Extended Forecast Discussion NWS Weather Prediction Center College Park MD 419 PM EST Wed Feb 27 2019 Valid 12Z Sat Mar 02 2019 - 12Z Wed Mar 06 2019 ...Pattern Overview... Broadly cyclonic mean flow across the CONUS at the start of the weekend should transition toward increasingly amplified troughing over the East by the middle of next week as an upper ridge reaches the Rockies and a trailing trough approaches the West Coast. ...Guidance Evaluation/Preferences... Forecast was kept fairly close to continuity in this update with only minor adjustments. Models continue to show good agreement at the larger scales through the period, but with some significant differences surrounding smaller-scale features. Fast-moving and low amplitude shortwave energy arriving at the West Coast on day 3 (Sat) is key in the evolution of a wave of low pressure across the eastern U.S. Sun-Mon. The deterministic 00Z ECMWF was a bit faster with this energy than the consensus throughout this time frame, which ends up resulting in a surface wave tracking farther north/west across the eastern U.S. by Sun. Models have shown a trend toward a less suppressed system across the East Sun, with the GFS lagging the trend. The solutions best clustered with the ensemble consensus throughout with respect to this feature seemed to be the 00Z UKMET/CMC and 06Z FV3-GFS, and a blend of these solutions formed a basis for the forecast during days 3-5. The eventual track of this surface low by Sun-Mon will have significant implications in terms of p-type for portions of the Mid-Atlantic and Northeast, including the major cities, the forecast for which is highly uncertain at this time. Later in the forecast period, as arctic high pressure continues to spread cold air south into the central and portions of the eastern U.S., the most active weather will shift to the West coast, with general consensus that a low pressure system should approach the coast by the middle of next week. Solutions differ rather substantially on timing, however, while the ensemble means show a bit more consensus. Thus, the forecast during days 6-7 (Tue-Wed) placed heavy emphasis on the 00Z ECENS/06Z GEFS ensemble means. ...Weather Highlights/Hazards... Across the West the primary axis for heaviest rain and higher elevation snow will extend from California eastward through the central/south-central Rockies with the Sierra Nevada likely seeing the highest 5-day totals. Two systems will affect the region during the period, one arriving early in the weekend and spreading moisture rapidly eastward then followed by a second arriving Tue-Wed. As moisture from the second system spreads northward there may be potential for wintry precipitation at low elevations over interior portions of the Northwest given the cold air likely to be in place. The system affecting the Northeast during the weekend should bring some snowfall to the region but weakening and possibly offshore trends for low pressure as well as fast progression may limit accumulations. Southern stream energy interacting with the trailing front may produce a period of heavy rainfall over parts of the Southeast with the weekend into early Mon time frame the most likely period for this activity. Guidance has not yet fully converged for details so confidence is only moderate thus far. Snow will be possible in the northern part of the moisture shield--from the south-central Mississippi Valley into the Mid-Atlantic. Lighter rainfall could linger over parts of the extreme Southeast/Florida into next week. Within the widespread area of extremely cold air over the central U.S., expect the most extreme anomalies to be over the northern-central Plains during Sat-Mon with anomalies of 30-45F below normal. Especially on Sun-Mon many places over the Plains and Midwest should challenge or exceed daily record values--especially for cold highs but also for morning lows at some locations. Less extensive coverage of daily records may continue into Tue. The eastern U.S. will see a modified form of this cold air by the first half of next week with readings "only" 10-20F below normal. Localized parts of the interior Northwest may see temperatures 10-30F below normal. Relative warmth will be confined to the southern half of the West and more so for morning lows than afternoon highs. Ryan/Rausch Hazards: - Heavy precipitation across portions of the Northeast, the Central Appalachians, the Middle Mississippi Valley, the Mid-Atlantic, the Great Lakes, and the Ohio Valley, Sun-Mon, Mar 3-Mar 4. - Heavy rain across portions of California, the Central Great Basin, the Pacific Northwest, and the Southwest, Sat-Wed, Mar 2-Mar 6. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Sat-Sun, Mar 2-Mar 3. - Heavy snow across portions of the Northeast, the Central Great Basin, the Pacific Northwest, the Northern Great Basin, California, the Great Lakes, and the Southwest, Sat, Mar 2. - Heavy snow across portions of the Southern Rockies, the Central Rockies, the Central Great Basin, the Central Plains, and the Southwest, Sat-Sun, Mar 2-Mar 3. - Flooding possible across portions of the Southeast and the Mid-Atlantic. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, the Mid-Atlantic, the Upper Mississippi Valley, the Southeast, the Pacific Northwest, and the Ohio Valley. - Flooding likely across portions of the Southeast, the Mid-Atlantic, and California. - Much below normal temperatures across much of the eastern two-thirds of the country including the interior sections of the Pacific Northwest, Sat-Wed, Mar 2-Mar 6. - High winds across portions of mainland Alaska and the Aleutians, Mon-Tue, Mar 4-Mar 5. - High winds across portions of the Aleutians, Sun-Mon, Mar 3-Mar 4. and Wed, Mar 6. - High significant wave heights for coastal portions of mainland Alaska, Mon-Tue, Mar 4-Mar 5. 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