Extended Forecast Discussion NWS Weather Prediction Center College Park MD 147 PM EST Sun Jan 07 2024 Valid 12Z Wed Jan 10 2024 - 12Z Sun Jan 14 2024 ...Another Major Storm to impact the East-Central to Eastern U.S. with Snow/Rain/Wind Threats as an Arctic Blast digs through the North-Central U.S.... ...General Overview... The powerhouse low pressure system that hammers the Eastern U.S. early in the week is expected to reach southern Ontario by Wednesday morning, with a trailing cold front exiting the East Coast except for New England, where lingering strong onshore flow will offer heavy rain and high winds. An improving weather trend commences in the wake of this storm system going into Thursday across the Eastern U.S. as high pressure briefly settles in. However, another potent low pressure system likely gets organized across the south-central U.S. by Friday morning in response to an amplifying upper trough approaching from the Rockies, and this low should track in the general direction of the Ohio Valley and Great Lakes region by next weekend with more bad weather expected for many areas. In addition, pattern variance will result in a strong Arctic front heralding the coldest weather of the season thus far across much of the north-central U.S. for the second half of the week and beyond. ...Guidance/Predictability Assessment... The 00/06 UTC model guidance suite remains in good overall agreement on the exiting low pressure system across the Northeast U.S. on Wednesday, although the GFS/UKMET remain a tad slower with pivoting the axis of the upper trough across New England compared to the ECMWF/CMC. The 12 UTC GFS/UKMET have trended toward the ECMWF/CMC. For the next main system developing across the southern Plains Friday, the GFS and to a lesser extent the UKMET remain slower and more amplified with the supporting upper trough compared to the much more progressive ECMWF/CMC, and therefore considerable timing differences with the surface low track (GFS slowest and ECMWF fastest). The ECMWF and GEFS ensemble means remain between these two solutions, albeit with the GEFS mean slightly less progressive than the ECMWF ensemble mean. Even so, an ensemble mean composite still seem to serve as a good proxy for the expected deterministic location of the surface low within a broad envelope of plausible timing solutions. Energy digging to the lee of east Pacific high amplitude ridging tends to support the less progressive solutions, while variable smaller wavelength spacing with individual systems embedded within the larger flow tend to support the more progressive side of the forecast envelope. 12 UTC guidance forecast spread has decreased to a degree, but is till ample. However, it is important to note that despite these uncertain timing differences, there continues to be a greater than normal signal for another major system and associated weather threats to develop in a continuation of a highly favorable pattern, but at this point not quite as potent as the lead early week storm. ...Weather/Hazards Highlights... Heavy rain and strong winds will likely be ongoing across central and eastern portions of New England Wednesday morning into the afternoon ahead of the strong cold front exiting the East Coast. An impressive south-southeasterly low level jet ahead of the front will likely mix down enough to produce surface wind gusts in excess of 50 mph at times across Maine, and storm force winds across the coastal waters. Rainfall will be maximized across the upslope portions of the northern Appalachians of Maine where 1 to 2 inches of rainfall is likely, and much of this will likely be falling over areas with snowpack. Given some existing uncertainty in snow levels and overall precipitation type, a broad Marginal Risk area for Day 4/Wednesday remains planned, although this could eventually be upgraded to a Slight Risk once things become more certain. For the Day 5 period Thursday, no risk areas are planned for this forecast cycle. Looking ahead to the Friday-Saturday time period, another potentially strong low pressure system is looking increasingly likely and this could affect similar areas of the central and eastern U.S. compared to the low on Tuesday/Wednesday, tracking from Texas to the Great Lakes based on the latest guidance. There will likely be a band of significant snow extending from Missouri to Michigan associated with the deformation zone/wrapping moisture northwest of the surface low, and the highest probabilities of this are generally southeast of the highest snowfall totals expected with the system early in the week. Focus slated to then work across the Appalachians into the interior Northeast with system progression should also again favor lake effect snows in the wake. Heavy rain and strong thunderstorms are also likely for much of the South/Southeast likely Friday, before lifting robustly up the Eastern Seaboard Saturday. Wet antecedent conditions across these areas suggest more widespread runoff issues to monitor. In terms of temperatures, an Arctic blast is expected for the Northern Plains/Rockies to close out the week with widespread double digit below zero lows for the Dakotas and into Montana, and some subzero highs, with anomalies as much as 40 degrees below normal. Cold Canadian air will also dig west of the divide trough the Intermountain West and also spread down across the central to east-central U.S., driven/modified in the wake and between main storm passages. Upstream, a continued series of highly dynamic Gulf of Alaska/western Canadian digging systems will maintain and periodically rejuvenate a highly unsettled/wintry pattern this week for a good portion of the West. There is a moderate signal to spread down from the Pacific Northwest to north-central California into the Sierra. There should also be widespread terrain/mountain focusing snows further inland across the Intermountain West into the Rockies with continued system progressions. Hamrick/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, 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