Extended Forecast Discussion NWS Weather Prediction Center College Park MD 338 PM EDT Wed Mar 13 2019 Valid 12Z Sat Mar 16 2019 - 12Z Wed Mar 20 2019 ...Overview... Guidance continues to show a strong ridge aloft that will build into western Canada and drift slowly eastward with time. Corresponding to this ridge the multi-day means at D+8 are remarkably similar in showing a strong positive height anomaly center over the northeast corner of Alberta. Teleconnections on this center support the consensus idea of a broad eastern North America trough. The western U.S. will likely see some degree of mean ridging aloft but mixed with one or more smaller scale shortwaves. Meanwhile some guidance is hinting that a trough aloft approaching the West Coast next week may try to separate and lead to an upper low reaching near California. Southern stream flow will carry along one or more shortwaves through the Southwest U.S./northern Mexico and locations to the east. ...Guidance Assessment/Preferences... While the forecast large-scale pattern evolution is agreeable, there are still meaningful uncertainties with embedded shortwaves. One shortwave possibly reaching the Mississippi Valley region around day 4 Sun may reflect at the surface as a leading wave/frontal system that crosses the eastern half of the country. CMC/UKMET runs have generally been on the strong side of guidance with this shortwave while latest ECMWF runs are on the faster/weaker side of the envelope. Fairly low predictability given the feature's scale favors maintaining an intermediate solution. Another ongoing question mark involves the weak Pacific energy heading into/around the mean ridge during the weekend--along with the exact character of the ridge. Over the past day the guidance has become more committed to the idea that the ridge will briefly close off a high over southwestern Canada early next week. This would allow for some southwestward elongation of shortwave energy after it passes by the ridge axis, as suggested by some earlier GFS/GEFS runs. However it remains to be seen whether as much energy gets pulled back as depicted in 00Z GFS/06Z FV3 GFS runs. Differences in specifics early in the week lead to spread for trough details from the eastern Plains eastward by day 7 Wed. The best clustering of guidance suggests that shortwave energy reaching the northern Plains by Tue should amplify and reach near the Mississippi Valley by Wed, supporting another wave/front at the surface. The 00Z ECMWF amplifies its trough in the western part of the envelope while the 00Z GFS is one of the most progressive (due to depositing more energy over the West). The 06Z GFS/00Z CMC are closest to the ensemble means but recent trends allow for some consideration of the 00Z ECMWF. These shortwave differences will play a greater role in the eastern U.S. forecast just after the end of the current extended period. Guidance as a whole still displays a slower and more separated trend with the upper trough nearing the West Coast next week. A combination of operational models and ensemble means would provide a reasonable starting point to balance recent trends toward more separation (i.e. possible closed upper low approaching California per GFS/ECMWF runs) along with typically increasing uncertainty that exists 6-7 days out in time. Based on the preference for a consensus/intermediate forecast, the updated blend started with the 06Z GFS/00Z ECMWF and lesser weight of the CMC/UKMET during the first half of the period and then trended toward a model/mean blend involving the 06Z GFS/00Z ECMWF/00Z CMC and 06Z GEFS/00Z ECMWF means. ...Weather Highlights and Hazards... The forecast pattern should lead to fairly low coverage of significant precipitation during the Sat-Wed period. The Florida Peninsula will be the primary focus for highest rainfall totals as a wavy front settles over or just south of the Keys and one or more shortwaves progress within southern stream flow aloft. Most guidance suggests there will be higher potential for a period of enhanced rainfall during the first half of next week versus the weekend. The southern stream energy may also generate some rain over southern Texas but with lower totals. One or two surface waves/frontal systems may bring scattered areas of precipitation to parts of the central/eastern states. Confidence in specifics is still fairly low but overall expect most rain/snow to be in the lighter half of the spectrum. Lake effect snow downwind from open waters will taper off during the course of the weekend. Slower trend with the system approaching California will delay the arrival of associated moisture into at least Tue-Wed. There is a fair degree of spread in current guidance, ranging between light rain reaching the coast by Tue and dry weather continuing through Wed. The north-central and southern Plains should see the most persistent cold anomalies with some locations 10-20F below normal for highs through the weekend. The most extreme anomalies should be over southern areas. The warming trend over/near the West Coast states should bring readings 5-10F or so above normal by Sun-Mon with above normal readings extending into the Interior West by Tue-Wed. The East will generally see cooler than normal temperatures through the period under mean troughing aloft. ...Bullet points from Medium Range Hazards... Hazards: - Heavy rain across portions of the Southeast, Tue-Wed, Mar 19-Mar 20. - Flooding possible across portions of the Plains, Mississippi Valley, the Tennessee Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Southeast, the Middle/Lower Mississippi Valley, and the Ohio/Tennessee Valleys. - Flooding likely across portions of the Northern/Central Plains, and the Upper/Middle Mississippi Valley. - Much below normal temperatures across portions of the Southern Rockies, the Southern Plains, and the Southwest, Sat-Sun, Mar 16-Mar 17. 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