Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Sat Dec 09 2023 Valid 12Z Tue Dec 12 2023 - 12Z Sat Dec 16 2023 ...Overview... A dry pattern will be in place at the start of the medium range period Tuesday. Then by Wednesday-Thursday, return flow from the Gulf of Mexico streaming into the southern High Plains ahead of a Southwest/Four Corners upper low should lead to increasing precipitation there. The upper trough pushing eastward into the central U.S. late in the week may spread some precipitation farther east while high pressure builds in behind. Additionally, moist easterly flow could spread convection into the Florida Peninsula with a tendency toward higher rainfall totals late in the week. ...Guidance Evaluation/Predictability Assessment... A composite of operational model guidance from the 00Z/06Z cycles provided a reasonable depiction of important features for about the first half of the period (Tuesday into early Thursday). These include an upper trough crossing the northeast quadrant of the lower 48 with leading cold front, the upper low closing off over the Southwest/Four Corners, and a weakening front that reaches the Pacific Northwest. Some north-south spread develops for the western upper low, with the GFS/UKMET leaning south and CMC/ECMWF north with the ensemble means generally in-between. New 12Z CMC/ECMWF runs lead to more clustering on the southern side. Multiple forecast uncertainties arise during the latter half of the period. In addition to the path/evolution of the western upper low, there have been varied solutions for the shortwave energy reaching the Northwest and vicinity by Thursday and how it may affect the leading low/trough. These issues plus differences in more subtle details of southern stream energy lead to widening spread for surface evolution from the Gulf of Mexico northeastward. Meanwhile solutions also differ within the amplifying upper trough over the eastern Pacific, affecting the West Coast pattern by next Saturday. Regarding the central-eastern U.S. pattern, the 00Z/06Z guidance essentially flipped from 24 hours ago in that the GFS (instead of the ECMWF/CMC) showed greater influence of northwestern U.S./southwestern Canada energy on the central U.S. upper low/trough later in the period. This likely plays a role in the GFS being the most aggressive with developing Gulf low pressure and tracking it northeastward. 00Z guidance showed the best clustering among the ECMWF/CMC and the ensemble means, with the core of upper troughing reaching the central Plains and a suppressed surface pattern with only a hint of weak low pressure over the Gulf by next Saturday. Most of the 00Z ECMWF-initialized machine learning models fell in line with the 00Z ECMWF/CMC/ensemble mean scenario but with uncertainty over whether the central U.S. trough would consist of just the emerging western upper low or include energy approaching from the Northwest. Over the eastern Pacific, GFS runs were in the minority for pulling off a closed low farther offshore than consensus. This led to northern stream flow dampening the upper ridge over the northwestern U.S./southwestern Canada much more than other guidance. Based on the above late-period comparisons, the forecast blend based on 00Z/06Z guidance quickly adjusted to a blend of ECMWF/CMC models with ensemble mean input split among the 06Z GEFS and 00Z ECens/CMCens by days 6-7 Friday-Saturday. The remainder of arriving 12Z guidance has lowered confidence considerably. The developing southern clustering for the western upper low continues into the Plains to varying degrees while the 12Z ECMWF now digs northwestern energy into the central U.S. trough. Farther west the ECMWF has switched to the GFS configuration but the 12Z GEFS/CMCens means are not very supportive of that idea yet. On the other hand, the 12Z CMC is on the opposite extreme with how much troughing extends into California by next Saturday. ...Weather/Hazards Highlights... The lower 48 will be mostly dry on Tuesday, though with perhaps some light lake effect and Interior Northeast snow. By later Tuesday night into Wednesday-Thursday, chances for precipitation should increase across the southern High Plains as an upper low settles over the Southwest/Four Corners and moisture interacts with a front. Some of this precipitation may be snow from the southern Rockies eastward, 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 by Friday. 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. The southern Rockies/High Plains should trend drier while rainfall briefly increases near the western Gulf Coast toward the end of the week as the system's moisture advances eastward. 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 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. The favored pattern would maintain a potential for areas of heavy rainfall near the eastern coast of Florida beyond Wednesday. 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 if at all. Increasing precipitation chances are then possible for California by Saturday as an eastern Pacific low pressure system edges closer to the coast. A minority scenario would keep the Pacific system far enough west of California to keep that region dry while some moisture reaches the Pacific Northwest. 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. Rausch/Tate 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