Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Sun Jan 15 2023 Valid 12Z Wed Jan 18 2023 - 12Z Sun Jan 22 2023 ...Mid-late week storm system likely to bring a variety of significant weather to the central/eastern U.S... ...Overview... Latest guidance still shows a significant pattern change expected to take place next week through the weekend, with a steadily building eastern Pacific upper ridge promoting the development of a positively tilted mean trough over the lower 48 by the late week/weekend time frame. This evolution should lead to a period of drier weather over California/southwestern U.S. with the ridge diverting most incoming Pacific moisture farther north and then into the Interior West/Rockies as shortwave energy drops into the mean trough. Ahead of this transition, the final system reaching southern California in the short range period will likely emerge over the Plains on Wednesday and track northeastward thereafter. This area of low pressure may bring a band of meaningful snow to the north/west of its track and the potential for heavy rainfall/strong convection in portions of the warm sector. The southern tier could see another episode of rainfall by next weekend, depending on details of upstream shortwave energy. ...Guidance/Predictability Assessment... As has been the case in previous model/ensemble runs, there is good agreement on the large scale pattern evolution but continued meaningful detail differences for individual features. Operational model runs suggest that pesky medium-smaller scale shortwave specifics with lower predictability, including energy to the north, may continue to plague the forecast for a time regarding the system tracking northeastward from the Plains Wednesday onward. Most of the models have displayed some oscillations over time, with latest consensus a little deeper than 24 hours ago and GFS runs still waffling for timing between the majority (or even a bit slower for a while in the 18Z GFS) and straying faster--especially as the system reaches the Northeast. Thus far the ECMWF/CMC runs and their means have generally been the most consistent in principle, with the 12Z run of the GFS being the latest version that is closest to that cluster. At least for the moment, guidance is coming together a little better for the mid-late week evolution of shortwave energy dropping into the West, with a most solutions indicating an embedded upper low may close off somewhere over the Great Basin/Four Corners area. However note that the new 00Z GFS shows more southwestward elongation of the trough versus other other guidance as it enters the West, leading to a slightly farther west upper low into early Friday before catching up. Model/ensemble spread for this energy as it exits the West increases dramatically during next weekend, leading to considerable uncertainty for the associated system that could affect the eastern half of the country. Ensemble members are dispersed enough to result in an ill-defined pattern in the means but a composite of GFS/ECMWF/CMC runs would suggest a more organized system to some degree. Greater splitting of the energy in the new 00Z ECMWF leads to a weaker surface reflection though. Also likely to play a role in next weekend's forecast will be the next bundle of shortwave energy expected to enter western North America. For forecasts valid early next Sunday, GFS/ECMWF runs have been converging toward each other by way of the GFS trending somewhat faster/less amplified and the ECMWF slower and more amplified. The GFS/GEFS are still a bit on the amplified side though, favoring about a 60/40 tilt toward the ECMWF/ECMWF mean. The new 00Z ECMWF has reversed its recent trend, going back to more pronounced ridging over the West next weekend. A composite of 12Z/18Z models provided a reasonable representation of consensus for the first half of the updated forecast period. Then the forecast blend transitioned to a model/ensemble mean mix with the aforementioned slight ECMWF/ECMWF mean tilt, while phasing out the 12Z CMC due to it becoming a slow extreme with the western U.S. trough/upper low at that time. ...Weather/Hazard Highlights... The system expected to track through the Plains and Midwest into the Northeast Wednesday-Friday should bring a variety of significant weather to the central and eastern U.S. Locations to the north and west of the storm track will likely see a band of meaningful snow, with recent forecasts suggesting a most likely axis from the central High Plains through the Great Lakes and New England. Farther south in the warm sector, there will be the potential for heavy rainfall with a Slight Risk area depicted on the Day 4 (12Z Wednesday-12Z Thursday) experimental Excessive Rainfall Outlook across parts of the Lower Mississippi Valley, Tennessee Valley, and Lower Ohio Valley. The Storm Prediction Center is monitoring the potential for severe thunderstorms over/near the Lower Mississippi Valley in this time frame as well. Some moderate to heavy rainfall may extend farther through the East after Wednesday but should become more localized as the best dynamics lift northeastward. Continue to monitor forecasts as guidance eventually resolves some of the finer details of this system. With the evolving pattern change later in the period, the next frontal system to reach the West Coast will bring a brief period of locally moderate to heavy rainfall/mountain snow to the Pacific Northwest and northern California around midweek with lighter totals then extending into the Rockies. As the supporting dynamics extend beyond the Rockies, the South may see a rebound in rainfall next weekend Saturday as a front lingering over the Gulf lifts back northward ahead of possible developing low pressure and cold front. Current confidence is low for the specifics of this episode of rainfall and surface evolution. Expect another front to spread some rain/mountain snow into the Northwest and northern Rockies next weekend. The Wednesday-Sunday pattern will favor persistently cool daytime highs over the southern three-fourths of the West and at times into the central-southern High Plains, with most locations 5-15F or so below normal. Friday-Saturday may be the coolest days during the period. On the other hand, the eastern U.S. and northern tier should be above normal, with anomalies as high as plus 20-30F for lows over parts of the South and East on Wednesday-Thursday ahead of the Plains/Midwest system. Frontal passage will bring a cooler trend to the East Friday into the weekend but with temperatures remaining above normal in most cases. Most of any daily records should be for warm lows, at least if such readings can hold on through the end of the calendar day. Rausch 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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