Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Sun Nov 17 2019 Valid 12Z Wed Nov 20 2019 - 12Z Sun Nov 24 2019 ...Heavy rain possible over parts of the Southwest and then south-central U.S. east of the Rockies... ...Pattern Overview and Guidance/Predictability Assessment... Guidance shows amplified and moderately progressive northern stream flow persisting across the CONUS from midweek through next weekend, with a tendency for periods of split flow as energy separates and cuts off south of the strongest westerlies. An upper-level trough will exit the eastern U.S. on Wed as a deepening coastal low moves quickly into or near the Canadian Maritimes. A second northern stream wave should quickly follow on its heels, moving a low pressure system and trailing cold front across the Midwest/Great Lakes Wed night-Thu and southeast Canada/eastern U.S. by Fri. Farther west the models and ensembles depict northern stream energy digging southward along the West Coast as the medium range period begins, with an upper low possibly closed off by around 12Z Wed. This upper low should then slowly progress eastward across the Southwest. This feature continues to be the most significant forecast problem as solutions have differed considerably over exactly how far south/west the feature will dig before moving east, and also on the speed with which it moves eastward. Adding further complexity is the potential interaction with northern stream shortwave energy--plus the fact that models/ensembles are all over the place with the ultimate evolution of upstream shortwave energy that splits after reaching near 140W longitude early day 4 Thu (with a compact upper low eventually reaching close to the West Coast as per 12Z UKMET/CMC, as far west as 140W as in the 00Z ECMWF, or somewhere between). The GFS has consistently been quicker to move the Southwest upper low eastward, with the ECMWF on the slower side of the spread and latest CMC runs along with ensemble means in the middle. Earlier CMC runs had represented the western extreme off the southern California coast. There have now been some converging trends though, with the 06Z GFS slower than the 00Z GFS and some of yesterday's runs versus the 00Z ECMWF adjusting somewhat faster than some of its previous runs. This appears similar to model behavior over the past month or so... the ECMWF cluster being the leader in signaling the idea of energy dropping into a closed low, and then once the GFS comes into better agreement the slower/closed cluster drifts somewhat faster. Taking a look at the hemispheric pattern, there appears to be very little in the way of significant blocking, with largely progressive flow prevailing from Asia across the North Pacific to North America. Additionally the teleconnections associated with persistent positive height anomalies becoming established across western Canada and/or eastern Pacific favor colder conditions/lower heights across the eastern half of the CONUS, suggesting that the flow should have a tendency to remain somewhat progressive. These considerations and aforementioned model trends/tendencies favor an intermediate approach for the Southwest upper low and the ejection of its energy to the east of the Rockies. This favored timing maintains continuity (albeit with slightly faster progression) for the idea of a frontal wave developing near the lower Mississippi Valley by Fri night-Sat and then moving northeastward thereafter. Based on the above, the updated forecast incorporated ideas from the 00Z ECMWF/CMC/UKMET and 06Z GFS early in the period and started to include modest 06Z GEFS/00Z ECMWF mean input by days 4-5 Thu-Fri. Diverging operational model details required increasing ensemble mean weight considerably by days 6-7 Sat-Sun. ...Weather Highlights/Threats... Especially in light of recent trends toward the slower part of the spread trending faster for the Southwest upper low, expect the heaviest precipitation over the southern half of the West to be in the early part of the period (Wed-Thu). Highest totals should be over favored terrain in Arizona. Snow will be possible in mountain areas, including some of the highest elevations of Arizona/New Mexico. Ejection of the upper low's energy beyond the Rockies late Fri through the weekend will likely reflect at the surface as a wave of low pressure and bring a band of locally heavy rainfall through parts of the East, most likely from the lower half of the Mississippi Valley into parts of the Southeast/Appalachians/Mid-Atlantic. Much of the West will be fairly dry behind this system though the northern Pacific Northwest/Rockies could see a little moisture arrive toward the end of the weekend. The mid-late week Midwest/Great Lakes system and trailing front will bring showers and perhaps a few thunderstorms to the southern-central Plains and mid-Mississippi Valley with some snow possible in the northern part of the precipitation shield across the Upper Great Lakes/interior Northeast. Currently anticipate highest totals with this system to extend from northern Missouri/southern Iowa into the southwestern Great Lakes region. Expect coolest temperature anomalies of 10-15F below normal for highs in a narrow band from the northern High Plains southward Wed-Fri, behind a front that drops through the central U.S.. The upper low over the Southwest during that time will support below normal highs but above normal morning lows in its vicinity. Temperatures will moderate over the West next weekend as the upper low departs and the pattern aloft becomes less amplified. The warm sector ahead of the initial central U.S. front--which should reach the East Coast by Fri--will provide a brief period of above normal readings, with plus 10F or greater anomalies especially for lows. The late week frontal passage over the East will bring a short period of modestly below normal highs. Rausch/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 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