Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM 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. But by Wednesday-Thursday, return flow from the Gulf of Mexico streaming into the southern High Plains ahead of a Four Corners upper low should lead to increasing precipitation chances there. The upper trough pushing eastward into late 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... Model and ensemble agreement remains good for the early part of the forecast period, showing energy digging through the West Tuesday serving to create a closed low over the Great Basin to Four Corners Wednesday-Thursday. Meanwhile a relatively shallow trough is forecast to push through the north-central to northeastern U.S. while upper ridging builds in between these features and over the West Coast. Given the general model agreement, a multi-model deterministic blend was used for the early part of the period. By the latter part of the week, model spread notably increases, starting with a shortwave that is set to come into the Pacific Northwest by Thursday. This feature has shown considerable variability in its strength and the potential interaction with the downstream upper low over the last couple of days. The 12Z ECMWF was a little less aggressive with the shortwave than its previous runs but still digs the energy farther west than the GFS runs into the northern Rockies by Friday, though the CMC shows some hints of this as well. Some EC ensemble members reflect some aspects of the operational run but the resulting mean does tone down the more extreme aspects. The bulk of other model and ensemble guidance (including the ECMWF-initialized machine learning models) as well as the WPC preference has been leaning away from a shortwave that is strong enough to significantly impact the track of the downstream low. However, now the 00Z GFS keeps the shortwave and the downstream low (in weaker form) as separate features within the broader trough. This also impacts the other main uncertain aspect of the forecast--the potential for a surface low to spin up in the Gulf of Mexico late week if it has enough support from energy aloft. The 12Z GFS and ECMWF runs were reasonably agreeable in showing this surface low, so even though this is a somewhat new trend, the WPC forecast trended toward it slightly. But the 18Z and 00Z GFS runs show an increasingly northeast position over the southeastern U.S. with the surface low, which seems quite aggressive. Meanwhile the ensemble means from the various model suites indicate a suppressed surface pattern, but the ensemble members are all over the place with a potential surface low. Clearly the model differences are not yet resolved. The latter part of the WPC forecast quickly transitioned toward a more ensemble mean-heavy blend, especially from the GEFS mean, while maintaining a bit of 12Z GFS and CMC in the blend. ...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, 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. Precipitation should continue into Thursday there, but increasing spread for specifics of western-central U.S. flow aloft by the end of the week lowers confidence in how precipitation may progress eastward over the Plains/Mississippi Valley on Friday and perhaps into the Southeast on Saturday. 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. A Marginal Risk is planned for the Day 5/Thursday Excessive Rainfall Outlook for southeastern Florida given these ingredients for heavy rainfall are in place atop the sensitive urban corridor. More widespread heavy rainfall may be a concern for late week for Florida, but this is dependent on low-predictability impulses in southern stream flow and whether they are strong enough to support a surface feature. Farther 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. Some cool temperatures a few degrees below normal are forecast across the Southeast on Tuesday behind a cold front, with near normal temperatures farther north. Above average temperatures will build atop the north-central U.S. underneath upper ridging there, likely peaking Thursday across the Dakotas and the Upper Midwest with temperature anomalies 15-25F above average. Some milder than average temperatures should push into the Ohio/Tennessee Valleys and the Northeast for the latter part of the week but in moderated form. Most of the West will tend to see near to somewhat above normal temperatures. 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). 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