Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 AM EST Thu Feb 08 2024 Valid 12Z Sun Feb 11 2024 - 12Z Thu Feb 15 2024 ...Overview... Split flow across the nation into early next week features a northern stream weak shortwave through the Great Lakes and more notable, an amplified upper shortwave over the Southwest. The latter will shift steadily through the Mid-South Monday and Tuesday associated with a deepening low pressure system at the surface bringing heavy rainfall potential to parts of the Southeast/Mid-Atlantic and snow across the Northeast. As this system exits the Northeast late Tuesday, the flow behind it should become more zonal with a weak shortwave through the northern tier Wednesday-Thursday and a couple of weak shortwaves into the West Coast. ...Guidance/Predictability Assessment... The latest guidance continues to show good agreement on the overall pattern shifting from amplified in the early period to more zonal/weaker systems late period, but there remain plenty of variability in the details. The main system of interest in the period concerns the potential development and track of a Southern to Eastern U.S., then Western Atlantic, coastal storm early next week with associated inland and maritime impacts. Models have shown much better agreement in this system compared to a few days ago, but there are still some notable timing issues. The 12z (Feb 7) ECMWF run was faster with the low through the Southeast and off the Mid-Atlantic Coast Monday-Tuesday compared to the deterministic GFS, UKMET, and CMC, but the 00z run tonight came in a little slower. Still some considerable run to run variability with the low, so additional adjustments are likely in the coming days. A nearly equal blend of the four deterministic solutions seemed to give a good middle ground starting point for days 3-5 with this system and maintained fairly good WPC continuity. Otherwise, as the flow turns more zonal and stable, used a blend of the ensemble means with the more agreeable GFS and ECMWF for the later periods. This helped to mitigate any of the smaller scale detail uncertainties. ...Overview and Weather/Hazards Highlights... Increased moisture return ahead of the main system over the Southwest will continue from the very end of the short range period into the start of the medium range period/Sunday to fuel heavy rainfall potential from the Lower Mississippi Valley into the Southeast. Repeat/training of activity along a frontal boundary, combined with anomalous moisture and some instability, may trigger a flash flood risk, and a Marginal Risk continues to be highlighted on the Day 4/Sunday Excessive Rainfall Outlook across the Southeast. As the main low pressure starts to deepen across the south-central Plains, heavy rainfall may be favored in this region/just to the north of the low given upper level dynamics. Latest runs of models have indicated some secondary QPF maxes across parts of central-eastern Oklahoma, so a small marginal risk was added to Sunday's outlook for this potential. Some accumulating snowfall is also possible across the far northern portion of the precipitation shield across the central Plains/middle Mississippi Valley as well. As the deepening system shifts east, heavy rainfall should lift northward into parts of the Mid-Atlantic and Northeast on Monday, but looks to be progressive enough to preclude any kind of risk area on the Day 5/Monday ERO at this point. Also increasing potential for wrap-back accumulating snows across the Appalachians/interior Northeast around Monday-Tuesday before the whole system exits offshore. 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 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