Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Mon Jan 6 2025 Valid 12Z Thu Jan 09 2025 - 12Z Mon Jan 13 2025 ...Arctic blast persists for the central/eastern U.S. this week... ...Overview... Lingering cold northwesterly flow will still support Great Lake effect snows into later this week. Meanwhile, organized precipitation will return later this week as a frontal system moves into the West and low pressure develops off the Gulf Coast. The frontal system in the West will approach the coast on Friday and move across the region this weekend with widespread precipitation from the Northwest to the northern and central Rockies. The downstream Gulf system is tricker given uncertain southern stream upper trough energy ejection from the Southwest U.S./Northern Mexico. The pattern eventually does seem to support the development of widespread precipitation from the Gulf Coast to the Southeast late this week, and precipitation may spread up the East Coast as it moves offshore this weekend. There is a significant amount of uncertainty surrounding the evolution of the system over the Gulf and especially as it moves towards/up the East Coast and western Atlantic, and the timing and location of this system will be important for determining impacts. ...Guidance/Predictability Assessment... Guidance continues to struggle overall with the specifics regarding the timing, evolution, and phasing of individual waves between streams during the medium range period (Thursday Jan 9 - Monday Jan 13). However, the overall pattern tends to be well- represented by the ensemble means and the envelope of the individual deterministic guidance, beginning with an upper-level trough over the southwestern U.S. that eventually lifts east- northeastward over the Southeast and off the East Coast this weekend as individual upper-level energies reinforce expanding broad-scale, positively tilted mean upper-troughing over the central/western U.S. by early next week. A more detailed review of the initial pattern reveals the more complicated specifics of the evolution of the system through the central/eastern U.S., with the deterministic guidance suggesting that northern and southern stream energies over the north-central and southwestern U.S., respectively, may eventually phase together over the Ohio Valley/Southeast before moving eastward off the East Coast. The evolution of this energy will have important ramifications at the surface as a low pressure system develops in the Gulf and then moves northeastward along/east of the East Coast, with a path of potentially impactful winter precipitation following along to the northwest. While the path and areal coverage of this winter precipitation are harder to pin down, the guidance is in good agreement on the evolution of the system initially as it tracks along the Gulf Coast and lifts northeastward towards the southeastern U.S. coast. Thereafter, specifics on the system and any eventual hazardous weather become much more uncertain. The latest 00Z guidance trends suggests a low that will remain well offshore which lowers the chances of hazardous weather further north along the East Coast this weekend. However, the 12Z GFS in particular was much more aggressive in deepening a coastal low along the northeastern U.S. which would not surpassingly lead to the potential for much more significant precipitation. Following the departure of this system, the next feature of interest will be the evolution of split stream energies to the west over the western/central U.S. as these energies generally tend to reinforce mean, broad-scale troughing. A northern stream system looks to pass across the north-central U.S., though following multiple frontal passages including the latest eastern U.S. system over the weekend, the main ramifications of this latter system look to be a reinforcement of a cold, Canadian airmass over the country amidst generally dry conditions except for some lingering snows in the Rockies. Given the notable differences with regards to the specifics and how the overall pattern is well-captured with relative detail in the ensemble means, opted for an updated forecast blend that relies on the 06Z GEFS/00Z ECens means with contributions early in the period from the 06Z GFS and 00Z ECMWF/CMC guidance while the guidance falls within the envelope of the ensemble means. One trend to monitor is that with the 00Z guidance this favors limited precipitation along the northeastern U.S., which would undoubtedly change if the forecast were to trend more in line with the 12Z GFS over the next day. ...Weather/Hazards Highlights... Lake effect snows will periodically be rejuvenated by deep cold surges into later this week as light to modest precipitation will focus down through the Rockies and north-central U.S. with system frontal system genesis. Precipitation chances will increase over the southern Plains, Lower Mississippi Valley, and Gulf Coast by Thursday as energy from an upper-low spreads northeast. Anomalous moisture will focus over the western Gulf Coast as a low pressure system strengthens over the western Gulf, and heavy precipitation will be possible in portions of Texas and Louisiana. While deepest instability and heaviest rains may focus offshore, guidance shows ample western then central Gulf Coast state precipitation potential, so Marginal Risk areas have been retained for the Day 4/Thursday and Day 5/Friday WPC Excessive Rainfall Outlooks to account for localized flood potential. While the Gulf low strengthens, cold air well in place over the southern Plains/ArklaTex will support potentially significant snow and mixed/icy wintry precipitation into Thursday, see WPC's key messages for southern U.S. winter storm for more details. Temperatures will drop below freezing overnight for most of the state, and even portions of South texas could be cold enough for some icy precipitation. More uncertain wintry precipitation may translate with the system towards the Tennessee Valley and central/southern Appalachians/Southeast on Friday. As the Gulf system progresses, precipitation may expand northeastward across the East Friday into Saturday. Heavy precipitation potential will be strongly dependent on the timing and placement of the low pressure system, which is highly uncertain at this time. If the low moves quickly offshore and strengthens away from the coast, then impacts will be limited, and impacts will be more likely with a slower low that strengthens closer to the U.S. coastline. The weather pattern will favor the continuation of well below normal temperatures east of the Rockies through at least this weekend. Temperature anomalies of 10-20 degrees below normal are likely from the southern/central Plains and Mid- Mississippi/Ohio Valleys through the Appalachians, especially as temperatures remain quite cold this week for snow covered areas from an ongoing major lead winter storm. The combination of colder temperatures/wind chills is most likely to be potentially dangerous over the southern Plains and from the Ohio Valley eastward through the Mid- Atlantic. Meanwhile, upper ridging in the western U.S. will offer slightly above normal temperatures prior to breakdown heading into the weekend. Putnam/Schichtel 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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