Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EDT Thu Mar 21 2019 Valid 12Z Sun Mar 24 2019 - 12Z Thu Mar 28 2019 ...Overview and Guidance Assessment/Preferences... Anomalous upper-level ridging extending from western Canada into the north central U.S. this weekend is forecast by gradually break down during the early to middle portion of next week. Some degree of ridging appears likely to become reestablished from the Gulf of Alaska north across Alaska - a position much more common in recent months. Mean troughing should persist across the north central Pacific, bringing a continuation of the recently seen pattern with Pacific shortwave energy crossing the CONUS every couple days. The reconfiguration of flow at the higher latitudes will have some implications farther south, with less split flow and greater likelihood of phased systems across the central U.S. by mid to late next week. In the meantime, one significant southern stream system remains during the medium range, affecting the south central and southeastern U.S. Sun-Tue. Models continue to struggle with the system crossing the southern U.S. from Sun onward, and the potential evolution of a coastal low along/off the Eastern Seaboard early-mid next week. In general, guidance over the past day has trended toward a quicker eastward progression, and somewhat less likelihood of a significant low along the Southeast/Mid-Atlantic coast early next week. Ensemble spread remains quite large, however. At 12Z on Tue, for example, ensemble members range from a low still over the Mississippi River, to others having energy already transferred to a new low off the Southeast coast. While the consensus of solutions has shifted faster, spread also seems to have increased over the past few ensemble cycles. Thus, confidence is low, and would expect continued run-to-run variability. Opted to tweak the forecast toward a faster progression rather than making a dramatic change given the low confidence. This resulted in a blend weighted a bit more toward the slower solutions (12Z ECMWF/CMC) and not much weight on the GFS which continues to lead the pack in terms of eastward progression of the system and quickly moving any low out into the open Atlantic early next week. Given significant uncertainty as to how quickly any low along the Southeast coast by the middle of next week moves out to sea or, perhaps up the coast as the 12Z CMC and a handful of ensemble members show, maintained at least low PoPs for portions of the Southeast/Mid-Atlantic into midweek. Elsewhere, models show better consensus that one weakening shortwave should reach the West Coast late Mon, with a more significant wave arriving late Tue, moving into the Great Basin or Rockies Wed night-Thu. Models handle the first wave similarly, but some larger differences emerge by the middle of next week with the second system. Some timing differences emerge with the second system Wed-Thu. Differences also exist as to structure of the north Pacific ridge, with impacts on how/if the second shortwave interacts with additional northern stream energy. In general, a more ensemble-based approach was taken to the forecast by Wed-Thu, with increased weight on the ECENS and NAEFS ensemble means. ...Weather Highlights and Hazards... The southern stream system is expected to bring rain and thunderstorms from portions of the Southern Plains and lower/mid-Mississippi Valley to the Southeast. Locally heavy rain is possible, but no strong signal for excessive rains is evident in the guidance at this point given the trend toward increased forward speed of the system. Cold temperatures on the northern end of the precipitation shield may marginally support snow or a rain/snow mix for some areas from the central Appalachians and Mid-Atlantic, and perhaps into the Northeast. Meanwhile, the West will see a couple rounds of precipitation from the incoming Pacific systems, bringing rain and mountain snows to much of the central/northern Rockies and from central California to the Pacific Northwest. The arrival of the more significant system by the middle of next week produces a stronger signal for potentially heavy precip (rain and mountain snow) across portions of central and northern California. Given the increasingly progressive flow pattern across the CONUS, temperatures will be highly variable, with much of the country seeing both above and below average temperatures at different points during the extended period. Ryan 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