Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Fri Mar 03 2023 Valid 12Z Mon Mar 06 2023 - 12Z Fri Mar 10 2023 ...Overview... Next week the lower 48 looks to generally be in a less amplified pattern compared to its west and east, where over the eastern Pacific into Alaska a strong upper ridge/high is likely, and in the western Atlantic a Rex Block pattern should be slowly retrograding. Through the early part of the week, some troughing will likely persist in the West for some precipitation chances and cooler temperatures there, while leading energy should support a surface low moving across the Plains to Mid-Atlantic/Northeast. Precipitation including some snow is likely across the Great Lakes region Monday and possibly spreading into the Interior Northeast by Tuesday, while the low's trailing front will provide a cooling trend across the central and eastern CONUS. By midweek and beyond the overall pattern becomes much more uncertain as it depends on the evolution of western Canada upper low energy as well as the details of the upper ridging over and south of Alaska. The resulting differences in upper flow pattern unfortunately lead to a wide array of possibilities regarding the sensible weather over the U.S. by the latter half of next week. ...Guidance/Predictability Assessment... Early in the forecast period Monday-Tuesday, model guidance remains in fairly good agreement with the overall pattern. This includes a western U.S. trough, a shortwave crossing the Midwest/Great Lakes region into the Northeast, and the latter's associated surface low. Thus early in the period, a multi-model blend of the deterministic 18Z GFS and 12Z ECMWF/CMC/UKMET was able to be used. But models and ensembles quickly diverged as issues with the amplified pattern atop Alaska/western Canada wreak havoc on the pattern downstream. There have been considerable differences from model to model and run to run within the same model suite. The 12/18Z GFS and the 12Z UKMET pull energy off into a closed low just offshore British Columbia by day 5/Wednesday and then the GFS has it sit over the Pacific, unlike the CMC and ECMWF. The 12Z ECMWF seemed to be fast with its trek of troughing moving eastward across the central U.S. by Thursday, and the new 00Z ECMWF did slow down with this. Overall the newer 00Z guidance seems to be in better agreement in principle fortunately. But for the WPC forecast with only the 12/18Z cycle available, had quickly transition to a blend heavy on the ensemble means from the GEFS, ECMWF, and CMC to create the forecast. Even the means were not terribly agreeable though in terms of the trough positions. There is some general agreement for ridging to come into the West by day 7/next Friday. Overall, predictability becomes quite low for the latter part of the forecast period and there may be more significant changes than usual in future forecasts during this time frame. ...Weather/Hazard Highlights... The upper trough over the western U.S. should continue to support periods of rain along the northern/central West Coast and snow in the higher elevations, but the best chances for higher accumulations have phased into the short range period. Light precipitation is also possible across the Rockies into the High Plains, with some slightly enhanced snow amounts in the central Rockies possible. Confidence rapidly decreases for coverage and intensity of precipitation over the West after Tuesday, with possibilities ranging from a drier trend as upper ridging approaches to persistence of precipitation with a longer duration of upper troughing, or even a transition to a more zonal and moist flow by Thursday or so. Farther east, the first system of note will be the low pressure system tracking from the Middle Mississippi Valley toward the Mid-Atlantic/Northeast Monday-Tuesday along with its trailing cold front. This will produce some rain and snow across the northern third of the central/eastern U.S., and then rain and thunderstorm chances look to increase across the south-central to southeastern U.S. by Tuesday as Gulf moisture increases. Since the airmass looks to be unstable, rain rates are likely to be high, and a Marginal Risk was introduced for day 5 in the experimental Excessive Rainfall Outlook. As similar areas are receiving heavy rainfall/flash flooding in the near term as well, this area may be primed to have flooding concerns, and the risk level may need to be upgraded in future cycles if there becomes better agreement in the axis of heavy rain. The forecast pattern will support breezy conditions over the Northeast Tuesday onward with some areas of terrain-enhanced snow. Late-period forecast issues over the West begin to affect areas east of the Rockies as well by Wednesday-Friday, with decreasing confidence in precipitation coverage and amounts. In general it seems that a rainy pattern may continue across the Southeast with some potential for snow around the Central Plains or so, but is quite uncertain. Below normal temperatures over much of the West should persist through the period while also spreading into the northern/central Plains next week. There will be decent coverage of highs 10F or more below normal within this area and there may be increased potential for some anomalies to reach 20F or more below normal over the central Rockies and High Plains toward next Wednesday-Friday. Meanwhile expect more above normal temperatures to extend across all but the extreme northern parts of the central/eastern U.S. during the first part of the week, in the warm sector ahead of Plains/Great Lakes/Northeast low pressure and trailing cold front. Greatest max/min temperature anomalies of plus 10-25F should be from early Monday into early Wednesday with a cooling trend coming in from the west/north with the cold front. By midweek or so, the blocky upper pattern setting up over eastern Canada and the Northeast should keep temperatures within a few degrees on either side of normal aside from above normal lows over northern New England and lows in particular near the Gulf Coast. Tate/Rausch 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