Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sun Dec 10 2023 Valid 12Z Wed Dec 13 2023 - 12Z Sun Dec 17 2023 ...Pattern Overview and Guidance/Predictability Assessment... The 18 UTC GFS and 12 UTC ECMWF/Canadian/UKMET solutions seem reasonably well clustered Wednesday into Thursday for much of the country and vicinity. This is evident in a pattern with a developing and separating southern stream closed low digging into the Southwest, well detached from a series of systems riding more progressively from the Pacific inland through southern Canada and the U.S. northern tier within northern stream flows. A model composite coupled with the 01 UTC National Blend of Models maintains great WPC product continuity in this period with at least average predictability. Latest 12 UTC guidance stays mainly in line. However, the models then increasingly show less than stellar model to model and run cycle to run cycle continuity issues later week into next weekend with the bulk of the differences/QPF associated with the potentially wet downstream ejection of the aforementioned Southwest southern stream system and the extent of phasing with northern stream energies. This leads to significant variance with the potential development and areal extent of Gulf of Mexico then Southeast Coast/vicinity coastal low genesis and associated moisture fueling rainfall pattern. The 12 UTC ECMWF showed stream/system phasing Friday, but the associated northern stream system offers timing issues that often portend quite uncertain phasing. The newer 00 UTC ECMWF has flipped away from phasing. The 18 UTC GFS waited until Saturday to offer some system phasing, but is also the most earnest of the models to develop leading activity/QPF over the eastern Gulf/Florida this week that leads into a more significant low up the Eastern Seaboard next weekend. The newer 00 UTC GFS offers very different system interactions, showing enhanced sensitivity and lowering forecast confidence. The 12 UTC and now 00 UTC Canadian runs maintain a separated southern stream system that eventually leads into very deep coastal low development off Florida/Southeast. Preference is to discount less likely stream phasing scenarios that rely on uncertain shortwave interactions at medium range time scales until a more common guidance signal is established. The 12 UTC and newer 00 UTC Canadian runs certainly offers a robust option, but instead prefer, given growing uncertainties in the flow, a broad composite of more compatible GEFS/ECMWF/NAEFS/Canadian ensemble means that generally maintains split flow with focus on the more predictable embedded systems. 01 UTC National Blend of Model QPF also seems a reasonable baseline for this pattern. ...Weather/Hazards Highlights... Accordingly and overall, chances for precipitation should increase across the southern Plains by midweek as an upper low settles over the Southwest/Four Corners and the inland return of Gulf moisture interacts with a wavy lead front. Some of this precipitation may be snow from the southern Rockies/High Plains, with typical elevation/coverage uncertainty. Details of precipitation coverage/amounts/type will depend on the upper low track, which at this time guidance suggests could be anywhere over the central or southern Plains. In the case of both snow and rain areas, some totals could be locally heavy. Most of the southern High Plains region has fairly dry antecedent conditions and short-term rainfall rates do not appear to be high enough to merit significant runoff concerns. However, introduced a WPC Excessive Rainfall "Marginal Threat area over the vicinity of the Texas Hill Country for Day 5/Friday given favorable southeast upslope flow longevity. Across Florida, expect easterly flow to set up south of strong eastern U.S. high pressure while moisture and instability increase for midweek and beyond. The Day 4/Wednesday and Day 5/Thursday Excessive Rainfall Outlook maintains a Marginal Risk area for southeastern Florida given these ingredients for heavy rainfall are in place across the sensitive urban corridor. There is still uncertainty over the magnitude and northward extent of instability/deep moisture though. The sensible weather forecast over a broader portion of the South/East becomes increasingly uncertain toward next Friday-Saturday, given the combined spread for upper level troughing expected to reach the central U.S. (anchored by the emerging western upper low but possibly containing energy approaching from the northwest) and southern stream impulses aloft along with how these features may influence the surface pattern. The most likely scenario would have some rainfall extending east through the Mississippi Valley and by the weekend lifting northward over the Southeast, but with lower totals/coverage than depicted in some recent GFS/GEFS runs. Over the West, a weakening front may bring some light precipitation to the Pacific Northwest around midweek or so, with only light/scattered activity extending farther inland by Thursday. Increasing precipitation chances are less likely now for California next weekend as an eastern Pacific low pressure system holds more off the coast. The most anomalous temperatures during the period will be of the warm variety across the northern Plains and Upper Midwest from Wednesday onward. More moderate warmth should also extend into the northern half to two-thirds of the East later in the week. Expect decent coverage of temperatures 10F or more above normal and portions of the Dakotas into Upper Midwest may be 15-25F above normal Thursday into early Friday. Meanwhile clouds/precipitation over and near the southern High Plains mid-late week will support a fairly narrow temperature range (above normal lows, below normal daytime highs). Much of the West will likely see near to slightly above normal temperatures during the period. 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