Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Fri Jan 5 2024 Valid 12Z Mon Jan 8 2024 - 12Z Fri Jan 12 2024 ...Intense low pressure system tracking from the Southern Plains to the Great Lakes expected to bring widespread rain, snow, wind, and severe thunderstorms for early in the week... ...General Overview... A major upper trough will be digging across an unsettled Western U.S. this weekend, focusing wintry threats for terrain/mountain snows and winds. Deep surface cyclogenesis is expected across the south-central Plains by Monday, with impressive 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 00Z model guidance suite has a good overall depiction of the synoptic scale weather pattern for the first half of the week and the evolution of the major low pressure system over the Plains and Great Lakes. The 12Z CMC that was available at the time of fronts/pressures composition was noted to be well south of the existing model consensus across the southern U.S., and also south of the prior CMC runs, so the 00Z Thursday run of the CMC was substituted for the 12Z run since it better matched the consensus. Taking that into account, a multi-deterministic model blend was used for the Monday-Tuesday time period with greater weighting towards the GFS/ECMWF. Going into the latter half of the week, increasing model differences in the wake of the strong low necessitated the use of more ensemble means, up to about 40-50% by Friday. ...Weather/Hazards Highlights... With the Northeast U.S. storm system moving well out to sea by Monday morning, attention then turns to the developing major low pressure system across the south-central U.S. that has been on the weather horizon for the past several days. Confidence continues to increase for a highly impactful event spanning from Texas to the Great Lakes and Northeast U.S. through the middle of the week as the surface low becomes anomalously strong over Arkansas and then lifts northeast towards the eastern Great Lakes by Wednesday morning. Heavy rainfall will indeed be one of the things making weather headlines as a very strong low level jet advects deep Gulf of Mexico moisture northward across the central Gulf Coast region on Monday, and with the potential for 2 to 4 inches of rainfall from multiple rounds of convection, a Slight Risk remains warranted for the new Day 4 ERO on Monday from southern Louisiana to the Florida Peninsula, where hourly rainfall rates could easily exceed one inch per hour. The heavy rainfall threat continues going into Day 5 on Tuesday with widespread 2-3 inch rainfall totals likely from the southern Appalachians to southern New England. A Slight Risk area is planned from extreme northeast Georgia to western Virginia where a strong southeasterly low level jet will advect highly anomalous moisture towards the terrain and result in enhanced rainfall totals and rates for these areas, increasing the threat of flooding problems. A second Slight Risk area is planned for eastern PA to western MA and most of CT and RI, where there is a strong deterministic model signal for 2 to 4 inch rainfall totals during this time with impressive IVT values expected. A broader Marginal Risk area includes all areas from southern Georgia to southern New Hampshire. In addition to the heavy rainfall will be the increasing threat of strong to severe thunderstorms in the warm sector of the low where the best combination of kinematics and instability will exist, and this is expected from the central Gulf Coast to the coastal portion of the Carolinas on Monday and into Tuesday. On the northwest side of the low will be the increasing prospects for an impactful winter storm from Kansas to the western Great Lakes, and given the strong pressure gradient expected, some blizzard conditions are expected. Widespread windy conditions are expected across much of the eastern half of the nation Tuesday and going into Wednesday as the surface low likely drops below 980 mb. In terms of temperatures, an arctic blast is forecast for the Northern Plains to close out the week with widespread subzero lows for the Dakotas and into Montana, and highs only in the 0s and 10s for many of those same areas. Hamrick 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