Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Thu Dec 28 2023 Valid 12Z Sun Dec 31 2023 - 12Z Thu Jan 04 2024 ...Overview... The latest models and ensembles still advertise a pattern transition toward a series of Pacific shortwaves (some with an embedded upper low) affecting some locations near the West Coast and then tracking across the southern tier. After a leading trough passes through the eastern U.S. early next week, Western Canada mean ridging should be persistent enough to support a downstream trough extending into the northeastern quadrant of the lower 48 by mid-late week. This pattern should confine the majority of precipitation and somewhat below normal high temperatures to parts of the West Coast and the Gulf Coast states/Southeast. The Great Lakes could see some light snow around Sunday and Wednesday-Thursday. Warmest temperature anomalies will likely be near the Canadian border. ...Guidance/Predictability Assessment... As has generally been the case lately, most guidance reflects similar ideas for the general pattern but shows some meaningful detail differences with significant features. These include the Upper Midwest trough progressing through the East early next week, a system reaching near southern California by Monday and continuing eastward thereafter, then followed by another system reaching the West Coast around midweek. Guidance has yet to decide on the details of the leading upper trough crossing the East. Recent GFS runs have generally been on the slow and closed side of the spectrum, though up through the 18Z run there had been attempts at convergence by some solutions, with that GFS a little faster and delaying closure of an upper low while the 12Z UKMET/ECMWF were close in principle. The new 00Z GFS has reverted back to pulling off a slower and more closed feature from the trailing part of the initial positively tilted trough. One 00Z GFS adjustment closer to consensus is toward weaker surface low development just off the East Coast versus prior runs. Majority clustering also supports the more southern GFS latitude versus the 12Z ECMWF. Ensemble spread has kept the means open and positively tilted with the upper trough, leading to less defined western Atlantic low pressure than most operational solutions. A compromise among the 18Z GFS and 12Z models provided a reasonable starting point given ongoing differences. Southern stream energy emerging from the southern Rockies on Sunday should shear out underneath the leading eastern trough. The best clustering with the next system in the progression is for an upper low track just offshore southern California as of Monday followed by a more open appearance as the shortwave continues eastward across the southern tier and possibly interacts with northern stream troughing. Currently the majority of guidance has a phased appearance with overall eastern U.S. troughing by day 7 Thursday, leaving the more separated 12Z ECMWF with a slower southern closed low in the minority. The uncertainty over eastern U.S. trough details at that time results in a broad spread for one or more surface lows--at least from the east-central Gulf Coast to near the Southeast/Mid-Atlantic coast. The 12Z ECMWF/GFS, latest GEFS/ECens/CMCens means, and to some degree CMC runs provide a sizable majority cluster for the next system of interest. This evolution would have an upper low reaching close to the central West Coast by early Wednesday and the southwestern U.S. by Thursday. The experimental 12Z ECMWF-initialized machine learning models differ somewhat for Wednesday but most converge with the aforementioned majority by next Thursday. The 18Z/00Z GFS runs strayed faster with northeastern Pacific flow and faster/southeast with the West Coast/Southwest system and thus were not favored. The first half of the updated forecast started with the 18Z GFS/12Z ECMWF with lesser weight of the 12Z CMC/UKMET. By days 6-7 Wednesday-Thursday, GFS input switched to the 12Z run to reflect preferences over the West while maintaining some weight of the ECMWF/CMC. Those three runs already compared fairly well to the ensemble means so the latter part of the forecast needed only a modest inclusion of the 18Z GEFS/12Z ECens to smooth the proverbial rough edges. ...Weather/Hazards Highlights... The weak system crossing the Great Lakes through Sunday may produce a little light snow near its path. The trailing front may produce areas of rain across the Gulf Coast/Southeast Sunday-Monday but with continued uncertainty over coverage/amounts due to detail differences in the guidance for southern stream energy emerging from the Southwest and the orientation/timing of northern stream energy. At the very least, this activity should be progressive enough to keep most totals in the light to moderate range. Another weak wave and trailing cyclonic flow may produce a little lake effect snow by mid-late week. It will be a close call as to whether a system brushing southern California will bring any moisture to that part of the state. As the upper trough continues eastward, expect some rain to spread along and near the Gulf Coast by midweek, with uncertain northward coverage by the time the moisture reaches the East Coast by Thursday. Currently expect most of this moisture to stay far enough south to maintain rain as the primary precipitation type. A better defined system forecast to approach the West Coast by Wednesday and move into the Southwest by Thursday may bring organized precipitation to the West Coast and some areas farther inland, with relatively higher totals most likely over favored terrain in portions of California. The greatest temperature anomalies during the period should be of the above normal variety across the far northern tier near the Canadian border with some locations seeing multiple days with readings 10-20F or so above normal. Morning lows will tend to be farther above normal than daytime highs. On the other hand, aside from the southern Plains being above normal on Sunday ahead of a front, most areas from California eastward across the southern tier should see near to below normal high temperatures during the period. Low temperatures will also be below normal on some days, such as Sunday morning when readings in the low to mid 30s may lead to frost or freezes over portions of northern Florida. 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 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