Extended Forecast Discussion NWS Weather Prediction Center College Park MD 208 AM EST Thu Nov 24 2022 Valid 12Z Sun Nov 27 2022 - 12Z Thu Dec 01 2022 ...Snow is likely across higher elevations of the West early next week and may spread into the north-central U.S. by Monday-Wednesday... ...Widespread, potentially heavy rainfall could affect the east-central U.S. Tuesday-Wednesday ahead of a cold front... ...Overview... A fairly progressive upper pattern is in store for much of next week. The upper low located in the Southern Plains during the short range period that has been very tricky to forecast for should be lifting into the Mid-Mississippi Valley as the medium range period begins early Sunday, while a surface low also tracks quickly northward across the northeastern quadrant of the U.S. and spreads precipitation along the low track and the trailing cold front in the Mid-Atlantic to Southeast on Sunday. Most precipitation will be rain, but there could be some light snow in higher elevations of the interior Northeast. Then troughing is set to dig across the western U.S. Monday-Tuesday, increasing precipitation chances there. As the trough shifts eastward, the current forecast shows a consolidating surface low pressure system may shift into the Midwest for Tuesday-Thursday, with some snow possible north and west of the low for the Northern Plains into the Upper Midwest, and potential for heavy rainfall ahead of a trailing cold front in the east-central CONUS. ...Guidance and Predictability Assessment... The initial upper and surface lows to begin the period 12Z Sunday continued to trend slower overall in the 12/18Z model cycle that was available for the overnight forecast--though the UKMET was a bit of a south outlier. The newer 00Z models are not too different from the earlier consensus, though the CMC moved from the eastern side of the spread to slower at 00Z. Thus this forecast pulled the low and frontal placement as well as the QPF a bit farther south and west compared to continuity given the model trend. The differences were not huge, but for day 3 perhaps a bit larger changes than normal. Attention then turns to the West with rounds of troughing expected. Even by day 3/Sunday a shortwave looks to be over the Four Corners region, passing quickly into the east-central U.S. by Monday. There is general agreement for energy to dig and deepen larger scale troughing in the West Monday-Tuesday with just some minor shortwave differences. Slightly larger differences arise by around Wednesday as the trough ejects into the central U.S., particularly as the 12Z CMC is faster than the better consensus of the GFS/ECMWF runs, but also with the details of potential minor stream separation--though guidance does not currently show an upper low forming and cutting off within the trough. There is also some variability with what may be a complex surface low evolution in the central U.S., which will depend on details of the energy that may take more time to resolve. These differences are pretty typical for this forecast lead time though, and it appears that the newer 00Z guidance shows better agreement for the eventual track of the consolidated low into southeastern Canada by day 7/Thursday. Models are also reasonably consistent with showing another bout of troughing coming into the Northwest late in the period. Thus the WPC forecast utilized a blend of the deterministic guidance led by the 18Z GFS and 12Z ECMWF for the early part of the period, with gradual transition to about half the GEFS and EC ensemble mean guidance by days 6-7. This served to maintain some strength of systems while also smoothing out individual model differences. ...Weather/Hazard Highlights... Gulf moisture is forecast to stream into parts of the central and eastern U.S. ahead of the upper and surface lows moving from the Mid-Mississippi Valley northeastward and produce widespread rain chances on Sunday for the East. With this low track, most areas are likely to see rain rather than wintry precipitation, but higher elevations of the Interior Northeast could be an exception where snow could fall. Since the low and fronts will be moving quickly by then, this should lessen overall flooding concerns. The Pacific Northwest into the Northern Rockies can expect increasing precipitation on Sunday in conjunction with a couple of frontal systems. Precipitation is then forecast to spread southward across California and eastward into the Great Basin and Rockies by Monday as the noted upper trough continues digging into the region. Lower elevation rain and higher elevation snow will be likely. At this point it looks like the heaviest snow should be across the Cascades into the Wasatch to Wind River Mountains and the Northern/Central Rockies, with more uncertainty for heavy precipitation in California/the Sierra Nevada. Yet another round of troughing could increase chances for additional precipitation in the Northwest by Wednesday-Thursday. Low pressure system development/consolidation still looks likely in the central U.S. by around midweek, bringing precipitation chances to central and eastern portions of the lower 48. Widespread, potentially heavy rainfall looks possible across parts of the Mississippi Valley into the Ohio/Tennessee Valleys and Appalachians. There also remains potential for snow on the backside of the low as it tracks northeastward into the Midwest Tuesday-Wednesday, with a possible focus of snow from eastern Montana into the Dakotas and the Upper Midwest. This is toward the end of the medium range period though so the details of the forecast may need to change with time, so stay tuned to future forecasts. Slightly cooler than normal highs could linger in the south-central U.S. on Sunday, while milder than normal temperatures are possible in the East with anomalies for lows around 10-20F and highs around 5-15F above normal. But more widespread cooler than normal temperatures are likely to expand across the western half of the U.S. Monday-Wednesday as deepening troughing takes hold there. Anomalies of 15-25F below average may focus across the northern High Plains Monday and spread south into the Great Basin and central High Plains Tuesday. Meanwhile, the south-central and east-central U.S. could see above normal temperatures ahead of the western trough. Then the cold air continues marching eastward to flip most areas to below normal by next Thursday. 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, experimental excessive rainfall outlook, 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/#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