Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Fri Feb 10 2023 Valid 12Z Mon Feb 13 2023 - 12Z Fri Feb 17 2023 ...Stormy flow with a series of upper lows in split southern stream flow from the Western U.S. northeastward... ...Overview... The early to middle part of next week will feature a continuation of the short range tendency for upper lows to close off in the southern stream and then track northeastward, followed by the potential for more phased troughing to reach the eastern half of the country as a ridge moves into the West. Within this pattern, a leading system will track away from the East Coast as a northern stream shortwave approaches. Another upper low should track out of the Southwest and reach the Great Lakes by Wednesday while a trailing upper trough digging into the West will likely close off a low around Tuesday-Wednesday with the upper dynamics and potentially fairly strong surface system tracking from the Plains into eastern Canada mid-late week. Expect the latter two systems to produce a broad area of active weather including heavy rain, snow, and severe weather across the lower 48 during next week. ...Guidance/Predictability Assessment... The deterministic forecast guidance continues to show good agreement in terms of the pattern evolution through most of the medium-range period between both the individual models and run-to-run. The ensemble guidance backs the above average confidence in the pattern with initially low spread between the individual members and a steady expected increase with forecast lead time. The period begins with a compact closed low departing the East Coast to the northeast and another wave over the Southwest ejecting out over the Plains. The main area of interest will be during the middle of the period (day 5) as a second trough digs deep in the West and a closed low develops. The trough shifts eastward over the Plains by day 6 with a surface low pressure system developing in the lee of the Rockies, moving to the northeast over the Mississippi Valley and eventually past the Great Lakes and into southern Canada. A ridge is expected to build over the West by the end of the period with another trough/potential cutoff low developing upstream over the eastern Pacific. Guidance differences relate mostly to the deepness of the closed low over the western U.S. and with the timing of embedded shortwave energy, with more agreement between the 06 UTC GFS/00 UTC ECMWF compared to the 00 UTC CMC/UKMET. Specifically, the 00 UTC UKMET does not fully develop a closed low. Both the overall agreement in the pattern and more subtle differences amongst the guidance are remarkably similar to continuity with the prior WPC forecast blend and thus a similar blend of the deterministic guidance weighted more towards the GFS/ECMWF was used early in the forecast period. The blend shifted to incorporate more of the 00 UTC GEFS/ECENS means following the end of the 00 UTC UKMET forecast period, which was timed well with the natural growing divergence in the solutions. This also coincided with more significant differences amongst the guidance with respect to the development of the upstream trough over the Pacific coupled with the degree of ridging over the west. A blend with the means captures the overall phasing of this pattern well with more specific details to be resolved later. ...Weather/Hazard Highlights... Most of the moisture associated with the system departing from the East Coast should stay offshore, though it remains a close call for southeastern New England where minor changes in track could make the difference between dry conditions or meaningful precipitation. The system initially tracking out of the Southwest early next week should produce some rain and higher elevation snow over Arizona and the southern Rockies, followed by an area of precipitation spreading across the central-southern Plains and most of the east-central U.S. This should be a fairly warm system with the chance for some light snow confined to the northern Plains/Upper Midwest. Rainfall may be moderate to heavy over the Plains/Mississippi Valley, particularly on Tuesday where forecast amounts have trended a bit upward as the supporting front may stall over the central Gulf Coast/Southeast by around Tuesday night. The combined threat of heavier rainfall and increasingly wet antecedent conditions has led to a Marginal Risk area on the day 5 experimental Excessive Rainfall Outlook for an isolated threat of flash flooding. The next system moving into the West should bring a brief period of enhanced rainfall/heavy mountain snow to the Pacific Northwest and to some degree in the northern Rockies early next week, with moisture then continuing southeastward through Tuesday/early Wednesday with the highest precipitation totals likely to be over favored terrain of Arizona/Utah/Colorado as an upper low passes over the region. Expected system emergence over the south-central Plains and then a track northeastward into eastern Canada mid-late week will likely renew a heavy precipitation pattern over the eastern half of the country. More meaningful snow is possible in the cold sector of this system relative to its predecessor, with the most probable axis for significant accumulations currently extending from the central High Plains into the central/upper Great Lakes. Meanwhile leading Gulf inflow ahead of the system's trailing front should promote areas of heavy rainfall for the Southeast/southern Appalachians and up the East Coast. Areas of severe weather are also expected from the Lower Mississippi Valley into the Southeast given strong low-level and deep-layer shear and sufficiently buoyant air spreading northward from the Gulf. Guidance is signaling this storm could become deep enough to produce fairly strong winds. High pressure will settle in over the Interior West as this prior system departs over the East. While there remains more significant uncertainty in the guidance, precipitation amounts have trended upward along the immediate West Coast late in the period as another system over the Pacific approaches the region. The upper trough/closed low digging over the West will bring highs down 10-25F below normal over the southern two-thirds of the region during Tuesday-Thursday. Cool temperatures should persist into Friday but with more moderate anomalies. Warm flow ahead of the two primary systems next week will bring well above normal temperatures to parts of the central U.S. during the first half of the week and the eastern U.S. through at least Thursday. Some readings may reach at least 20-25F above normal for both high and low temperatures. Widespread record warm lows are possible especially Wednesday-Friday (if they hold through the end of the calendar day), while some record highs could be possible around Thursday over New England and the Florida Peninsula. Putnam/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