Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Mar 31 2019 Valid 12Z Wed Apr 03 2019 - 12Z Sun Apr 07 2019 ...Overview... Today's guidance remains fairly agreeable and consistent in showing a progression of shortwaves from the Pacific into and across portions of the Lower 48, with a corresponding series of surface systems. The northwest periphery of strong low pressure off the East Coast on Wed may have some influence on New England's coastal areas as the system tracks northeastward. Then shortwave energy over the West on Wed will make steady progress and push a wave/frontal system across the central/eastern states during the latter half of the week. The northern part of incoming Pacific energy Thu-Fri should weaken as it heads into a developing western Canada ridge but the southern part will likely continue across the southern U.S. and northern Mexico. Then a fairly strong shortwave should reach the West Coast around Sat with continued progression thereafter, perhaps followed by yet more energy brushing the Pacific Northwest late next weekend. The combination of these systems will likely bring precipitation to most of the CONUS at some point during the Wed-Sun period, with the southern third or quarter of the West the most likely area to stay dry. ...Guidance Assessment/Preferences... Model/ensemble clustering has improved dramatically for the storm off the East Coast as of early Wed, and with somewhat of a westward adjustment from 24 hours ago after that time. However note there are some smaller but locally significant differences that persist even in the short-range time frame. Otherwise guidance continues to agree fairly well with the existence of each feature in the Pacific/CONUS stream but displays considerable spread/uncertainty for specifics. This favors keeping an approach that emphasizes components of the operational models during the first half of the period followed by a transition toward more ensemble mean input as operational details increasingly diverge. The shortwave and associated low pressure reaching the Plains around Thu agrees fairly well in the guidance up to that time. However after early Thu solutions rapidly go astray for exactly how energy will be distributed within the overall trough aloft--along with strength and track of best-defined surface low pressure. Fast moving northern tier energy may also feed into the upper trough as it reaches the eastern U.S., providing added complexity to the forecast. This type of evolution tends not to be easily resolved in the extended time frame. By early Fri ensemble members are still widely varied north-south (northern Gulf Coast states into the Upper Great Lakes). At that time the 12Z ECMWF is in the southern third of the spread while the best density of models/members is between the Ohio Valley and Lower Great Lakes. Established consensus has generally held within that area but with some north-south waffling from day to day. Further lowering of confidence near the East Coast by Sat favors a model/mean blend that is fairly close to continuity. Farther upstream a model/mean blend represents established consensus with little continuity change for the southern stream Pacific shortwave that eventually heads into the southern Plains, as well as for the vigorous shortwave reaching the West Coast around early Sat. Models and ensemble spaghetti plots show a more rapid increase in detail spread over the entire eastern Pacific through Plains area by next Sun. This favors increasing weight of the similar 18Z GEFS/00Z ECMWF means toward the end of the forecast period. Note that the 12Z ECMWF leans a bit to the strong/slow side of the spread for the energy over the West by Sun. ...Weather Highlights/Hazards... The series of systems affecting the West will bring at least two or three periods of enhanced rain and high elevation snow. Expect highest totals for the five-day period to be along favored terrain from northern California/Sierra Nevada through the Pacific Northwest. Moisture will extend through the Intermountain West and northern-central Rockies, with some significant precipitation over these areas as well but with less extreme totals relative to areas near the coast. The system emerging into the Plains on Wed and reaching the East Coast toward the end of the week will spread a broad shield of precipitation across the eastern half of the country. There is still less than desired agreement on areas of heaviest rainfall but at least localized pockets of moderate to heavy rainfall may be possible over parts of the sensitive eastern Plains/Mississippi Valley region and then into the Northeast. The northern periphery of the moisture shield may contain snow with best potential from the north-central Plains into New England. Today's guidance is showing a stronger signal for heavy rainfall potential over parts of the southern Plains and possibly into the Lower Mississippi Valley from about Fri night through the weekend. This activity should be encouraged by the combination of an approaching southern stream shortwave aloft and low level Gulf inflow interacting with a front expected to stall near the Gulf Coast. During the day Wed most precipitation with the western Atlantic storm should be mostly confined to areas near the New England coast. Steady progression of systems across the Lower 48 will provide some day-to-day variability of temperatures. Within this pattern the general evolution of mean flow will likely be toward more above average heights aloft with time, corresponding to increasing coverage of above average temperatures from midweek through the weekend. The areas of clouds and precipitation accompanying each system will tend to favor greater warm extremes for morning lows with some areas of plus 10-20F anomalies. In terms of five-day averages the Interior West into the Plains should see warmest temperatures relative to climatology. Plus 10F or greater anomalies for highs are most likely over the parts of the Plains Fri-Sun. Rausch 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