Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Jan 04 2024 Valid 12Z Sun Jan 07 2024 - 12Z Thu Jan 11 2024 ...Major Winter Storm to continue bringing Hazardous Weather across parts of the Northeast into Sunday... ...Another Major Storm to bring heavy rain/strong convection and gusty winds across the South-central U.S. and into the East early to middle of next week, with a heavy snow threat to the north... ...Dynamic Storm Systems to focus coastal rain/inland heavy snow threats over the Northwest/West next week... ...Overview... An extremely active medium range period remains on tap with a series of major storm systems in the forecast. A leading system will be exiting the East on Sunday, with some potential for lingering hazardous weather across parts of the Northeast. In its wake, another major upper trough will be digging through a highly unsettled West this weekend as well, focusing wintry threats for terrain/mountain snows and winds. Deep cyclogenesis then remains likely across the south-central Plains by Monday, with impressive lead moisture and instability returning to fuel widespread heavy rainfall/convection and runoff issues across the south-central to eastern U.S. through the middle of next week. To the north of the deep low's track, there is also a significant snow/ice threat from the central Plains to the Midwest/Great Lakes and Northeast Monday-Wednesday, along with strong and gusty winds. Additional dynamic systems in the series look to move into the Pacific Northwest early next week before digging across the West into mid-late next week. ...Guidance/Predictability Assessment... The latest guidance continues to suggest better than average confidence for significant large scale features in a very active January storm pattern across much of the nation for at least the next week. There are some detail differences for leading systems affecting the eastern half of the country while the latter part of the period features typically increasing spread for some specifics within the upper trough amplifying over the western U.S. and northern Plains. For the system expected to be just south of New England as of early Sunday, the primary difference at the moment is that the new 12Z GFS is on the southwestern side of the guidance spread (with a corresponding shift in the axis of heaviest New England snowfall). Prior GFS runs were less extreme in that regard. Then the primary focus during the period will be on the storm likely to develop and impact much of eastern half of the country next week. Models agree on a deepening upper trough across the West Sunday, with 00Z/06Z guidance clustering fairly well for the overall track and timing as the system kicks out into the Plains and tracks through the eastern Great Lakes by Wednesday. The new 12Z CMC has strayed from established consensus and its 00Z run in showing a farther south track of the upper low/surface system. The strength of this system will be another forecast consideration. Especially by early Wednesday the 00Z ECMWF shows a deeper surface low than other guidance or its previous runs with a central pressure in the mid 960s mb (tempered a bit in the new 12Z run) while in contrast latest GFS runs are somewhat on the weaker side of the spread in the low 980s. Most of the 00Z ECMWF-initialized machine learning models favor a depth close to 975 mb and the UKMET/CMC are similar. Thus for now will depict a central pressure just under 976 mb as of early Wednesday. Verification on the deeper side of the guidance spread could challenge January sea level pressure records within an area from the south-central Mississippi Valley into Lower Ohio Valley. Records over areas farther east in the Ohio Valley/Great Lakes are from the historic late January storm of 1978 and appear much more difficult to reach. Behind this system, there is reasonable consensus for a frontal system to reach the Pacific Northwest around Monday. The developing medium-smaller scale shortwave differences lead to some spread for an immediately trailing system, with this energy and perhaps upstream flow from western Canada differing Wednesday-Thursday within a more agreeable large scale western trough. The 12Z GFS has changed some details from its 00Z/06Z runs while the 12Z CMC has strayed the most from the overall model/ensemble majority, becoming quite fast with incoming Pacific Northwest energy. By next Thursday there is a good signal for low pressure to reach the Plains but specifics will be sensitive to shortwave details that may take a while to resolve, while a strong cold front should dip into the far northern tier. A 00Z/06Z model composite provided a good start for the updated forecast through the first half of the period, reflecting consensus and/or the preferred intermediate solution. Even by days 6-7 Wednesday-Thursday a blend of the GFS/ECMWF/CMC already toned down some less confident aspects of any particular solution with general support from the ensemble means, requiring only moderate incorporation of 06Z GEFS/00Z ECens. ...Weather/Hazards Highlights... The major surface low bringing snow and ice to parts of the Mid-Atlantic/Northeast will be exiting the East Coast by Sunday, but there could still be some hazardous weather across parts of New England, including moderate to heavy snows and gusty winds. Currently expect the best potential for meaningful snow on Sunday to be over eastern portions of New England. The next system affecting the West will bring some rain and mountain snows across the Four Corners region on Sunday with a decent signal for meaningful snow accumulations across parts of the Southern Rockies. As the parent trough and embedded closed low eject into the Plains, an area of low pressure will deepen as it tracks northeastward into the Mississippi Valley and eastern Great Lakes next week. This system will tap Gulf of Mexico moisture to fuel widespread moderate to heavy rainfall across much of the Mid/Lower Mississippi Valley and the South on Monday with some severe weather potential as well along the central Gulf Coast, per the latest outlook from the Storm Prediction Center. Given ample moisture and instability, and wet ground conditions due to rainfall expected in the short range period, the Day 5 Excessive Rainfall Outlook depicts a Slight Risk area across the central/east-central Gulf Coast region--expanded somewhat relative to prior issuance based on the best consensus of latest model/ensemble signals. A broad Marginal Risk area covers the South and extends farther northward into the Mississippi Valley where there could be a secondary maximum closer to the low track. On Tuesday rain will move into the Eastern U.S. and depending on snow accumulations across parts of the East this weekend, this could bring an increased risk for flooding due to excessive rain and snowmelt. Meanwhile severe weather potential may continue eastward into the far Southeast/Florida. Expect at least a couple days of strong and gusty winds around the deep low across the East-Central U.S. to Northeastern U.S. as this system strengthens. There is also an increasing threat for a widespread swath of heavy snow and ice to the north of the low from the Central Plains to the Midwest/Great Lakes then Northeast. The strong winds could pose a coastal flooding risk along the East Coast ahead of the storm, while blowing and drifting of snow would be a threat in the cold sector. Additional dynamic systems in the series look to move into the Pacific Northwest early next week with rounds of moderate to heavy rainfall expected for the coastal ranges and foothills of the WA/OR Cascades. Given the ongoing wet pattern across this region later this week/this weekend, the Day 5/Monday Excessive Rainfall Outlook maintains a Marginal Risk area for this region. Details of this event are currently ambiguous though, with the average of guidance suggesting the best persistence of anomalous moisture in the southern half of the risk area versus some ensemble/model guidance showing the best QPF signal farther north. Expect heavy snow in the higher terrain of the Cascades. Rain and mountain snow should also extend into parts of the interior West/Rockies through mid-late week. Precipitation of various types could develop over the central U.S. by around Thursday as low pressure reaches the region. Temperatures should generally trend cooler across much of the Western U.S. and at times extending into the High Plains next week as mean troughing aloft remains anchored over the region. There should be decent coverage of daytime highs 5-15F below normal Sunday-Monday with some slight moderation around Tuesday, followed by another colder trend Thursday that would lead to more highs 5-15F below normal. Some highs may be up to 20-30F below normal over the northern High Plains at that time. Ahead of the next storm system in the Central U.S. early next week, temperatures across the East will likely be moderately above normal with the greatest anomalies being for morning lows across the Great Lakes/Northeast. Rausch/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