Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Fri Dec 08 2023 Valid 12Z Mon Dec 11 2023 - 12Z Fri Dec 15 2023 ...Overview... Surface low pressure will be in place in the Northeast as the medium range period begins Monday, bringing possibly heavy rain and snow to the region before the system exits by Tuesday. After a bout of light to moderate precipitation in the Northwest on Monday as well, much of the lower 48 will trend drier by Tuesday. By midweek and beyond though, return flow from the Gulf of Mexico streaming into the southern High Plains ahead of a Four Corners upper low should lead to increasing precipitation chances there, while moist easterly flow could spread convection into the Florida Peninsula. ...Guidance Evaluation/Predictability Assessment... Model guidance continues to be agreeable for the early part of the forecast period, including initial troughing in the eastern U.S. supporting surface low pressure in the Northeast. Models indicate this trough should exit quickly while another trough that is not as deep pushes through the Great Lakes region Tuesday-Wednesday. Guidance also shows notable agreement regarding energy dropping into the Southwest and forming a closed low by Wednesday, just with some minor placement differences in the models and ensemble members. Overall a multi-model deterministic blend was used for the first part of the medium range forecast period. By Thursday-Friday, the 12Z and the previous 00Z run (both from Dec 7) of the ECMWF were quite aggressive with another shortwave coming in behind the Southwest upper low, which then takes over in coming through California and the central Great Basin and merges with the preexisting low. Though the deterministic run is supported by some of its ensemble members, other guidance does not favor this solution, and neither do the AI/machine learning ECMWF versions. Thus removed the ECMWF from the forecast blend but kept some EC ensemble mean as a nod to this possibility. The newer 00Z ECMWF is sort of in between, with a weaker shortwave than its previous run but stronger than other guidance, but ending up with an upper low in the Great Basin where GFS runs have a ridge on Friday. The 00Z CMC trended toward an EC-like solution after its 12Z run was more like the GFS runs. So there is still some uncertainty here. There is at least some agreement with ridging downstream across the central U.S. pushing toward the eastern U.S. by Friday. The latest WPC forecast for the latter part of the period was based on a blend of the 18Z GFS and GEFS mean along with the 12Z CMC and some 12Z EC ensemble mean. ...Weather/Hazards Highlights... Precipitation will be ongoing across the Northeast on Monday. Much warmer than average temperatures are possible across Maine in particular early in the day, which along with a bout of heavy rain could cause quickly melting snow for possible flooding there. A Marginal Risk is planned for Day 4/Monday in the Excessive Rainfall Outlook for this potential, and river flooding is also a threat. On the backside of the low, probabilities for notable snow are increasing especially in the higher elevations of the interior Northeast on Monday. Some brisk to strong winds are also possible from the south ahead of the cold front and from the west behind it. The precipitation should clear out by Monday night. Some light to moderate precipitation is likely across the Northwest on Monday, with some enhanced snow totals lingering in the Northern Rockies. After that, precipitation will be at a minimum across the lower 48 on Tuesday, but chances for precipitation should increase on Wednesday-Thursday across the southern High Plains as an upper low settles over the Southwest and moisture interacts with a front. Some of this precipitation may be snow, with typical elevation/coverage uncertainty. Meanwhile easterly flow setting up to the south of strong eastern U.S. high pressure will promote periods of rain over Florida. Finally, light precipitation is possible at times in the Pacific Northwest for the latter half of next week. As noted above, temperatures will be well above average in the Northeast on Monday, and several record high minimum temperatures could be broken. Behind the cold front, temperatures will be around 5-10F below average across the Gulf Coast to central Appalachians on Monday and pushing into the Southeast on Tuesday, while the Northeast moderates to near average. After that the East should see generally near normal temperatures, perhaps reaching above normal by Friday. Farther west, the most persistent above average temperatures will be across the north-central U.S. (other than a brief cooler spell on Tuesday) as upper ridging builds. The greatest anomalies look to be on Thursday-Friday for the Dakotas into the Upper Midwest as temperatures reach 10-20F above normal. Tate 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw