Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Sat Oct 23 2021 Valid 12Z Tue Oct 26 2021 - 12Z Sat Oct 30 2021 ...A stormy pattern continues across the nation next week... ...Overview... The weather pattern during the medium range period should remain quite active and amplified as a couple of systems impact the CONUS next week. An upper low initially over the lower Great Lakes on Tuesday will induce a deepening cyclone with heavy rainfall over the Northeast U.S. which should lift north and east into the Canadian maritimes by Thursday. Meanwhile upstream, the next trough will be ejecting out of the Rockies Tuesday-Wednesday with another possible closed upper low/deep surface cyclone likely to form and track from the north-central Plains into the Ohio Valley and eventually the Northeast by late week/early next weekend. Organized rainfall will accompany the cold front as it traverses the eastern half of the nation with rounds of heavy rainfall possible. A couple of shortwaves may also move into the Pacific Northwest during the period bringing more heavy rain/mountain snows to parts of the Northwest. ...Guidance Evaluation/Preferences... For the most part, the guidance continues to show good agreement on the overall longwave pattern evolution, but significant uncertainties in the details regarding strength and track of individual systems. For the intial Northeast closed low on Tuesday-Wednesday, there remains some subtle differences in placement but a general model blend should give a good starting point. The biggest uncertainties surround the amplified trough exiting the Rockies into the Plains at the beginning of the period. By late Wednesday/early Thursday, much of the guidance has been trending towards closed upper low development, but rather significant differences in placement and track of the low. The notable outlier at least right now seems to be the 12z (yesterday) ECMWF as well as the new 00z run both of which are displaced well south of the better consensus with the low moving through the Southern Plains/Tennessee Valley into the Mid-Atlantic. The latest runs of the GFS and CMC suggest the low center farther north (into the Midwest/Ohio Valley and Northeast) which is also consistent with the latest ensemble means as well. The WPC blend leaned more towards the GFS/CMC along with the ensemble means, which also maintained fairly good continuity with the previous WPC forecast as well. Out West, some late period timing differences arise with another trough moving towards the Pacific Northwest later next week, so a blend of the ensemble means was preferred. ...Weather/Hazards Highlights... The leading closed low into the Northeast on Tuesday will allow for heavy rainfall potential to continue from the end of the short range period into much of New England early next week. Deepening coastal low development may induce pockets of enhanced rainfall on the backside of the low as it lifts northward towards the Canadian Maritimes but there remains some uncertainty in exact low placement which will impact where the heaviest rains fall. The cold front reaching the Plains by Tuesday and onward into the east-central U.S. Wednesday/Thursday is expected to present an emerging and expanding lead rainfall/convective focus as ample Gulf of Mexico moisture streams northward ahead of the system. The latest guidance suggests the best potential for heavy rainfall will be across portions of the Gulf Coast states and inland on Wednesday and then into the Mid-Atlantic/Northeast Thursday-Friday with closed upper low development and a slowed frontal system progression. The Storm Prediction Center also is highlighting the potential for severe weather across parts of the central/southern Plains on Tuesday, and the central Gulf Coast on Wednesday. Meanwhile, out west, a sustained moisture plume from the short range period will continue into the Northwest U.S. on Tuesday bringing a continued threat for heavy rainfall and mountain snows to especially the Pacific Northwest/northern California but also farther inland across the northern Rockies. Another system moving towards the coast mid next week will continue the active weather with another round of heavy rainfall and mountain snow impacting parts of the Pacific Northwest Wednesday into Friday. In terms of temperature highlights, the southern/central Plains should be above normal to start, but quickly trend back towards normal following the passage of the cold front. Daytime highs could be 10 to 15 degrees above normal on Tuesday for parts of Texas, Oklahoma, and western Kansas. Successive closed lows/troughs into the east-central U.S. will keep temperatures near or below normal through the entire week. Out West, temperatures may initially begin cold (minus 10 to 15F anomalies), but should trend warmer and back to normal values as some weak upper ridging moves through. The Central U.S. may again warm to slightly above normal by late week as the ridging reaches the Plains. Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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