Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM 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 this 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 southeastern quadrant of the lower 48 could see another episode of possibly heavy rainfall by next weekend, depending on details of upstream shortwave energy. ...Guidance/Predictability Assessment... Following the pattern of recent model/ensemble runs, there is good agreement on the large scale pattern evolution but continued meaningful detail differences for individual features. One such feature is the surface low tracking from the Plains eastward into the Middle Mississippi/Ohio Valleys Wednesday-Thursday. Overall models have trended faster with the low in the 00/06Z model cycle and persisting into the newer 12Z models, with the exception of many GFS runs that have been on the slower side. The 00Z GFS seemed to be in better alignment with other guidance than the 06Z/12Z runs. The speed of the surface low and its exact track will affect the forecast for snow on the northern side. Winter weather probabilities thus shifted northeast from the previous forecast in conjunction with the short range winter weather desk. By around Thursday, models favor shortwave energy dropping into the West, but solutions vary on how deep the energy dives. GFS runs and GEFS members show a dive farther south, bringing precipitation to the southern half of the interior West, while 00Z CMC/EC solutions are farther north with the energy/500mb heights and the precipitation. By Friday-Saturday, models vary with the closing off of a low within the positively tilted trough, but at least are generally agreeable for a gradual shift eastward. But by Sunday considerably more differences arise with the trough as central Canada shortwave energy may phase with the trough (00Z EC) or rush eastward without phasing (GFS runs). Additionally, major differences upstream play a role--namely another round of energy perhaps spilling south into the Northwest, serving to erode the eastern edge of the ridge/shift the ridge westward, which most models show other than the ECMWF/EC ensemble means. All these differences aloft lead to uncertainties with frontal positions across the south-central and southeastern U.S. and potential for heavy rainfall. Given this variety in solutions, confidence goes down by next weekend with the pattern. The WPC forecast utilized a multi-model blend of mainly 00Z deterministic guidance early in the period, given the 06Z GFS surface low position seemed slow as mentioned earlier, but did not necessarily favor the 00Z GFS's rainfall axis in the warm sector Wednesday-Thursday. Gradually blended in and trended up the ensemble means for days 5-7. The 00Z CMC mean seemed like a good middle ground between the EC mean that stayed amplified in the West and the GEFS mean showing digging energy. ...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 the Interior Northeast. 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 by Saturday as a front lingering over the Gulf lifts back northward ahead of possible developing low pressure and cold front. Current confidence remains low for the axis and amount specifics of the rainfall and the surface frontal 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 underneath the upper trough/low. 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. Tate/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