Extended Forecast Discussion NWS Weather Prediction Center College Park MD 208 AM EST Sat Nov 19 2022 Valid 12Z Tue Nov 22 2022 - 12Z Sat Nov 26 2022 ...A couple of low pressure systems including a possible nor'easter could cause meaningful precipitation across eastern parts of the U.S. late next week... ...Overview... A quasi-zonal upper pattern with embedded shortwaves should be in place as the medium range period begins Tuesday with reasonably quiet weather in place across the country, save for the possibility of heavy rainfall across northeastern Florida in easterly flow just behind a surface low and frontal boundary and a round of precipitation in the Pacific Northwest. By the latter half of the week, the pattern is forecast to amplify considerably with a potent upper trough moving across the central U.S. into the East, while ridging builds behind across the West. At the surface, this will lead to a low pressure/frontal system traversing the central U.S. and crossing the Great Lakes while the western Atlantic low pressure system lifts north. These features will support increasing chances for precipitation late next week, but given the model spread in the exact low tracks, there is uncertainty regarding precipitation types and areas of heavier amounts across the East. ...Guidance and Predictability Assessment... Fortunately model guidance is showing better agreement than it was a day or so ago with troughing amplifying somewhat aggressively in the central U.S. by Wednesday-Thursday as shortwave energy dives from the northeastern Pacific into southwestern Canada and into the northern Rockies and High Plains, causing the trough to dig. There remain some differences with the placement of the trough with time and potential for a closed low to form within it, but these are relatively minor for the medium range period. A surface low should also track toward the Upper Midwest while the pattern would also support cyclogenesis near the East Coast. While there is some model spread for the tracks of each surface low, at least models are reasonably agreeable for these systems to form. There may be some differences in the potential for their phasing. Meanwhile model guidance also shows good agreement for an overall upper trough across the West for the latter part of the week. The feature with perhaps the most model spread is eastern Pacific upper energy that could form a southern stream closed low but with a large variety among models/ensemble members for where it tracks. The UKMET had seemed to be an outlier with its drift west into the Pacific rather than gradually east, until the new 00Z ECMWF took the same sort of track as the UKMET had been taking. The amplifying pattern and reasonable agreement on the large scale meant the WPC forecast could be based on a blend of the deterministic 12/18Z GFS, 12Z CMC, and 12Z ECMWF runs through day 5, with incorporation of the EC/GEFS ensemble means by days 6-7. This approach led to better definition of the eastern U.S. surface lows though with the caveat that their tracks may change. ...Weather/Hazard Highlights... Precipitation is possible across the Pacific Northwest and farther east into the Northern Rockies on Tuesday-Wednesday with a frontal system passing through. Model trends have remained generally drier for the Northwest and into the Rockies later in the week as surface high pressure and upper ridging build in. Santa Ana winds across southern California may be a concern though Tuesday-Wednesday. Showers and thunderstorms are possible across Florida and into the Southeast through Tuesday-Wednesday as a low pressure system along a front meanders nearby and focuses moisture across those areas. Isolated flash flooding could be a concern particularly across urban areas. Then as the upper pattern amplifies later in the week by Thanksgiving Day and beyond, Gulf moisture will stream into the eastern half of the U.S. ahead of the trough. The southeastern quadrant of the U.S. northward into the Ohio Valley and Great Lakes may see some precipitation around Thanksgiving, while heavier precipitation looks more likely across the northeastern U.S. on Friday. Current forecasts indicate that wintry precipitation is a better possibility for the Interior Northeast while the metropolitan areas along the I-95 corridor are more likely to get rain. But stay tuned to future forecasts as these uncertain details like precipitation types in particular areas could change as model guidance hopefully comes into better consensus with low tracks and temperatures. Temperatures in the medium range period should be overall more seasonable compared to those of this past week. Cooler than normal temperatures could continue across the southern tier for Tuesday, especially across southern Texas where highs could be 10-20F below normal. Temperatures are forecast to warm across the West Coast by midweek with above average temperatures by around 5-15F expanding into the Rockies and central U.S. later in the week as the upper ridge builds in. Conversely, post-frontal cooler temperatures especially in terms of highs are forecast for the Mississippi Valley toward the East Coast Friday-Saturday as the trough moves through. 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