Extended Forecast Discussion NWS Weather Prediction Center College Park MD 215 PM EST Sun Feb 05 2023 Valid 12Z Wed Feb 08 2023 - 12Z Sun Feb 12 2023 ...Guidance/Predictability Assessment... Latest guidance continues to exhibit higher-than-normal uncertainty for much of the medium-range period especially across the eastern two-thirds of the country. The medium-range period will begin on Wednesday with the models generally agree that the shortwave/upper low that is forecast to eject out from the Southwest will develop into a low pressure system over the southern Plains and track toward the northeast. However, models continue to show a great deal of spread regarding the timing and amplitude of this system thereafter, with the GFS being the fastest, the CMC being quite slow, while the ECMWF is in between these extremes. The subsequent evolution of this system over the eastern U.S./East Coast later this week will be further complicated by a simultaneous flow amplification across the U.S., with a much amplified ridge over the western U.S. while a large closed low is forecast to develop somewhere near/off the West Coast with a high degree of uncertainty on its location. In addition, a shortwave rapidly moving across the Pacific Northwest on Wednesday would further complicate the pattern evolution in the eastern U.S. late this week as the wave could positively reinforce a surge of polar air down the Plains during the latter part of the week, which could trigger the development of a smaller scale low pressure system somewhere in the Mid-South as indicated by the deterministic model solutions. This system may then interact with residual energy on the back side of the first system off the New England coast to form another low pressure system near the East Coast. By next Sunday, models generally agree that the low pressure system(s) should begin to move away from East Coast but the whereabouts of the upper low near/off the West Coast remain highly uncertain. The WPC medium range product suite through medium-range period was primarily derived from a composite blend of 40% from the 00Z GEFS/00Z & 06Z GFS, 40% from to 00Z ECMWF/EC mean, and 20% from the 00Z CMC and CMC mean. The 06Z GEFS was not included due to a much faster northeastward track of the low from the South to the Great Lakes. As for the low near/off the West Coast next weekend, a solution between the 00Z GEFS and the 00Z EC mean was adapted, which places the system near the southern California coast by next Sunday. This solution is supported by the latest 12Z ECMWF with a faster eastward motion of the system. ...Weather/Hazard Highlights... An initially closed southern stream upper trough/low will eject into the southern Plains by midweek as a kicker shortwave trough digs southeastward through the Great Basin to support some potential for snow there into the Rockies. Downstream, a surface frontal wave over Texas with the lead system will organize, deepen and occlude while lifting northeast into the Midwest/OH Valley Thursday before exiting through the Northeast Friday. Gulf moisture inflow/instability and supportive upper jet/height falls will combine with the associated frontal system to produce a swath of heavy rain/convection this period across the Lower/Mid MS and TN/OH Valleys, with widespread moderate rains then shearing across the Appalachians and Eastern Seaboard. The WPC experimental Day 4 Excessive Rainfall Outlook (ERO) will show a "marginal" threat area over the mid MS Valley and vicinity through Wednesday, with a noticeable northeastward shift compared with the previous forecast. Cold air draped to the north of the system and several potent shots of Canadian air in the wake of system passage will also offer a threat for heavy snow/ice on the northern periphery of the expanding precipitation shield this period from the central Plains through the Great Lakes and interior Northeast and southern Canada. Expect lake-effect snows into the weekend given lingering cold/windy flow. Upstream, there is a growing guidance signal for the amplification of the overall flow significantly into later week across the nation and adjoining oceans, but with limited precipitation re-emergence overall at this point. Organized rains stay mainly off the West Coast this period, with significant eastern Pacific storm development held just offshore. Renewed shortwave approach out from the West and potentially the northern stream will meanwhile help carve out an amplified central to east-central upper trough position later week into next weekend, but responding surface system development does not seem to have deep moisture inflow in the wake of the aforementioned lead main system, so expect an expanding area of overall moderate rainfall signature with a main focus likely to lift from the Southeast to up the East Coast next weekend with modest coastal low genesis and Atlantic inflow that could offer a late period interior snow threat over the Northeast to monitor. Kong/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