Extended Forecast Discussion NWS Weather Prediction Center College Park MD 304 AM EDT Sat Sep 10 2022 Valid 12Z Tue Sep 13 2022 - 12Z Sat Sep 17 2022 ...Overview... The medium range period begins Tuesday with a fairly amplified upper pattern with good model agreement for an upper low centered over the Great Lakes region, a ridge across the west-central U.S. causing warm temperatures, and troughing overall for the West Coast. At the surface an occluded low pressure system in the Great Lakes will sweep a cold front across the Eastern Seaboard Tuesday-Wednesday, with rain and thunderstorms along and ahead of it, while the western U.S. flow pattern directs 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. Expect some flattening in the large scale pattern for midweek and beyond, though some ridging should persist and track at least as far east as the Mississippi Valley and perhaps the Tennessee/Ohio Valleys, while rounds of shortwave troughing come into the West. Energy perhaps creating an upper low or at least a weakness in the flow could linger in the the Southeast while a front meanders nearby, which could produce some heavy rain but with uncertainty regarding how much rain falls onshore of the extreme Southeast. ...Guidance Evaluation/Predictability Assessment... Good model agreement persisted in the 12/18Z yesterday model cycle with the Great Lakes upper low and other aspects of the synoptic scale flow at the beginning of the medium range period Tuesday. One minor exception was that the 12Z UKMET was a bit faster than consensus with the upper low lifting across the Northeast late Tuesday into Wednesday. While ridging initially centered over the Rockies Tuesday shifts east with time toward the east-central U.S. with reasonably good agreement in the guidance, the details of energy and perhaps a weak closed low settling somewhere in the Southeast Thursday-Saturday are still uncertain. Though this feature is small scale, the positioning and strength of the energy would affect the rainfall forecast, i.e. the amount of rainfall that reaches at least coastal areas of Georgia/the Carolinas as well as portions of the Gulf Coast. This forecast update attempted to strike a middle ground between the more suppressed ECMWF and the wetter GFS/CMC runs, much like the last forecast. The new 00Z GFS is particularly aggressive in spinning up a surface low near the Carolina coast late week. In the West, northern and southern stream shortwave troughs will provide a messy mean troughing pattern. Most model differences are relatively minor through the period, though by late week the CMC became a more extreme outlier in the Pacific, cutting off a deep upper low and affecting its pattern downstream over the CONUS. The WPC forecast utilized a multi-model deterministic blend favoring the ECMWF and GFS early in the period, and incorporated the EC and GEFS means with increasing proportions to about half by the end of the period to account for increasing detail differences. This maintained reasonably good continuity from the previous forecast. ...Weather/Hazards Highlights... Though Kay is now a post-tropical cyclone per the National Hurricane Center and the low should continue to dissipate through the weekend into early next week, it will still provide increased moisture (with precipitable water values of at least 2-3 standard deviations above normal) flowing into the Interior West through much of next week given deep layer flow from the southwest. Rain and some thunderstorms are forecast across the Central Great Basin and the Four Corners states Tuesday with a gradual trend farther north into the Northern Rockies by Thursday. Locally moderate to heavy rainfall is a possibility in some areas, but with refinement in the details likely to come in the shorter range time frame. Farther east, rain and thunderstorms are likely in conjunction with the Great Lakes low pressure system and its attendant cold front tracking across the East. The front is forecast to be pushing offshore of the Mid-Atlantic by early Tuesday and clear out rain chances there, but remain across the Northeast Tuesday, ending sometime Wednesday. The southern end of the front should linger near the Gulf Coast and off the Southeast Coast through the latter half of the week. This front and any embedded waves may combine with a possible developing upper feature over the Southeast to increase rainfall over the far southeastern U.S., but with low confidence in the details of how much rain will fall onshore vs. remaining offshore. Continue to monitor forecasts as slight adjustments in specifics could significantly influence rainfall coverage and amounts over some locations. Regardless, several rounds of convection are possible across Florida with the front stretching across the state with possibly a wave of low pressure as well. The most notable temperature anomalies next 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 increasing coverage of highs 5-10F below normal, likely a welcome change from the recent heat. Overnight/morning lows should remain near to above average there. The east-central U.S. should be cooler than normal behind the cold front Tuesday but moderate to near normal for the latter half of the week. 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