Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Apr 01 2019 Valid 12Z Thu Apr 04 2019 - 12Z Mon Apr 08 2019 ...Overview... The forecast of the overall pattern has changed little over recent days, with a steady progression of Pacific features into the West and continuing onward producing one or more episodes of active weather over a decent portion of the Lower 48. Heaviest precipitation during the Thu-Mon period should extend from the central/northern West Coast into the northern Rockies as well as over and slightly east of the southern Plains/Lower Mississippi Valley. The southern third of the West has the best potential to stay dry during the period. Parts of the High Plains may see little precipitation as well. ...Guidance Assessment/Preferences... Latest models and ensembles have been remarkably stubborn in maintaining their spread for the exact details of diffuse shortwave energy aloft and associated surface low pressure tracking across the eastern half of the country late this week into the start of the weekend. As has been the case in recent days there is decent clustering for low pressure over the south-central Plains as of early Thu. Then individual models/ensemble members rapidly diverge for the latitude of best-defined low pressure farther east. In forecasts valid early Fri the one trend of note is a reduction of ensemble members in the northern part of the spread in the Great Lakes. On the other hand the 12Z ECMWF/CMC are in the southern part of the spread. By early Sat there is still the potential for low pressure to be anywhere from northern New England to southern Mid-Atlantic. Confidence remains low for system specifics due to the sensitivity to small scale details aloft that include a combination of initial Plains shortwave and northern tier energy that could interact by Fri. Preference is to minimize continuity changes until a better consensus emerges, yielding a surface wave tracking through the Ohio Valley and northern Mid-Atlantic. In spite of the model spread, a multi-model blend ended up providing a reasonable template for the desired evolution. Farther westward the guidance has been fairly consistent in weakening shortwave energy that crosses the Northwest and heads into a downstream ridge, while a southern stream shortwave cross the Southwest U.S./northern Mexico and then the southern Plains into Mississippi Valley. A fairly strong northeastern Pacific storm should bring a decent front (with vigorous upper shortwave support) into the West by Sat, potentially followed a day later by another system tracking near/northwest of Vancouver Island and bringing a front/energy aloft into the Northwest. By the latter half of the period there is still a rapid increase in spread for details of flow aloft from the Pacific into the western U.S. In forecasts valid next Mon the GFS/GEFS show the most upper ridging over the Interior West while the past couple ECMWF runs and 12Z CMC depict flat or even slightly cyclonic flow. The 12Z ECMWF/CMC means show a modest ridge. Teleconnections relative to a negative height anomaly center over the west-central Pacific in D+8 charts and a positive center east of northern Greenland hint at a split flow type of pattern over western North America. The established pattern of eastern Pacific troughing and relative agreement among the means for a western ridge ultimately lead to a preference close to the means. Including some weight of the differing 18Z GFS/12Z ECMWF for the full forecast domain essentially balances out near the desired scenario over the West. The specifics of flow over Pacific/western U.S. late in the period will affect how energy heading into the West on Sat will ultimately interact with the southern stream shortwave reaching the Plains during the weekend. It will likely take a while to resolve this aspect of the forecast. Currently the eventual combination of shortwaves is most likely to support a surface wave tracking from the southern Plains into Midwest during Sun-Mon. ...Weather Highlights/Hazards... Aside from the typically variable finer details, there is good continuity in the forecast of multiple periods of significant rainfall/higher elevation snow from northern California/Sierra Nevada and Pacific Northwest into the northern Rockies. Highest five-day totals will likely be over favored terrain in the West Coast states with somewhat less moisture reaching the Rockies. Heaviest precipitation should occur along the West Coast during Fri-Sun. Meanwhile the system tracking out of the Plains Thu onward will spread a decent moisture shield across the eastern half of the country but with continued lack of confidence in specifics. Today there is somewhat more of a signal for heavy rainfall potential over the Lower Mississippi Valley/Gulf Coast region, and some convection over this area could be strong to severe. Check SPC outlooks for latest info on severe threats. Some snow may be possible from the central Great Lakes into New England. Behind this system, most guidance continues to suggest a heavy rainfall threat next weekend over the southern Plains/Lower Mississippi Valley. This activity will be encouraged by the approaching southern stream shortwave aloft and a healthy flow of Gulf moisture interacting with a stalled front that should return north as a warm front. Rainfall of varying intensity should also spread farther northward across the central and eventually eastern U.S. Temperatures will vary with the progression of individual systems but there will be a noticeable trend over time toward greater coverage of above normal readings across the Lower 48. The one exception should be over the northern and central parts of the West where clouds and precipitation will tend to keep highs near to below normal. Highest anomalies (plus 10-20F with some higher values especially for min temps) will be most likely from the northern half of the Plains into east-central U.S. during Sat-Mon. A large part of the West should see plus 5-15F anomalies for min temps as well. 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