Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Jan 03 2024 Valid 12Z Sat Jan 06 2024 - 12Z Wed Jan 10 2024 ...Major storm system to bring hazardous weather across parts of the Mid-Atlantic and Northeast this weekend... ...A second significant storm system could bring heavy rain across the South-central U.S. and into the East early to middle of next week, with heavy snow to the north... ...Overview... Guidance continues to advertise an active medium range period with two major storm systems in the forecast. The leading system moves into the East this weekend, with increasing potential for heavy rain near the coast, and heavy snow across interior portions of the Mid-Atlantic and parts of New England. In its wake, the next trough drops into the West this weekend as well bringing another round of valley rain and mountain snows progressing from California into the Four Corners region. Cyclogenesis appears likely across the south-central Plains by Monday, bringing another round of heavy rainfall to parts of the South/Tennessee Valley region Monday-Tuesday, with increasing snow potential to the north of the low track from the central Plains into the Great Lakes and interior Northeast Monday-Tuesday. Another trough/closed low looks to move into the Pacific Northwest early next week. ...Guidance/Predictability Assessment... The latest model runs continue to maintain above average confidence for the overall large scale pattern, but offer plenty of uncertainty in the details/timing of individual systems. The strengthening surface low lifting through the Southeast and off the Mid-Atlantic coast on Saturday has trended faster but key differences in exact track would have huge implications for precipitation type, especially for more eastern areas (including the DC-NYC corridor). There is plenty of wobble in the surface low track run to run, and will likely take until the near term to fully sort out. A blend of the latest deterministic guidance offered a good starting point for the forecast and maintained good continuity with the previous WPC forecast until more confidence is gleaned in any substantial adjustments. Meanwhile, another trough enters the Pacific Northwest early Saturday and the guidance shows generally good agreement that it will amplify over the West before ejecting into the Plains around Monday. A surface low likely spins up near the Texas/Oklahoma panhandle early Monday, deepening as it generally tracks east-northeastward into the Middle Mississippi Valley on Tuesday and the Great Lakes by Wednesday. There is plenty of uncertainty though on the exact track and a solution closest to the ensemble means seems to suffice at this time. Yet another system looks to enter the Pacific Northwest late in the period/early next week, but there is a lot of uncertainty in both the strength and timing of this system. WPC prefers a blend of the ensemble means to help mitigate the differences at these longer time scales. ...Weather/Hazards Highlights... A major surface low tracking off the Mid-Atlantic coast on Saturday continues to show increasing snow potential to the north of the low, primarily for portions of the Central Appalachians and interior Mid-Atlantic. There may be a period of freezing rain/ice as well for parts of the Central Appalachians. Closer to the coast, and including the major DC-NYC corridor, there is plenty of uncertainty regarding precipitation type and amounts, highly dependent on the exact track of the low and availability of sufficiently cold air which will take well into the short range to fully resolve. There is some guidance suggesting these major cities could see moderate to heavy snow accumulations, but also potential for little accumulation and more rain. A marginal risk for flash flooding remains on the Day 4/Saturday Excessive Rainfall Outlook but was confined mainly for the coastal Carolinas and eastern Mid-Atlantic given a overall faster progression and lack of significant rainfall. Accumulating snow may impact parts of southern/eastern New England as well into Sunday. The next trough into the West will bring expanding coverage of rain and mountain snows across the Northwest and into the Four Corners region this weekend. There is a decent signal for meaningful mountain snows across parts of the Cascades, Sierra Nevada, and interior mountain ranges/southern Rockies. As the parent trough ejects into the Plains, an area of low pressure looks to form and deepen as it tracks northeastward into the Mississippi Valley and Great Lakes next week. This 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 from the Storm Prediction Center. Rain will move into the Eastern U.S. on Tuesday and depending on snow accumulations across parts of the East early in the period, this could bring an increased risk for flooding due to excessive rain and snowmelt. A period of gusty winds looks likely as well across the East-Central U.S. as this system strengthens. There is also an increasing threat for heavy snows to the north of the low from the Central Plains towards the Great Lakes, and possibly into parts of the interior Northeast as well. This weekend above seasonal average temperatures can be expected across the far northern tier states with anomalies generally plus 5-15F for highs, and perhaps as high as plus 15-25F for morning lows over parts of Minnesota/Wisconsin. Above normal temperatures also look to return to the Southern Plains/Lower Mississippi Valley and eventually the East early next week. The southern two-thirds of the West will likely have below normal temperatures late this week, with some cooler anomalies at least 10F below normal for highs spreading across the West into the High Plains by Sunday-Tuesday as upper troughing moves over the region. Parts of the Northwest and northern Plains should trend below normal as well, with modestly below normal temperatures across the Southern Plains/Southeast following passage of the strong cold front early next week. 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