Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Sep 19 2018 Valid 12Z Sat Sep 22 2018 - 12Z Wed Sep 26 2018 ...Heavy rainfall potential across the Arklatex region into the Ozarks for Saturday through Monday... ...Considerable cool down expected in the middle of the country next week... ...Pattern Overview/Model Guidance/Preferences... To commence the weekend, a strong band of height falls will be lifting up through Newfoundland driving an impressive area of low pressure toward the higher latitudes. The associated height falls will graze New England with the attendant boundary stalling across the mid-Atlantic. Farther south across the Gulf of Mexico, an easterly wave is forecast to slide from Cuba back toward the Gulf Coast while likely shearing in time. Over the Southern Plains, a broad weakness is expected to slowly drag eastward toward the Arklatex region this weekend. The guidance show fairly impressive precipitable water values the disturbances will have access to thus aiding in the heavy rainfall threat. Across western North America, models are not as clear with model spread plaguing the forecast. Ultimately, a complex array of shortwaves ejecting across the western U.S. should gradually establish a more defined longwave trough that progresses toward the nation's mid-section. This would lead to a fairly amplified mid/upper pattern for next week with a trough across the central U.S. while a pair of ridges inhabit the Eastern Seaboard and eastern Pacific, respectively. While the guidance vary with how quickly the easterly wave will shear across the Gulf of Mexico, its impacts are primarily restricted to the offshore waters of the Gulf. One key feature with a great deal of precipitation uncertainty is the slow moving shortwave across the Southern Plains. Increasingly robust QPF amounts are suggested by the past few runs of the ECMWF as well as the 00Z CMC/UKMET solutions. Only the GFS and its parallel runs seem to downplay this signal. The ingredients appear to be in place with slow moving vorticity centers traversing an axis of 1.50 to 2.00 inch precipitable water values. Based on the 500-mb comparisons, recent runs of the GFS are flatter with the trough and more progressive than the ECMWF/UKMET forecasts. This area bears watching during the upcoming days given the potential for flooding rains. Elsewhere, the mentioned uncertainty with the evolving western U.S. trough is further complicated by the jumpy nature of the ECMWF ensembles. More specifically, the past four runs of the ECMWF and its ensembles have shown a marked quicker trend, most notably in the 12Z/00Z cycles. Other global models have tended to hold the trough back more and ultimately been more stable. A quick look at the most recent 00Z ECMWF run shows it has stuck to its guns with the more progressive solution. Really found it difficult to utilize any significant contributions from the 12Z ECMWF/ECMWF ensemble mean. Kept a 30 percent fraction of its forecast in the picture throughout to account for any future 00Z guidance which moved that way. While the 00Z GFS remained on the western side of the spread, the 00Z CMC did make a considerable jump eastward although this model can exhibit notable run-to-run jumps. The medium range forecast was led by the 18Z/12Z GFS, 12Z UKMET, and 18Z GEFS mean but again did keep non-zero contributions of the 12Z ECMWF/ECMWF ensemble mean in the mix. Given this assessment, the forecast confidence is below average from Day 6/Tuesday onward. ...Sensible Weather and Hazards... As mentioned in the headline, a striking signal in the guidance is evident across the Southern Plains eastward into the Ozarks region this weekend. While it is difficult to trust some of the more robust solutions (6 to 10 inch 24-hour amounts), the threat remains there. The manual QPF downplays this scenario but could see future forecasts raise numbers if the GFS joins the heavier camp. Gradually this rainfall threat will shift northeastward, particularly focusing along the stalled backdoor front across the Carolinas and mid-Atlantic region. Moving into next week, while considerable timing issues have been well documented, some heavy precipitation threat would be possible along the advancing baroclinic zone. The big story with temperatures will be the considerable cool down across the north-central U.S. eventually into the middle of the country next week. By Day 6/7, September 25/26, forecast anomalies will be around 15 to 20 degrees below climatology over the Northern Rockies into the Northern/Central Plains. If this were to verify, highs would only be in the low to mid 50s these days allowing for a true feel of Autumn. Rubin-Oster WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml