Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EST Mon Nov 18 2019 Valid 12Z Thu Nov 21 2019 - 12Z Mon Nov 25 2019 ...Heavy rain possible later this week for portions of the lower Mississippi Valley and the Southeast... ...Pattern Overview and Guidance/Predictability Assessment... Split flow will be present across the CONUS through much of the medium range. Initially on day 3 (Thu) an upper low is expected to be in place across the Southwest, with progressive northern stream flow extending from the North Pacific east into Canada and the U.S. northern tier. A northern stream shortwave and associated low pressure system are forecast to move from the Upper Midwest/Great Lakes on Thu quickly northeast into Quebec by Fri morning, with the trailing frontal boundary crossing the eastern third of the CONUS. Models consensus surrounding this feature was quite good, with only minor timing and intensity differences noted during days 3-4. The evolution and progression of the southwestern upper low continue to pose the biggest challenges during the medium range. Model/ensemble consensus surrounding this feature has shown notable improvement over the past 24-48 hours, but some significant differences remain. These include the speed of eastward progression, interaction (or lack thereof) with the northern stream shortwave, and the resultant impacts on the development of a frontal wave across the lower Mississippi Valley by Fri-Sat. While model spread on the timing of the upper low has reduced somewhat, earlier GFS runs had remained on the fast side of the spread (but trending slower), while the previously slow ECMWF has sped up a bit, now more closely aligned with a cluster of solutions near the 00Z UKMET and most ensemble members. Given the current trends, opted to split/smooth the timing differences between the ECMWF and GFS/GEFS camps, with a gradual trend toward heavier weighting (50%) of 00Z ECENS/06Z GEFS ensemble means by day 7 (Mon). This solution shows a wave of low pressure developing along the trailing surface front across the lower Mississippi Valley Fri/Fri night, moving northeast into the Tennessee Valley by Sat morning, and off the Mid-Atlantic coast by early Sun. Trend has been for a bit quicker progression with the 00Z/06Z ensembles. Farther west, models/ensembles show agreement on the idea that upper-level ridging should begin to weaken over the weekend as the primary center for positive height anomalies shifts well offshore, allowing an active upper jet across the North Pacific to sag southward toward the northwestern U.S. Guidance suggests that shortwave energy traversing this flow should amplify across the western U.S. by early next week, potentially allowing a cold front to press into the western states. Models show a large degree of run-to-run variability around this scenario despite at least some agreement on the overall evolution. Thus, 50% ensemble weighting by late in the extended period was warranted given the setup despite the good agreement between the 06Z GFS and 00Z ECMWF. ...Weather Highlights/Threats... The heavy precipitation event across the Southwest will wind down by the extended period as the upper low begins to move east, but some areas of heavy rain and mountain still will still be possible across portions of the Four Corners region and the southern Rockies on Thu. Farther east, rain and snow will accompany the low pressure system crossing the Midwest/Great Lakes on Thu and the Northeast on Fri. As the aforementioned wave of low pressure develops along the front across the lower Mississippi Valley Fri-Sat, a significant area of overrunning precipitation is expected to develop, with guidance continuing to suggest the potential for a couple inches of rain in a swath from Arkansas/northern Louisiana eastward into portions of the Tennessee Valley and the Southeast/southern Appalachians/southwestern Mid-Atlantic. As the low pressure system crosses the Appalachians and exits the Eastern Seaboard Sat night-Sun, modest rains should spread into portions of the Mid-Atlantic region and New England, although faster forward motion of the system by that time should preclude a significant heavy rainfall threat. Also by Sun-Mon, as lower heights and a cold front reach the Pacific Northwest, rain and mountain snows should begin to return to portions of the Cascades and coastal ranges. Below average temperatures are forecast to spread across portion of the central and eastern U.S. in the wake of the mid/late-week cold front. Highs are forecast to be 5 to 15 deg F below average from the Plains Thu, reaching the Midwest/Great Lakes on Fri. The air mass should moderate somewhat as it spreads east, but highs 5 to 10 deg below average are expected to reach the eastern U.S. over the weekend. Much of the West will see some moderation in temperatures late this week into the weekend before additional cooling is possible next week. Fracasso/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 rain across portions of the Lower Mississippi Valley, Tennessee Valley, and Southern Appalachians, Thu-Fri, Nov 21-Nov 22. - Flooding occurring or imminent across portions of the Northern Plains. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Thu-Fri, Nov 21-Nov 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