Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 PM EST Sat Feb 25 2023 Valid 12Z Tue Feb 28 2023 - 12Z Sat Mar 04 2023 ...Significant storm system to eject across the Northeast U.S. and offshore early to mid next week with heavy inland snow... ...A series of active storms for the West through the period also may lead to emerging late next week storm development across the southern then eastern states... ...Guidance/Predictability Assessment... Models and ensembles paint a similar pattern evolution Tuesday through Thursday, albeit with some pesky smaller scale system differences in active and split flow over the lower 48 and vicinity. A composite of best clustered guidance aloft and at the surface from the 00 UTC ECMWF/UKMET/Canadian seemed to offer the best forecast starting point in a period with overall above normal forecast predictability. The pattern seems well forecast into longer time frames from the Pacific into the West, but especially offers increased forecast spread with ejecting and potentially significant system energies and main weather focus across the east-central to eastern U.S. with the models and ensembles remaining more out of sync than usual. Prefer a solution closer to, but not fully toward, recent runs of the GFS that lift energies over the East more overtop what may prove to be a persistent upper ridge from the Southeastern U.S. into the Caribbean. Guidance oftens underestimates the effect from this type of upper ridge. The latest 12 UTC guidance, especially the ECMWF has trended this way, bolstering forecast confidence to some degree. WPC product continuity was reasonably good across the West through medium range time scales and also downstream Tuesday-Thursday, but now does show a more threatening southern U.S. to an east coast storm Friday into next Saturday to monitor. ...Weather/Hazard Highlights... A well organized low pressure system lifting in split southern stream flow is expected to approach the Northeast U.S. by Monday night, with secondary coastal cyclogenesis south of Long Island into Tuesday morning that becomes the new dominant low. Moderate to occasionally heavy snow is appearing likely for much of interior New England on Tuesday in lingering wedged cold high pressure, and a rain/snow mix closer to the coast depending on the exact track of the low and expected upstream interaction with a northern stream system also nearing the region. Although the strongest winds should remain offshore, winds could gust 30 to 50 mph at times during this event and may result in coastal flooding at times of high tides. The precipitation is expected to end Wednesday for the Northeast in the wake of these systems. The Western U.S. will continue to remain unsettled going into the middle of next week with upper trough rebuilding, with moderate coastal rains and heavy snow likely for the Sierra and northern California mountains. The heavy snow threat will then reach the mountain ranges of the Intermountain West/Southwest as height falls and associated surface system track inland. High winds are also a good possibility ahead of the front with the potential for elevated fire weather concerns for the western High Plains Wednesday and into Thursday. There is a grwoing signal for another amplified system to develop off/into the West Coast next weekend and early indications offer a renewed precipitation threat. This seems reasonable given amplified upstream ridging over the east-central Pacific to support ample downstream digging. Meanwhile, heavy rain and thunderstorms are likely to develop across the Mid-South and Lower Mississippi/Tennessee Valleys and vicinity late Wednesday into Thursday/Friday as lead return flow increases in advance of a dynamic/separated southern stream upper trough and associated surface system. There remains an emerging Day 5 "Marginal Risk" area in the WPC experimental medium range Excessive Rainfall Outlook (ERO) in a region with ambient wet soils and potential for repeat downpours. Additional rainfall is expected going into the end of the week, and some of this may be heavy at times downstream across the east-central to eastern U.S., but the latitudinal axis of the heaviest rainfall continues to remain uncertain at this juncture. However, system tendency to lift/shear northeastward Friday into Saturday overtop of a peristent far Southeast U.S. upper ridge position suggests emergence of a swath of heavy snow/ice on the cooled northern periphery of the expanding precipitation shield, especially for the Upper OH Valley/Appalachians to then the northern Mid-Atlantic to Northeastern states as per a potentially deepening coastal storm threat to monitor. In terms of temperatures, much of the central/eastern U.S. should see multiple days of readings 10-20F above normal, and potentially up to 25 degrees above normal on Wednesday across the Ohio Valley. The West will see a persistent cool pattern through the entire week with the Intermountain West and Rocky Mountains most consistently having highs on the order of 10-20F below normal. Hamrick/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, 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