Extended Forecast Discussion NWS Weather Prediction Center College Park MD 442 PM EDT Wed Mar 20 2019 Valid 12Z Sat Mar 23 2019 - 12Z Wed Mar 27 2019 ...16Z Update... The array of 00Z/06Z guidance yields a forecast rationale similar to previous cycle--a consensus approach for days 3-4 Sat-Sun and then emphasis on the 00Z ECMWF/CMC and NAEFS/ECMWF means by days 5-7 Mon-Wed. GFS/FV3 GFS runs and GEFS means are still on the faster side of the spread with the shortwave crossing the southern half of the Lower 48 (though the UKMET hedges in that direction by Mon). At the same time the GFS/GEFS mean are more amplified with flow dropping into the Great Lakes by Mon, leading to more phasing between streams than the majority scenario. From late Mon onward the 00Z ECMWF has trended back to the north with its eastern U.S. surface low to agree better with ECMWF/CMC means. The average of members within the CMC/ECMWF cluster has adjusted a tad faster from continuity and this is reflected in the updated forecast. Over the eastern Pacific/West Coast there is reasonable consensus into day 5 Mon. After that time the majority of guidance generally suggests the upper low off the coast will be more consolidated than depicted by the 06Z GFS/FV3 GFS. The 00Z GFS is closer to consensus. Including the NAEFS mean in the blend accounts for the possibility that the ECMWF/CMC cluster could be slightly fast to bring the upper trough inland next Wed, given the overall blocky nature of the evolving western North American pattern at that time. Models and ensembles are stable with the overall signal for one or more episodes of heavy precipitation focused over/near northern California and Sierra Nevada during Mon-Wed. Some of this moisture and/or the leading system crossing the West during the weekend will spread rain and higher elevation snow over remaining parts of the northern half or more of the West. Spread and variability in model guidance keep confidence on the low end of the spectrum for coverage/intensity/timing of precipitation from the Plains to East Coast, as well as for details of any snow that may fall in the northern fringe of the moisture shield. Clouds/precip will contribute to above normal morning lows (by 10-20F in some cases) from parts of the West into the Plains during the weekend and central/southern Plains into lower Mississippi Valley early next week. Highs will tend to be near to below normal over the West during the weekend. The Interior West and Rockies/Plains should warm up again by next Tue or Wed. On the other hand below normal readings will likely push southeastward from the northern Plains/Great Lakes during Mon-Wed. Rausch Previous Discussion: ...Overview and Guidance Assessment/Preferences... A split flow pattern is expected to persist through the medium range. Anomalous upper ridge axis extending from Alaska and northwest Canada into the north central U.S. is expected to persist, with downstream mean troughing centered east of Hudson Bay - making for highly amplified, if not blocked northern stream flow. Meanwhile, the stream will be fairly progressive across the CONUS, with shortwaves spaced a couple days apart as energy ejects from an upstream mean trough across the north central Pacific. Days 3-5 In general, a multi-model blend of the latest deterministic guidance served as a good forecast starting point during days 3-5 (Sat-Mon). Model solutions still differ to some degree on the position of a deep surface low near/east of Maine Sat morning, but solutions are generally well-clustered around the ensemble means, indicating that a consensus approach is likely best. As the Sat system quickly pulls away, another shortwave should traverse the broader mean trough, diving into the Great Lakes/Northeast Sun-Mon. Solutions have trended more amplified with this wave over the past several model cycles, and a consensus approach is also favored here at this time. Farther west, models show good agreement that a Pacific shortwave should reach the West Coast early Sat, then cross the Four Corners Sun, reaching the Southern/Central Plains by Mon. At least through Sun, models handle this feature similarly, with differences beginning to emerge by Mon as the system moves into the Plains. Nonetheless, there is at least modest consensus that a surface low should develop across the Southern Plains Sun night-Mon. Days 6-7 The forecast gets quite tricky by next Tue-Wed, particularly with respect to the southern stream system moving from the Plains toward the East Coast. The GFS and ECMWF have both shown significant difficulty handling this system. The ECMWF has been highly variable from run-to-run, while the GFS has consistently been a fast outlier, only seeming to get faster with each run. A sizable number of ECENS and CMCE members have shown good clustering for a few runs now around a surface low tracking from the Southern Plains into the Tennessee Valley Mon night-Tue (a solution starkly different from the 12Z ECMWF which seemed to be an outlier in the context of its ensemble). The 12Z CMC/GEM global and NAVGEM were the only deterministic solutions representing this significant cluster, which is also reflected in the ECENS mean. By Tue night-Wed, energy appears likely to transfer to a new surface low along the Southeast U.S. coast, in a "Miller-B" scenario. This process seems to be best represented by the ECENS and CMCE means, with the 12Z ECMWF solution also in the ball park (if you ignore its outlier solution for the Gulf Coast low). Finally, farther west, consensus remains relatively good that strong height falls should reach the West Coast Tue-Wed as a significant trough moves onshore. While timing among models is similar, there are structural differences that remain, leading to the conclusion that an ensemble approach for that system is likely best. Given these considerations, the forecast for days 6-7 was based heavily on the ECENS and NAEFS ensemble means, with components of the CMC/GEM global and NAVGEM also included, given their aforementioned handling of the central/eastern U.S. system. ...Weather Highlights and Hazards... Showers and thunderstorms are expected to develop across the Southern and Central Plains Sat-Sun. A leading upper shortwave Sat and a surface warm front/dry line will initially focus convection across these areas. The potential exists for some severe weather as well as locally heavy rainfall threats. Please refer to products issued by the SPC for further information on the severe weather threat. By Mon-Tue, showers and thunderstorms should spread east into the lower/mid-Mississippi Valley as well as the Ohio/Tennessee Valleys and the Southeast. Development of the surface low along the Southeast U.S. coast Tue night-Wed would also likely bring an enhancement of rainfall to portions of the Carolinas and southern Mid-Atlantic. Ensemble probabilities suggest some potential for wintry weather on the northern periphery of this system Tue-Wed, from portions of the upper Ohio Valley to the Mid-Atlantic and Northeast. Confidence in the extent of cold air in place by that time is very low, and only low-end probabilities were shown at this time on the Day 7 Winter Weather Outlook. Farther west, rain and mountain snows are expected across the Great Basin/Rockies with the passage of the shortwave Sat-Sun. A more significant area of precipitation could spread into northern California by next Mon-Tue as the stronger upper trough approaches, with some signal for a heavy precipitation event in the model guidance. Ryan 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 precipitation across portions of California, the Central Great Basin, and the Pacific Northwest, Mon-Wed, Mar 25-Mar 27. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat-Sun, Mar 23-Mar 24. - Heavy rain across portions of the Middle/Lower Mississippi Valley, the Central/Southern Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southeast, and the Ohio Valley, Mon-Tue, Mar 25-Mar 26. - Heavy snow across portions of the Mid-Atlantic and the Central Appalachians, Tue-Wed, Mar 26-Mar 27. - Heavy snow across portions of the Northeast and the Great Lakes, Sat, Mar 23. - Severe weather across portions of the Southern Plains, Sun, Mar 24. - Flooding possible across portions of the Northern/Central Plains, the Upper/Middle Mississippi Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Northern/Central Plains, the Mississippi Valley, the Tennessee Valley, the Great Lakes, and the Ohio Valley. 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