Extended Forecast Discussion NWS Weather Prediction Center College Park MD 131 PM EST Thu Nov 16 2023 Valid 12Z Sun Nov 19 2023 - 12Z Thu Nov 23 2023 ...Pattern Overview... The medium range period begins with split flow across the CONUS as the holiday week begins. Systems in the northern stream will track across Central/Southern Canada/U.S. northern tier while a potential closed low develops and tracks through the Central and East-Central U.S. around Monday-Tuesday. Towards the middle to latter part of next week, models seem to be trending towards these two streams combining resulting in a large and deep upper low over/near the Great Lakes. This should fuel an emerging wet pattern from the Central Plains, into the South, and eventually into the Northeast Sunday-Tuesday. ...Guidance Evaluation/Predictability Assessment... Model and ensemble guidance continue to be in relatively good agreement, allowing for a purely deterministic model blend through Tuesday between the latest runs of the GFS, ECMWF, and CMC (the 00z UKMET was a little fast than the better consensus). There are some lingering differences in the details, which shows up most prominently in the QPF forecasts, but the National Blend of Models seemed to do a good job smoothing these out. By Wednesday-Thursday, there is growing spread and a trend towards more combined northern/stream southern stream flow. Still a few lingering timing differences and run to run variability though, and so the WPC late week progs were based more on the ensemble means. Overall, maintained good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... Heavy rainfall potential will increase across the south-central to eastern U.S. for the upcoming holiday week as a main low pressure system deepens and tracks across the region. The low will develop in the lee of the Rockies over the central/southern Plains on Sunday and move to the east-central states early next week before curving towards the Northeast mid-week. The tightening pressure gradient as the low spins up will likely produce a period of high winds across the High Plains early next week and instability ahead of the system could bring severe weather potential to parts of the Central Plains and central Gulf Coast Sunday-Monday, as highlighted by the latest outlooks from the Storm Prediction Center. There is also a signal for organized and possibly heavy rainfall by Monday with focus over the central Gulf Coast states region. A WPC Excessive Rainfall Outlook Marginal Risk area was maintained to address this emerging threat. Ample moisture will stream into the southern and eastern U.S. ahead of the system as it works downstream, so despite progression over time may lead to some localized runoff issues in the vicinity of the central/southern Appalachians on Monday and the Mid-Atlantic and Northeast on Tuesday. Urban flooding potential may be a concern especially for the metro areas up through Mid-Atlantic and the Northeast. There is some possibility for snow on the northern periphery of this precipitation shield. In the West, unsettled weather including a burst of mountain snows is expected Sunday as northern stream upper trough energy digs through the north-central Intermountain West/Rockies. An amplified upper ridge will then build over the West Coast early next week, resulting in more stable and dry weather. However, upstream Pacific system energies may work into this ridge enough to bring another opportunity for organized precipitation into the Pacific Northwest next Wednesday-Thursday. Santorelli/Schichtel 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