Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Thu Nov 30 2023 Valid 12Z Sun Dec 03 2023 - 12Z Thu Dec 07 2023 ...An active and wet pattern on tap for the Northwest including heavy rain and inland heavy snow threats... ...Overview... An amplified and active weather pattern is expected to continue into the medium range period, especially across the Western U.S. as a series of Atmospheric Rivers impacts the region raising concerns for heavy rainfall along the Washington/Oregon coast and favorable western facing slopes of the Cascades with heavy snow for parts of the northern Rockies. Downstream, an amplified trough, periodically reinforced by shortwave energy, will slide from the Central U.S. into the East Sunday-Monday, with a second shortwave across the north-central U.S. to the Northeast mid-next week. In between this trough and the next system out of the East Pacific, a strong ridge looks to build initially over the West on Tuesday, moving into the Plains Wednesday-Thursday bringing well above normal temperatures, especially across the Northern and Central Plains. ...Guidance Evaluation/Predictability Assessment... On the large scale, the guidance shows pretty good agreement across the board, but with plenty of variability in the details. Out West, models agree on the presence of multiple ARs during the period (the first on Sunday, and the next moving in late Monday/early Tuesday) but still with some uncertainty in the AR timing/strength/duration and in the details and exact locations of maximum QPF. Continuing to feel the NBM is too wet with QPF, so the WPC forecast tempered that down some using deterministic guidance to account for the lingering uncertainties. In the Midwest and the East, run to run variability and timing of individual shortwaves this weekend/early next week still translates to even some regional sensible weather/precipitation uncertainties. Through yesterday's 12z/18z runs, the GFS is notably fast with an initial shortwave into the East on Sunday/Monday, while the UKMET is a little slower than consensus with the overall trough progression. With the next system dropping into the north-central U.S. Tuesday-Wednesday, the CMC gets too amplified/slow while the GFS/ECMWF and ensemble means suggests a flatter wave and faster progression. Accordingly, the WPC forecast blend for days 3-4 favored the ECMWF and the CMC with some ensemble mean guidance even in the early part of the period to help smooth out discontinuities. By day 5 and beyond, replaced the CMC with the GFS and increased more of the ensemble means to accounts for larger scale differences. Overall, this maintained reasonable continuity with the previous WPC forecast. ...Weather/Hazards Highlights... Out West, precipitation will be ongoing as this forecast period begins Sunday, with a series of atmospheric rivers into the Northwest continuing well into next week, with growing totals and impacts possible. Some modest rainfall totals are likely along the coast with both events Sunday and again late Monday-Wednesday. With heavy rain also in the forecast during the short range, the soils and rivers may be primed increasing the risk for flooding and flash flooding. Snow levels with both systems will be very high so especially western facing slopes of the Cascades may be subject to an additional risk of flooding due to snow melt as any accumulating snow will be confined to the very highest peaks (or farther inland). Given this, a marginal risk for excessive rainfall was included on the Day 4 and 5 EROs across this region. It is possible a slight risk will be needed in subsequent updates of the ERO, but at this point there was still enough uncertainty to preclude that. Farther inland across the Northern Rockies, heavy snow will be possible into Sunday, and possibly again mid next week with the next system. Downstream, ejecting shortwave energy and a leading/wavy frontal system may continue to support rainfall into Sunday and Monday across the Southeast, with much of the Eastern U.S. also looking unsettled through next week, albeit with higher uncertainty on rainfall amounts given shortwave differences. However, the approach and passage of several shortwaves and advance of an amplified main upper trough should spawn several wavy fronts. Some of the precipitation could be in the form of snow across parts of the Great Lakes and Northeast. The terrain of Northern New England may focus a possible snowfall threat given longevity of the pattern and cold air availability later weekend/early next week. Through Sunday, temperatures across the East may be moderately above normal, with slightly below normal temperatures across the interior Southwest. The Western U.S. should trend warmer with time and especially by next Tuesday-Thursday in the Northern High Plains/Plains where daytime highs could be 15 to 25 degrees above normal. Meanwhile, the Eastern U.S. should trend back towards normal/slightly cooler than normal next week as reinforced troughing affects the area. 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 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