Extended Forecast Discussion NWS Weather Prediction Center College Park MD 338 PM EDT Tue Apr 16 2019 Valid 12Z Fri Apr 19 2019 - 12Z Tue Apr 23 2019 ...Heavy rain threat for parts of the east Friday... ...16Z update... The narrow trough moving across Mississippi Valley on day 3 appeared to be better handled by the 00Z and 06Z GFS than in previous runs. Deterministic models and means across keep one trough in both the northern and southern stream at day 3, with the vort max then spinning into an upper low in the southern stream by day 4. Thus the medium range forecast used a multi-model blend through day 3-4. This pattern still supports heavy rain across the East early in the period. Split flow remains an issue with the latest guidance across the western/central U.S., and once again leaned away from the operational GFS during the later forecast period as it strengthens and slows down a trough in the western U.S., unlike consensus and suggestion from teleconnections. Favored the EC/NAEFS/GEFS means with some element of the operational 00Z ECMWF. For details on sensible weather, please see the previous discussion. Tate ...Previous Discussion... ...Overview... Quasi-progressive pattern for the medium range as atmospheric spindown continues during this transition season. Deep trough/closed upper low will push east of the Mississippi late this week and perhaps cut off from the flow off the Mid-Atlantic/New England coast as western troughing (from both the subtropics and the mid-latitudes) push out of the Rockies and onto the Plains next Mon/Tue. Heavy rain threat at the end of the short range will shift to the Appalachians --> coastal plain on Friday into early Saturday. ...Guidance Preferences... Models and ensembles continued to struggle with exactly how the eastern system evolves during this period, contingent upon the degree of northern/southern stream separation and exact track of the upper features. 12Z/18Z GFS runs seemed to depart most from the otherwise modest clustering of the ensembles and 12Z ECMWF/UKMET/Canadian with the track of the upper low (GFS was farther north and thus quicker to bring the system back into the westerlies than the ECMWF-led cluster). Preferred to side with the non-GFS guidance given the trend in especially the ECMWF ensembles but also the GEFS members. Nature of the flow does not instill high confidence in the details given the sensitivity to shifts in the short range forecast, so the GFS was still within the realm of possibilities. To the west, ensemble consensus has proven its worth in minimizing shifts in the forecast over the past several days. Again here, the GFS departed from the majority consensus with the trough in the Southwest late Sunday (slower vs a quicker consensus). With split flow out of the Northeastern Pacific, uncertainty increases in the forecast, necessitating an increase in the ensembles. ...Weather Highlights/Threats... Deterministic models continue to paint stripes of modest/heavy rainfall northeastward along and east of the Appalachians as the flow becomes increasingly meridional. This should focus a relatively narrow but intense plume of moisture from the subtropics (Yucatan/Cuba/Greater Antilles) along/ahead of a cold front. With PW anomalies +3 to +4, rainfall could be lead to flooding concerns, especially in urban areas. Severe weather threat will accompany the front, likely focused south of the Mason-Dixon line (please consult the SPC outlooks for more information). GEFS mean QPF continues to be much higher than typical values for this lead time in mid-April, suggesting a higher likelihood of an anomalous event. Mild temperatures will precede the front (10-15F above average) but will be replaced by cooler than average temperatures after its passage. That will be short-lived (1-2 days) as milder air in the west Friday pushes through the Plains this weekend and into the east early next week. Fracasso 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 Hazards: - Heavy rain across portions of the Southern Plains, Tue, Apr 23. - Heavy rain across portions of the Northeast, the Central/Southern Appalachians, the Mid-Atlantic, the Southeast, the Great Lakes, and the Ohio Valley, Fri-Sat, Apr 19-Apr 20. - Severe weather across portions of the Southeast, the Central/Southern Appalachians, and the Mid-Atlantic, Fri, Apr 19. - Flooding possible across portions of the Northeast, the Northern Plains, the Tennessee Valley, the Southern Plains, the Northern Great Basin, the Upper Mississippi Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Northeast, the Northern Plains, the Mississippi Valley, and the Ohio Valley. - Flooding likely across portions of the Southeast, the Mid-Atlantic, the Northeast, the Upper Mississippi Valley, and the Northern Plains. - High winds across portions of the Central/Southern Plains, Sun-Mon, Apr 21-Apr 22. 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