Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 PM EST Wed Feb 12 2020 Valid 12Z Sat Feb 15 2020 - 12Z Wed Feb 19 2020 ...18z Update... As far as the upper level, synoptic scale, flow is concerned, there were no major changes with the latest suite of model guidance since last night and the fronts/progs required only minor adjustments from the previous shift. The biggest forecast question in the medium range continues to be interactions between stream flow with a lingering frontal boundary from the Lower Mississippi Valley into the Tennessee/Ohio Valley the first half of next week. The latest model runs have backed off on QPF across this region, and the latest forecast reflects that. However, it's important to note that due to model run to run variability, this is a low confidence forecast. Regardless, there is at least a model signal for potentially heavy rainfall from portions of the Southern Plains through the Tennessee Valley, but details on amounts and the best rainfall axis have a long way to be resolved. This cycle of the WPC medium range progs utilizes the latest deterministic runs (ECMWF/GFS/CMC) through day 5, increasing contribution from the ECENS mean days 6 and 7. Incorporated 20 percent of WPC continuity through the entire period. Santorelli ...Previous Discussion below (issued at 0700z)... ...Overview and Guidance/Predictability Assessment... During the weekend guidance shows progressive northeast Pacific through lower 48 flow, with one shortwave tracking eastward from the Plains/Rockies and an upstream feature heading into the West. Then after Sun the majority scenario has a strong Aleutians into mainland Alaska/Gulf of Alaska storm helping to amplify the downstream pattern such that eastern Pacific ridging aloft builds into western Canada and the northwestern U.S. while troughing amplifies as it settles into the southwestern U.S.--part of a larger scale mean trough that should extend southwestward from eastern Canada by next Wed. Meanwhile an upper ridge likely to meander around Cuba and the Yucatan Peninsula should strengthen/expand during Tue-Wed. The forecast for the more progressive part of this evolution during the first half of the period has been difficult over recent days, especially regarding rainfall potential over the southeastern quadrant of the country. Recent trends for shortwave specifics and surface evolution, affecting the details of moisture return from the Gulf, have been toward later onset and more limited coverage of rainfall during the first half of the period. One wild card continues to be what happens to energy that collects just west of Baja California by early Sat as models differ significantly on when it ultimately ejects northeastward. The 12Z ECMWF was on the fast side of the full spread with this low-predictability feature. During the latter half of the period there has generally been better clustering/continuity thus far with the idea that a northern stream component of the upper trough that reaches the West Sun-Mon should continue onward and push surface low pressure from the Central Plains region through the Great Lakes and New England during the first half of next week. There is still some sensitivity though, as exhibited by the new 00Z ECMWF that has trended 12-18 hours slower. With typical detail differences there is also a moderate level of agreement that troughing should sharpen over the Great Basin and Southwest, possibly closing off an embedded upper low. A key difference that develops late in the period is that recent GEFS and CMC means become flatter and more progressive with the overall eastern Pacific into western U.S. pattern versus nearly all operational models and the ECMWF mean that has been stable for the past three runs. Strength of the upstream storm seems to favor the more amplified eastern Pacific/western U.S. solution of the operational models and ECMWF mean. In contrast to the shortwave sensitivity that has plagued the precipitation forecast for the weekend/early week period over the eastern half of the country, latest guidance signals for a potential event over some areas from the Southern Plains northeastward after Mon could be somewhat better behaved. This is due to the existence of a well-defined cold front and potential for upper flow to become oriented nearly parallel to the front for a period of time. The updated forecast based on data available through the 12Z/18Z cycles incorporated ideas from the 12Z ECMWF/ECMWF mean and 12Z CMC/18Z GFS. These solutions provided the best overall clustering and continuity during the first half of the period and for the system/front affecting the East into midweek, while representing preferences for the late-period pattern over the Pacific/western states. ...Weather/Hazard Highlights... Northwesterly flow and embedded shortwave energy/frontal system will produce a period of locally moderate to heavy rain and mountain snow affecting the Pacific Northwest during the weekend with some of this moisture also reaching eastward/southeastward through the northern-central Rockies. Over recent days most guidance has been consistent in advertising highest totals over favored coastal terrain and westward-facing slopes of the Cascades, with significant but slightly lower amounts reaching the northern Rockies. The upper ridge likely to build into the Northwest should bring a drier trend during the first half of next week. The system forecast to emerge over the Plains by Mon and continue northeastward should produce a broad precipitation shield with various weather types. The best snow potential currently exists from parts of the Upper Midwest/Great Lakes into New England. Expect rain to fall farther south. At this time the best signal for heavier rainfall extends from near the Southern Plains into Kentucky/Tennessee as there may be a period of time when upper flow may become nearly parallel to the cold front trailing from the surface low--which would lead to the possibility of training/repeat activity. The eastern U.S. will start the weekend on the chilly side with lows up to 10-20F below normal from the Lower Great Lakes into Northeast early Sat and daytime highs 5-15F below normal over the East Coast states. Amplification of the upper pattern and strengthening southerly flow ahead of the emerging Plains into Northeast system should then bring a pronounced warming trend to the central and then eastern states. There should be decent coverage of plus 10-20F anomalies for highs and plus 10-25F anomalies for morning lows. Meanwhile expect temperatures to be within 10F of normal over the much of the West during the weekend, aside from some warmer mins over/near the Great Basin on Sun ahead of an approaching front. Upper troughing that amplifies over the West and progression of the cold front anchored by the central-eastern U.S. system will lead to increasing coverage of below normal temperatures over the western-central states Mon-Wed. Coldest readings versus normal should be over the central-southern Rockies and eventually the southern High Plains with highs 10-20F below normal on Tue-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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sat, Feb 15. - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Mon-Tue, Feb 17-Feb 18. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Sat-Sun, Feb 15-Feb 16. - Heavy snow across portions of the Central Rockies, the Central Plains, the Central Great Basin, the Northern Rockies, and the Northern Great Basin, Sun, Feb 16. - Heavy snow across portions of the Great Lakes and the Upper Mississippi Valley, Mon, Feb 17. - Heavy snow across portions of the Northeast and the Great Lakes, Tue, Feb 18. - Flooding possible across portions of the Lower Mississippi Valley, the Tennessee Valley, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southeast, and the Ohio Valley. - Flooding likely across portions of the Southeast, the Mid-Atlantic, the Pacific Northwest, and the Tennessee Valley. - Much below normal temperatures across portions of the Northeast, Sat, Feb 15. 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