Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Sat Sep 10 2022 Valid 12Z Tue Sep 13 2022 - 12Z Sat Sep 17 2022 ...Overview... The medium range period begins Tuesday with guidance agreeing upon a fairly amplified upper pattern featuring an upper low centered over the Great Lakes region, a ridge across the west-central U.S. causing warm temperatures, and general weak troughing near 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 multiple shortwaves reach the West within the overall trough. Energy that may create an upper low or at least a weakness in the flow could linger in the Southeast while a front meanders nearby, potentially leading to some areas of heavy rain but with very low confidence regarding how much rain will fall offshore versus over land. ...Guidance Evaluation/Predictability Assessment... Overall the guidance remains fairly well clustered for the upper low and surface system tracking out the Great Lakes Tuesday-Wednesday. If anything, the latest array of guidance (adding in the 12Z runs) suggests perhaps a modest nudge faster to continue the trend from recent days. Then there is still a fair amount of spread with respect to weak energy that may settle over the Southeast later in the week as well as for the corresponding surface reflection. On the positive side, there is moderate clustering in principle aloft (ECMWF/GFS/UKMET runs and 00Z CMC along with most ensemble means). The prominent exception now is the 12Z CMC that whisks the feature quickly into the Atlantic. Within the majority scenario, recent GFS runs were much stronger to incorporate some energy on the eastern side of the overall circulation which led to much deeper and northward surface low pressure by Friday-Saturday. The 12Z GFS has continued a southward trend started by the 06Z run, bringing it closer to other guidance at least into Saturday. Preference continues to be with a conservative intermediate solution given the uncertainty of the evolution. The forecast over the West is reasonably agreeable/consistent from Tuesday into early Thursday, with shortwave energy originating from Kay ejecting through the West then followed by another feature crossing the Northwest and a farther south shortwave that arrives Wednesday-Thursday. Then models and ensemble members rapidly diverge with flow details over the eastern Pacific into western North America. 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. By late in the period the possibilities among the models range from a progressive and low-amplitude shortwave crossing western Canada per the 00Z ECMWF (but with a trailing shortwave reaching the West Coast) to multiple CMC runs pulling off an upper low well offshore the West Coast. The new 12Z GFS is a slightly faster version of the ensemble means which depict a more amplified phased trough reaching the West Coast by early Saturday. The 06Z/00Z GFS runs were somewhat deeper and had an upper low closing off near Vancouver Island or Pacific Northwest during the day Saturday. Low confidence in specifics recommends keeping the forecast closest to the means for now. Based on the above considerations, the latest forecast update based on data through the 06Z cycle incorporated an operational model composite for the first half of the period (more 06Z GFS and 00Z ECMWF than 00Z CMC/UKMET) followed by a transition toward about half total weight of the 06Z GEFS/00Z ECMWF means by day 7 Saturday. ...Weather/Hazards Highlights... The surface low associated with former tropical cyclone Kay should continue to dissipate over the eastern Pacific in coming days, perhaps losing definition by next Tuesday. However deep layer southwesterly flow will likely carry the lingering moisture from Kay into the Interior West/Rockies next week, leading to precipitable water values of at least 2-3 standard deviations above normal. Expect rain and some thunderstorms to spread across the Central Great Basin and the Four Corners states Tuesday with a gradual trend farther north into the Northern Rockies by Thursday. The anomalous moisture may increase the potential for some areas of 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 trend lighter toward the end of the week. 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 should push offshore of the Mid-Atlantic by early Tuesday and clear out rain chances there, but it will take another day for the rain to cross the Northeast, 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 versus remaining offshore. Keep watching for updates to the forecast 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. Elsewhere, locations over and near southern Texas may see periods of rain due to persistent easterly low level flow and a lingering front. At the same time a wavy front will set up over the Northern Plains for multiple days, providing a potential focus for some episodes of rainfall. Thus far there is not an agreeable signal for any specific heavy rain episodes along this boundary but this region will require watching since some of the moisture over the West through midweek should eventually spread over the Northern Plains. 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 fairly broad coverage of below normal highs (by up to 5-10F) Tuesday-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, east-central locations will be cooler than normal behind a leading cold front Tuesday and New England should see moderately below normal highs behind a reinforcing front later in the week. Otherwise expect near normal temperatures on most days. Rausch/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, 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