Southeast Coast will continue into Thursday as the surface low exits into the Atlantic. Northern extent of the heavy rainfall into the southern Mid-Atlantic is highly depending on exact surface low track. The Storm Prediction Center continues to highlight severe weather potential from southern South Carolina to northern Florida along the surface cold front. The system supported by the amplifying northern stream trough reaching the East late this week will brush parts of the Upper Great Lakes with some rain/snow and then trailing cold cyclonic flow should promote a period of lake-effect and terrain-enhanced snow between the central Appalachians and New England. Uncertainty is too great at this time to specify effects from deepening Atlantic low pressure Fri-Sat. There is greater confidence in a period of brisk to strong winds in the gradient between the low and surface high to the west, versus coverage/intensity of any snow, given the current spread for low track. Over the western states precipitation will initially focus over the Northwest, associated with low pressure that develops off British Columbia. A broader moisture shield will likely spread across the West by next weekend as some eastern Pacific upper trough energy ejects inland. Highest totals should align with favored terrain from the Sierra Nevada into the northern-central Rockies. As the energy enters the central U.S. on day 7, precipitation chances should increase across the Central/Southern Plains. A large portion of the lower 48 should see above normal temperatures for the overall Thur-Mon period. Expect the highest warm anomalies to be over the northern-central Plains Fri into the weekend as the pattern becomes favorable for some areas to reach at least 15-25F above normal. Much of the West will see readings 5-15F above normal and locally higher, though clouds/precip should keep highs moderately below normal over the Pacific Northwest. The South will see morning lows up to 10-20F above normal early Thursday in the warm/moist flow ahead of the storm system affecting the region while northern parts of the East will likely see min temps 10F or more above normal late in the week. Cool air behind the Eastern U.S. through should keep daytime highs moderately below normal Friday into the weekend for the East Coast states. Santorelli 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