Extended Forecast Discussion NWS Weather Prediction Center College Park MD 137 PM EST Thu Feb 08 2024 Valid 12Z Sun Feb 11 2024 - 12Z Thu Feb 15 2024 ...Overview... It generally remains the case that split flow across the nation into early next week features a northern stream weak shortwave through the Great Lakes and more notably, an amplified upper shortwave over the Southwest. The latter will shift steadily through the South Monday and Tuesday associated with a deepening low pressure system at the surface, leading into possible re-development into a coastal storm. This would bring moderate to heavy rainfall potential to parts of the Southeast/Mid-Atlantic and snow across the Northeast. As this system exits offshore the Northeast later Tuesday, the flow behind it should become more zonal across much of the nation, with a weak shortwave through the northern tier Wednesday-next Thursday and a couple of weak shortwaves into the West Coast. ...Guidance/Predictability Assessment... The latest GFS/ECMWF/Canadian/UKMET solutions continue to offer reasonably good forecast clustering this weekend into Monday. A model composite seems to offer a solid forecast basis along with the National Blend of Models and WPC product continuity. Forecast spread rapidily increases next week as models and ensembles remain highly sensitive to embedded system and potential stream phasing uncertainties in complex and transitional flow. The main issue in this time frame still concerns the potential development and track of a main southern U.S. low to eastern U.S./Western Alantic coastal storm by Tuesday and associated inland and maritime impacts. Machine learning models still offer varied solutions with this main system in this period. WPC products have over the past few cycles trended to show a somewhat less progressive and moderately deeper low track given potential support along with a composite track based on all guidance. This plan maintains good WPC product continuity despite rampant run-run and model-model system variances recently, and ensemble means that seem too progressive given the historical bias of well defined systems working through an at least initially separated southern stream flow. ...Overview and Weather/Hazards Highlights... Increased moisture return well ahead of a main system slated to exit eastward from the Southwest will fuel potential for heavy rainfall from the Central Gulf Coast states into the Southeast Sunday. Repeat/training of activity along a frontal boundary, combined with anomalous moisture and some instability, may trigger a flash flood risk. Accordingly, a Marginal Risk area continues to be highlighted on the Day 4/Sunday Excessive Rainfall Outlook (ERO). Suspect the threat over the Southeast may linger into Monday whose Day 5/ERO now shows a small Marginal Risk area. Additionally, as the main low pressure starts to deepen across the south-central Plains Sunday, heavy rainfall may also focus locally just to the north of the low given upper level dynamics. Latest runs of models have indicated some secondary QPF maxes across parts of Oklahoma. Some accumulating snowfall is also possible across the northern portion of the precipitation shield across the central Plains/middle Mississippi Valley as well. As the system progresses and potentially redevelops as a coastal storm Monday into Tuesday in a period with uncertain stream phasing, heavy rainfall potential should lift northward into parts of the Mid-Atlantic and Northeast. Increasing progression should limit runoff risks over much of this broad region. There is however an increasing potential for the spread of wrap-back accumulating snows and windy conditions across the marginally cool Appalachians/interior Northeast Monday and Tuesday before the whole system works increasingly offshore as a continued maritime hazard. Behind this system, conditions across much of the CONUS should dry out mid next week besides periodic intrusions from the Pacific to bring some modest precipitation to parts of the Northwest. Ahead of the Eastern U.S. system, temperatures Sunday into Monday could be 5-15 degrees above average, with morning lows even more anomalously warm given clouds and humidity. The East should moderate back closer to normal Tuesday and beyond. In contrast, the Southwest/California should be cool with upper system passage, especially daytime highs that may be 10-15F below normal on Sunday before moderating. Some ridging in the upper pattern may also support above normal temperatures across parts of the Northern tier through about Wednesday. Santorelli/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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw