Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Apr 02 2019 Valid 12Z Fri Apr 05 2019 - 12Z Tue Apr 09 2019 ...Overview... Expect continued moderate progression of Pacific flow into/across the Lower 48 during the period. This pattern should bring multiple episodes of precipitation to areas from the northern-central West Coast into the northern Rockies, while the eastern half of the country will likely see two broad areas of moisture--one with a fairly weak leading system crossing the East and another with a system tracking from the southern Plains northeastward during the weekend and early next week. To the east of the Rockies, best potential for highest rainfall totals should be within an area extending from Texas through the Lower Mississippi Valley into the Southeast/southern Appalachians. Some areas farther north could see significant rainfall as well. ...Guidance Assessment/Preferences... During the first couple days or so of the period a multi-model/multi-run consensus (past two 12-hourly ECMWF runs and 6-hourly GFS runs through the 12-18Z cycles, with some 12Z CMC included) represented the most likely solution across the Lower 48 and vicinity. Guidance agrees in principle on individual features but shows typical differences for smaller details that have low predictability. The system initially over the East as of Fri still looks diffuse aloft and as a result models are having trouble resolving surface specifics. Long-term trend has been toward a weaker surface reflection which was advertised earlier by ECMWF-based guidance. There is still a lot of spread though. Meanwhile a strong storm to the west/northwest of the Pacific Northwest will bring a front into the West Fri-Sat and into the Plains by Sun. Trailing bursts of energy aloft/surface features should continue into the Northwest through the weekend. The 12Z UKMET seemed to differ the greatest from other solutions for details over the eastern Pacific/Northwest U.S. and thus was excluded from the blend. Leading shortwave energy over the West late this week will focus in the southern stream as it heads east, reaching the southern Plains by Sat. By the latter half of the period there are two prominent overall uncertainties: 1. How energy reaching the West on Sat ultimately evolves and interacts with leading southern stream shortwave energy over the Plains--affecting the forecast over the eastern half of the country, 2. The strength of ridging that builds into the West by Sun-Tue and resulting speed of an upstream eastern Pacific trough. For this latter western issue, the majority of guidance shows less pronounced western ridging and faster Pacific trough versus the GFS/GEFS runs from the past couple day. A positive height anomaly center to the east of Greenland and a negative height anomaly center over the west-central Pacific in recent D+8 charts teleconnect to a split-flow pattern over western North America (mean ridging generally confined to Canada and northern parts of the western U.S.). This seems to favor an evolution closer to the ECMWF/CMC and their means, at least in terms of having lower heights than the GFS/GEFS over southern parts of the West. Farther east GFS runs vary from consensus/means in varying ways, the 18Z GFS straying on the fast side and the new 00Z run becoming much stronger with northern tier energy aloft. However confidence in other model details is not great either. These considerations favor trending the forecast toward a blend of recent ECMWF runs, 12Z CMC, and 12Z NAEFS/ECMWF means. The NAEFS provides a modest hedge to account for the possibility of slightly slower Pacific/western U.S. progression. ...Weather Highlights/Hazards... Continue to expect periods of enhanced rainfall/higher elevation snow from the central/northern West Coast into the northern Rockies. Some locations along the most favored terrain from northern California/Sierra Nevada into western Washington/Oregon may see at least several inches liquid over the Fri-Tue period. The northern Rockies should see lesser but still significant totals. Individual systems affecting the Pacific Northwest coast may also be accompanied by periods of strong winds. Confidence in precipitation coverage and intensity decreases during the first half of next week as guidance diverges for amplitude/timing of significant features. Leading area of precipitation over the East, possibly including some snow over New England, should depart after Fri night. Some pockets of locally heavier precipitation are possible but there is not a clear signal for where such activity may occur. From Fri night onward the shortwave aloft approaching/reaching the southern Plains along with strengthening flow of Gulf moisture interacting with a stalled front will likely lead to a heavy rainfall threat over the southern Plains/Lower Mississippi Valley and possibly extending to the east/northeast. Low pressure tracking northeastward from the southern Plains should eventually help to spread moisture over much of the eastern half of the country. Depending on how this system evolves there may be some bands of significant rainfall in middle/northern latitudes between the Plains and East Coast. Plains/Mississippi Valley areas sensitive to additional rainfall should monitor forecasts for this system. The overall pattern should favor modestly below normal highs over the West Coast states into the northern Rockies but with a warming trend over southern areas. Lows should be above normal through the period (some plus 10-15F anomalies). Most areas east of the Rockies should see above to much above normal temperatures, though with a cooling trend behind the front reaching the eastern states late in the period. Morning lows may be 15-25F above normal over parts of the northern Plains and Midwest around Sun-Mon with some daily record warm low values possible. 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