Extended Forecast Discussion NWS Weather Prediction Center College Park MD 401 PM EDT Fri Mar 29 2019 Valid 12Z Mon Apr 01 2019 - 12Z Fri Apr 05 2019 ...Nor'easter likely for midweek, but with a large timing discrepancy... ...Overview... Strong upper ridging over Alaska will slowly migrate into northeastern Russia, favoring northern stream troughing across much of Canada and initially over the eastern CONUS but then across the central CONUS late next week. With the block in the northeastern Pacific, southern stream systems will move through the Great Basin/Four Corners region and across the mid-Mississippi and Tennessee valleys. This supports a lead East Coast Nor'easter (still encumbered in uncertainty) at the same time as a western system moves into the Plains. ...Guidance Assessment/Preferences... Within the fairly agreeable mean pattern there are some significant issues with specifics. Differences in handling the initial Alaska ridge play at least a partial role in the divergence of solutions for flow across southern Canada and northern U.S. At question is the shape/strength of the northern stream and its interaction with a shortwave moving through the Southeast on Tuesday. This feature has had poor continuity back several days and, coupled with uncertainty in the northern stream, has led to high uncertainty. Ongoing WPC forecast has always had this coastal system lying just off the coast after organizing east of GA/FL but it was unclear if it would lift northeastward close to the New England coast or only slowly saunter east-northeastward toward Bermuda if the northern stream trough passed by to its northeast. Trend in the ensemble guidance favors a closer/coastal track though the 00Z UKMET/Canadian were notable exceptions on the far southern side of the envelope of solutions. In addition, the quicker speed tied to the northern stream may allow to system to race northeastward much quicker than previously forecast, but did not jump to the quickest solutions (00Z ECMWF/ECMWF ensemble mean and 06Z FV3-GFS) given the spread/continuity. As such, the 00Z/06Z GFS represented a best intermediate solution for now with the system. Looking farther westward, most solutions expect energy offshore British Columbia to progress through the West around Tue-Wed and the Plains/Mississippi Valley Thu-Fri. At the surface low pressure will likely track into the Northwest early in the week and then a wavy frontal system should reach the central states around Thursday. With continued differences in the deterministic models at the surface and aloft by the latter half of the period, favored greater emphasis on the ensemble means at that time. In the wake of this system expect a modest ridge to build into the West Coast/Interior West while more trough energy pushes into the eastern Pacific. The ensemble means were fairly close and merited the greatest weight in the forecast by late in the period. ...Weather Highlights/Hazards Expect rain/higher elevation snow to spread through the northern half to two-thirds of the West in the Mon-Wed period, followed by additional precipitation reaching the West Coast late in the week. Highest precipitation totals are most likely over northern California/Sierra Nevada with enhanced activity reaching into the northern/central Rockies. Locations across the South and East Coast regions may see a period of rain (and quite possibly some snow over higher elevations) with the system forecast to track just off the East Coast. Wide guidance spread for track and timing keeps confidence low with respect to precipitation type, coverage, intensity, and timing. Pockets of locally moderate/heavy rainfall may be possible and perhaps snowfall along and east of the Appalachian crest. Given that it will be early April, temperatures will be marginal and time of day of precipitation will influence actual accumulation. Areas of western NC, southwestern VA, and eastern WV were highlighted in the day 4-7 Winter Weather Outlook probabilities as well as higher terrain of the Northeast. The system emerging from the West may spread precipitation along a wavy frontal system into the Plains/Mississippi Valley region during the latter half of the week. Thus far signals for significant amounts are not agreeable enough to provide meaningful confidence in specifics. Temperatures will vary in accordance with system progression. The South and East will be on the chilly side early in the week with some areas of minus 10-20F anomalies in the wake of the coastal system. Then above normal readings (mostly plus 5-15F anomalies) should progress from the West and into the central/eastern states over the course of the week as heights rebound. Fracasso/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