Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Thu Mar 02 2023 Valid 12Z Sun Mar 05 2023 - 12Z Thu Mar 09 2023 ...Overview... Expect the upper trough over and inland from the West Coast to persist at least into the first half of next week, while leading energy initially ejecting from the West should support a Plains through Northeast wave whose trailing front (combined with a weakening northern tier front) will ultimately bring a colder trend after another episode of central/eastern U.S. warmth. The energy from this system will likely feed into a slowly retrograding North Atlantic Rex block pattern. By next Thursday the upper low near the Canadian Maritimes may start to progress east a bit but the higher latitude upper ridge may reach close to Hudson Bay while elongated troughing persists over the Northeast. Meanwhile the eastern Pacific/western North America pattern rapidly becomes more uncertain by next Wednesday-Thursday depending on the details of upper ridging over and south of Alaska and the ultimate evolution of western Canada upper low energy. The resulting differences in upper flow pattern lead to a wide array of possibilities regarding the coverage and intensity of precipitation over the West at that time. ...Guidance/Predictability Assessment... A composite of 00Z/06Z operational models provided a good representation of the most common elements of guidance during the first half of the period, with solutions clustered well in principle and only a small number of questionable regional details to downplay. Of particular note, the 00Z CMC was a suppressed extreme with the wave reaching the East by day 5 Tuesday (significantly improved in the 12Z CMC) while 00Z/06Z GFS runs held some trailing energy over the Upper Midwest/Great Lakes for a time in a manner unlike other guidance. The 12Z GFS looks more like other guidance in being more progressive with this energy but is somewhat of a deep extreme at the surface and aloft, and latest GFS runs have been depicting a Great Lakes QPF maximum that is quite extreme versus other solutions. Models are more compatible detail-wise over the eastern Pacific/western U.S. during the first half of the extended period. From late Tuesday onward, the spread among models and ensemble members increases rapidly over the eastern Pacific and western North America. This divergence relates to differences in evolution/position of initial Pacific through Alaska upper ridging western Canada upper low/shortwave energy to the east. Beyond the spread in the current guidance, consecutive GFS/ECMWF runs have varied considerably as well. Recent GEFS means and the more amplified 00Z/06Z GFS keep the upper trough close to the West Coast into day 7 Thursday while the 00Z ECMWF/ECMWF and CMC/CMC mean are all farther inland to varying degrees (and the new 12Z ECMWF adjusting much faster than the 00Z run). Note that all of the ensemble systems exhibit a lot of spread among members even though the means are somewhat more coherent. The 00Z ECMWF mean is actually slower than most of its prior runs. By day 7 the 12Z GFS now brings an upper low offshore the Pacific Northwest (12Z CMC near the Alaska Panhandle) with both of those runs showing fairly flat mean flow over the lower 48. Interestingly, some earlier GFS runs also depicted such an offshore upper low in some fashion. Teleconnections relative to the core of positive height anomalies near Bristol Bay in some D+8 multi-day means could support some troughing at least over the northeastern Pacific (and are consistent with the cold temperatures forecast over the Plains by mid-late week). Forecast preference was to follow a general model/ensemble mean blend tilted a bit more in the ECMWF mean direction by the end of the period, to yield the best intermediate forecast in light of what seems like above average potential for significant future changes. The blocky pattern farther east should tend to favor a more suppressed pattern but with uncertain details of western U.S. trough energy providing a wild card that could override that trend for a time. ...Weather/Hazard Highlights... The deep upper trough persisting over and inland from the West Coast at least into early next week should continue to support periods of rain along the northern/central West Coast and snow in the higher elevations, with highest accumulations likely to be over the Sierra Nevada. Lesser totals should extend into the Great Basin/Rockies. 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 primary system of note from Sunday into early next week will be Plains through Northeast low pressure and trailing front. This system will produce an area of snow north/rain south across the northern third of the central/eastern U.S. with some rain possible farther south along the front by Tuesday. This area of rain could become locally heavier over parts of the South with an increase of moisture and possible waviness along the front. Late-period forecast issues over the West begin to affect areas east of the Rockies by Wednesday-Thursday, with decreasing confidence in precipitation coverage and amounts. The forecast pattern will support breezy conditions over the Northeast Tuesday onward with some areas of terrain-enhanced snow. 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-Thursday. 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 corresponding to system progression. 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. 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