Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 AM 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 first system of note for model diagnostic purposes is an upper low likely near the Arizona/New Mexico border as the period begins early Thursday. Recent models start out with reasonably good agreement with its position but show increasing spread with time. The 12Z UKMET was the first to diverge from consensus with a slower upper low track than other guidance by Friday, and thus was not preferred in this forecast. A complication with the way this upper low evolves is 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. over the weekend. 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. Thus the WPC forecast started with a blend of the 18Z GFS, 12Z ECMWF, and 12Z CMC early in the forecast period, but quickly reduced the amount of the GFS in favor of the ensemble mean guidance. The 12Z CMC actually seemed to be a reasonable middle ground for the track and timing of the Gulf low compared to the fast GFS and the ECMWF that became pretty slow with lingering its upper and surface low features. Transitioned to using half the EC and GEFS ensemble means by Days 6-7 given the increasing spread. Regarding QPF, the NBM seemed to favor a more GFS/GEFS-like low track with its precipitation forecast. Since the preference was for a more suppressed and slower solution with the low, toned down the high totals it showed over Florida late week. ...Weather/Hazards Highlights... The southern stream upper low tracking across the Southwest to the south-central U.S. late week combined with ample moisture will provide support for widespread rain in Texas. Marginal Risks are in place in the Excessive Rainfall Outlook for Thursday and Friday as upper-level dynamical support will be good and precipitable water anomalies are likely to be near/over the 95th percentile. Instability may be somewhat meager, but with these factors and potentially wetter ground from rain on Wednesday, some flooding risk seems possible. On the western side of the precipitation shield, snow is likely from the southern Rockies into the southern High Plains including much of the eastern half of New Mexico and into the Texas and Oklahoma Panhandles. 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. The WPC forecast shows a Marginal Risk for far southern Florida into the Keys on Friday as there is potential for ample moisture to pool there, but this will have to be refined in future forecasts depending on model trends. For the weekend, some rain looks to spread into the Southeast, but there is high uncertainty particularly with how far north the precipitation could spread. At the very least, trends continue to favor a slower northward progression of moisture than recent GFS/GEFS runs. 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. Much of the northern tier of the lower 48 east of the Rockies should see above normal temperatures during the period. Expect fairly broad coverage of plus 10F or greater anomalies which could spread gradually farther south by the weekend, with warmest readings (15-25F above normal) tending to be centered over the Dakotas into Upper Midwest Thursday-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). 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