Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 AM EDT Sun Sep 11 2022 Valid 12Z Wed Sep 14 2022 - 12Z Sun Sep 18 2022 ...Overview... As the medium range period begins Wednesday, an upper ridge will be in place over the central U.S. causing warm temperatures for much of the Plains/Mississippi Valley and separating troughing in the Northeast and mean troughing in the West. The warm ridge is forecast to track slightly eastward and flatten somewhat through late week while energy stalls in the Southeast, which along with a meandering frontal boundary will cause rain and thunderstorms at least for Florida and perhaps reaching the Georgia/Carolina coasts. Meanwhile persistent southwesterly flow coming into the West through Wednesday-Thursday will continue to direct ample moisture originating from what was tropical cyclone Kay into the Intermountain West and parts of the Rockies, leading to some locally enhanced rainfall from showers and thunderstorms. Then rounds of rain are possible late week into the weekend for the north-central U.S. with frontal systems passing through. ...Guidance Evaluation/Predictability Assessment... Recent model guidance continues to show generally good agreement with the pattern at the start of the medium range period, with two shortwaves within the trough in the northeastern U.S., a common ridge axis over the Plains/Mississippi Valley, and troughing in the West with embedded shortwaves. Models are also reasonably similar with energy and a weakness/trough/possible closed low at times in the Southeast Thursday-Friday and tracking slightly northeastward through the weekend, considering the small scale of the feature. However even small differences in this energy could lead to QPF differences, specifically how much rain makes its way onshore from the heavier offshore western Atlantic totals. The 18Z GEFS mean that was available at the time of forecast production showed more QPF onshore than the bulk of other guidance, leading to the WPC in house ensemble bias corrected data and the 01Z NBM to show some higher totals. Other than those, the trend was perhaps for lower QPF onshore for this forecast. But confidence is low considering the model waffling and that these Georgia/Carolinas coastal areas are right on the QPF gradient, where small shifts lead to big differences locally. The 18Z yesterday and 00Z today GFS runs show a stronger surface low developing near the coast next weekend compared to consensus. Model guidance is also reasonably agreeable in the western U.S. Wednesday-Thursday before differences grow late week. Some of this is likely due to differences that develop for the handing of a potentially strong Bering Sea storm as well as for a recurving west-central Pacific tropical system. CMC runs over the past couple of days have been persistent in showing strong upstream energy cutting off over the Pacific Friday-Sunday. This has appeared to be an outlier solution as it was even more aggressive with containing this energy offshore than basically all 12Z ensemble members from the EC, CMC, and GEFS. The new 00Z CMC has also gone away from this solution...however the 00Z GFS has started to show a similar closed low forming in the Pacific like the older CMC runs. There is still ample uncertainty especially considering the origins of the energy. The ECMWF runs have appeared to be the most consistent with their splitting of the far northern stream energy over Canada and thus a couple of weaker troughs extending into the CONUS next weekend. This is also somewhat similar to the ensemble means. Thus the WPC forecast began with blend of deterministic guidance favoring the ECMWF and GFS runs with some incorporation of the CMC and UKMET initially, but dropping those in favor of the GEFS and EC ensemble means (to 50-60 percent by days 6-7) along with the operational ECMWF in particular considering its relative consistency. ...Weather/Hazards Highlights... While the surface low associated with former tropical cyclone Kay should continue to dissipate over the eastern Pacific during the short range period, deep layer southwesterly flow will likely carry the lingering moisture from Kay into the Interior West/Rockies this week, leading to precipitable water values of at least 2-3 standard deviations above normal. Expect rain and some thunderstorms to spread across the Great Basin and Northern Rockies Wednesday-Thursday, with potential for locally moderate to heavy rainfall. Refining the areas of best focus for this potential will depend on exact shortwave details/timing along with other ingredients, so continue to monitor forecasts over the coming days. Rainfall should shift eastward into the Northern Plains and Midwest in the vicinity of a couple of frontal boundaries for the end of the week with generally light amounts currently forecast but with potential for embedded higher amounts, while an earlier event Wednesday-Thursday in the Upper Midwest could contain some moderate to heavy amounts. A quasi-stationary front and perhaps a low along it will be stalled across the Florida Peninsula and back across the Gulf of Mexico and western Atlantic through the latter half of the week into the weekend, and combined with a possible developing upper feature over the Southeast could focus rain chances across these areas. Florida is most likely to see rounds of convection with possibly heavy rainfall totals. Rain amounts across areas like coastal Georgia and the Carolinas are much less certain. Model guidance agrees that the heaviest QPF should be offshore but differs in terms of how much rain could sneak onshore. Keep watching for updates to the forecast as slight adjustments in specifics could significantly influence rainfall coverage and amounts over some locations. Southern Texas can expect showers and thunderstorms with the back end of the front. The most notable temperature anomalies this week will be over the Plains, with above normal temperatures by 5-15F leading to highs in the 90s for the southern half of the Plains. Some above average temperatures could also stretch into the Midwest at times. Meanwhile the western U.S. will see fairly broad coverage of below normal highs (by up to 5-10F) Wednesday-Thursday, likely a welcome change from the recent heat. Highs over the West may return closer to normal Friday-Saturday while staying a little on the cool side over the Pacific Northwest. Overnight/morning lows should remain near to above average over the West through the period. Over the East, near normal temperatures are mostly forecast, though some moderately below normal highs are possible in the Northeast behind a reinforcing front Thursday-Friday. Tate/Rausch 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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