Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1152 AM EDT Wed Apr 3 2019 Valid 12Z Sat Apr 6 2019 - 12Z Wed Apr 10 2019 15Z Update: Similar to yesterday, there remains considerable model spread across the eastern half of the U.S. regarding the structure and evolution of the low pressure system crossing the Great Lakes region and its associated frontal boundaries. Teleconnections with the positive anomaly over Greenland is supportive of a trough over the western Pacific and also over Quebec, Canada. The FV3 GFS was substituted for the operational GFS for this forecast as part of the overall blend given larger run-to-run differences with the operational GFS. The UKMET and CMC begin exhibited differences from the ensemble means as early as this weekend regarding the trough over the central U.S. and Midwest states. Given lower than average confidence for the latter half of the forecast period, the WPC forecast remained fairly close to previous continuity while also taking into account the EC mean, GEFS mean, FV3, and ECMWF. Hamrick ...Overview... Expect the continuation of a moderately progressive pattern dominated by Pacific flow from the weekend into the middle of next week. There two largest scale features of note will be western U.S./Rockies energy that should lead to a trough reaching the East by next week and an upstream trough most likely moving into the West next Tue-Wed. Between these two features, fast flow will bring multiple impulses across the Pacific Northwest. Guidance has been consistent in highlighting an area from the northern/central West Coast eastward into the Rockies for significant precipitation. The upper trough progressing into the East will spread a broad shield of moisture across the eastern half of the country but with a lot of uncertainty over intensity and duration of precipitation at any particular location. ...Guidance Assessment/Preferences... The details are as uncertain as ever for evolution of the overall trough forecast to progress eastward from the western U.S./Rockies Sat onward. At the start of the period this trough should consist of a combination of southern stream energy reaching the southern Rockies/northern Mexico along with strong northern stream shortwave energy pushing through the Northwest. Operational models as a whole are signaling a decent potential for an embedded closed low to form by Sun-Mon but with tremendous north-south spread for the path of such a low. Solutions among latest runs valid 12Z Mon range between the Canadian border (12Z ECMWF and 00Z FV3 GFS) and central/south-central Plains into Mississippi Valley (recent GFS runs and 00Z UKMET). The old 00Z/02 ECMWF run in fact had a compact upper low tracking even farther south. The wide spread in guidance required earlier than usual inclusion of a significant percentage of the ensemble means to provide stability for the deterministic forecast--even if confidence in the exact ensemble mean scenario is not particularly high either. Based on the ensemble mean forecast a complex area of Plains low pressure should consolidate as it tracks into/across the Great Lakes-Northeast during the first half of next week. Low track would differ significantly if some of the operational runs were to verify. Over the eastern Pacific/Western U.S. during Mon-Wed, faster/more amplified trends in the GFS/GEFS mean over the past day have narrowed the guidance spread for the trough expected to move into the West. Through the 12-18Z cycles the GFS/GEFS mean still seemed to be in the trending process, while teleconnections relative to prominent height anomaly centers over the western Pacific (negative) and Northeast Atlantic/Greenland (positive) have been promoting height falls reaching the southern half of the West. Thus the forecast tilted more toward the ECMWF mean/ECMWF/NAEFS mean. The 12Z CMC was on the extreme side of the envelope for trough amplitude. The new 00Z GFS/GEFS mean appear closer to forecast preference. Also worth noting, guidance rapidly diverges for details of fast North Pacific flow by next Mon and these differences migrate into the northeastern Pacific by Tue-Wed. Issues across those areas may also affect the forecast for the West at that time. ...Weather Highlights/Hazards... Within the favored area for significant rainfall and higher elevation snow from the northern/central West Coast into the Rockies, highest totals will likely occur over northern California and western Oregon/Washington where the Sat-Mon period should feature strong flow aloft with multiple embedded shortwaves. Predictability in small-scale details is low but there is potential for one or more narrow bands of very focused precip. By Tue-Wed the upper trough moving into the West will shift the focus for rain/snow farther inland--most likely across the north-central Intermountain West and Rockies. Some of this moisture may reach the central Plains as well. Eastern slopes of the Rockies could see activity enhanced by low level upslope flow around midweek. Strengthening low level flow of moisture from the Gulf of Mexico ahead of the upper trough emerging from the West/Rockies will spread a broad area of precipitation from portions of the Plains through the East during the period. There is at least a moderate degree of consensus that areas from the eastern half of Texas through the Lower Mississippi Valley and possibly into the southern Appalachians will see a threat of heavy rainfall. There are indications that the Northeast may see significant amounts as well. Otherwise the wide disparity in guidance for specifics of upper level/surface evolution will make it difficult to have much confidence in determining areas that will see highest precip totals. Parts of New England may see wintry weather with this system. Over the West expect modestly below normal daytime highs from the West Coast into northern Rockies. The Southwest/southern Rockies into Great Basin will see a brief period of warmth Sun-Tue (some plus 10F or greater anomalies) before upper troughing expands the coverage of below normal temperatures over the West by midweek. Most of the West should see above normal mins during the period. Farther eastward expect an area of much above normal min temps to progress from the Plains through the East Sat-Tue. Plus 15-25F anomalies may challenge daily record warm low values at some locations. Anomalies should be less extreme for highs. Cool flow behind the system crossing the East should bring some modestly below normal highs into the northern half of the central/eastern states by next Wed. 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