Extended Forecast Discussion NWS Weather Prediction Center College Park MD 205 PM EST Mon Dec 11 2023 Valid 12Z Thu Dec 14 2023 - 12Z Mon Dec 18 2023 ...An emerging wet pattern over the South/Southeast... ...Overview... A southern stream upper low traversing the southern U.S. during the medium range period will be a primary driver of the sensible weather, including rain and snow in the southern Plains late this week, with rain potentially spreading into the Southeast into the weekend and early next week though with more uncertainty as models struggle to agree on a possible Gulf surface low forming. Ahead of this possibility, moist easterly flow into southern Florida could cause localized heavy rain. Fairly progressive mid-upper flow in the northern stream is likely with a series of shallow troughs moving through the north-central and northeastern U.S., while mean ridging builds in the West ahead of eastern Pacific troughing. ...Guidance/Predictability Assessment... The latest guidance continues to favor a dominant southern stream system that spread several inches of moisture from Texas to the Southeast states albeit with increasing model spread. As previously noted, the UKMET is the first to diverge from consensus with the slower upper low track and there are varying degrees of potential interaction with a northern stream shortwave upstream that should move near the U.S./Canadian border Thursday-Friday and then potentially dive southeastward into the east-central U.S. The strength and track of this shortwave will have implications downstream. Namely, the GFS/GEFS suite lately has been interacting/combining the preexisting southern stream upper low and this shortwave, which forms an east-central U.S. trough that supports a strong and faster moving surface low in the Gulf and progressing northeastward. This is opposed to the remaining majority that keeps the northern stream shortwave progressive and north of the southern stream upper low. Without the phasing of these features, the southern stream upper low will be slower and the only dynamical support for surface low pressure, so the surface low forms/strengthens later and stays more suppressed. WPC has been generally favoring the more suppressed and slower track given it is the larger cluster with multiple model suites (the ECMWF and CMC). Ensemble members tend to favor with their operational runs but do show a lot of spread. Elsewhere, upper ridging seems well predicted in the West behind these features and ahead of large scale eastern Pacific troughing. The timing of the troughing edging toward the West Coast early next week still shows some spread but nothing too egregious for the Days 6-7 timeframe. WPC maintained continuity by leaning toward the CMC, ECWMF and GFS initially before decreasing inclusion of the GFS while boosting the EC ensemble and GEFS means for the middle and latter periods. ...Weather/Hazards Highlights... Ample moisture advecting northward in conjunction with a southern stream upper low traversing from the Southwest to the south-central states will help spread rain from Texas, across the Gulf Coast and to the Southeast later this week and into early next week. Dynamic support, abundant moisture and the presence of some instability should support periods of moderate or heavy rainfall across the Southern Plains, Gulf Coast and portions of the Southeast. Marginal Risks for excessive rainfall were already in effect through Friday were some flooding risk seems possible. Only minor adjustments were needed to reflect the latest trends and WPC forecast. Portions of New Mexico, the Southern High Plains and Southern Rockies, which will be on the west/northwest side of the precipitation shield will likely have snow. Meanwhile on Thursday, moist easterly flow will likely persist into southeastern Florida with some instability in place, so a Marginal Risk persists for that area from Wednesday/Day 3, especially for the sensitive urban corridor. By Friday and beyond, confidence with the extent of precipitation rapidly lowers as model guidance diverges. Some model guidance shows widespread heavy rain over Florida and the Southeast by late week, but the more favored/larger cluster of guidance maintains a focus for heavy rainfall in the southern part of the state (or even perhaps fully south of Florida) to the south of high pressure at the surface. WPC maintains a Marginal Risk to account for the potential for pooling moisture. High uncertainty persists in regards to how far north the precipitation could spread across the Southeast. Elsewhere, mainly dry conditions are forecast through late week, other than some light showers in the Upper Midwest/Upper Great Lakes and some Pacific Northwest precipitation. By early next week, eastern Pacific upper troughing could sneak energy into California and spread precipitation there, but there remains uncertainty in timing when this will occur which affects the precipitation amounts. The Interior West is forecast to stay dry. A vast portion of the northern states east of the Rocky Mountains are forecast to have above seasonal normal temperatures through the extended period with broad coverage of plus 10F or greater anomalies. Some of the warmest values will be in the 15 to 25F range and focused in the vicinity of the Dakotas and Upper Midwest through Saturday. Mean ridging aloft over the West will favor moderately above normal temperatures and a very gradual warming trend with time. In contrast, clouds/precipitation during the latter half of the week will produce chilly highs over the southern High Plains during the latter part of the week. Temperatures there should rebound closer to normal next weekend. Areas near the East Coast should see a day or so of below normal highs Thursday as strong and cool high pressure builds in behind a cold front. Highs over northern parts of the East should trend above normal Friday into the weekend while the South stays near normal (but with Florida above normal for lows due to persistent clouds/rainfall). Campbell/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