Extended Forecast Discussion NWS Weather Prediction Center College Park MD 312 PM 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. Away from this ridge, troughing will prevail over the Northeast and in the West. The warm ridge will track slightly eastward and likely flatten somewhat through late week, though some ridging could persist over parts of the far southern Plains/Rockies through the weekend. Meanwhile guidance continues to suggest that weak upper level energy may settle over the Southeast for a time, which along with a meandering frontal boundary will cause rain and thunderstorms at least for Florida and perhaps reaching the Georgia/Carolina coasts. 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 surface waves and frontal systems passing through. With low confidence in the details, the mean trough near the West Coast may deepen with time--possibly enough to increase coverage of precipitation over the Northwest. ...Guidance Evaluation/Predictability Assessment... Once again, larger scale differences in the forecast develop over the Northeast Pacific by early Thursday and propagate into the rest of the eastern Pacific and North America through the rest of the week and weekend. This spread originates from major differences in the handling of a strong Bering Sea storm (ranging from the ECMWF being quite progressive and the GFS keeping it far westward), then compounded by differences in the track of a recurving Pacific tropical system. Differences with the Bering Sea storm show up as the ECMWF being flat and progressive with the leading Northeast Pacific shortwave versus other guidance amplifying more into the eastern Pacific/West Coast. While hinted at by the GEFS, the operational 00Z and 12Z GFS runs are the most extreme in pulling off an upper low well offshore. Interestingly the 06Z GFS brought the upper low closer to the Pacific Northwest and while the 00Z ECMWF brought an upper low to a similar location the latter model's low was from a different trailing source. CMC runs highlight the volatility of the forecast, with some prior runs looking like the 00Z/12Z GFS, the 00Z run possibly too open/progressive with the trough, and now the 12Z CMC showing a closed low reaching the West Coast during the weekend. Perhaps not surprisingly, the new 12Z ECMWF develops some notable changes from the previous run later in the period. The high uncertainty of the forecast favored a blend approach that increasingly trends toward the ensemble means with time. Issues with this part of the pattern ultimately affect the details farther east across the northern U.S. and southern Canada, as well as exactly how much upper ridging persists southern Rockies/Plains. Prefer to stay as close to continuity as possible where permitted by the favored forecast blend. The other significant forecast issue continues to involve the weak upper level energy that may drop into the Southeast for a time, with continued uncertainty over its shape/strength and eventual progression as well as how it may interact with a wavy surface front. The operational GFS continues to be on the strong side of the spread for the upper energy and the strength/northward extent of surface low pressure along the East Coast. The GEFS mean has been hinting at the GFS idea but in much less pronounced form while recent NBM runs have been showing enough of a GFS influence to favor dampening the higher amounts in the northern periphery of its rainfall shield in the latter part of the period. The early part of the updated forecast started with a blend of 06Z/00Z operational models (more GFS/ECMWF relative to the UKMET/CMC). The rapidly increasing guidance spread favored incorporating somewhat more 06Z GEFS/00Z ECMWF mean input than average mid-late period, already 30 percent by day 5 Friday and 70 percent by day 7 Sunday. ...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. At this time there is at least enough of a signal to favor introducing a Slight Risk area in the day 4 experimental Excessive Rainfall Outlook (12Z Wednesday-12Z Thursday) covering parts of Utah and southeastern Idaho. The best focus for activity may shift a little northward into the Rockies over the following 24 hours but confidence is not quite high enough to depict a Slight Risk area yet. Rainfall should shift eastward into the Northern Plains and Midwest in the vicinity of a couple of frontal boundaries and some embedded waves after midweek. There is a developing signal for some heavy rainfall potential with a wave/warm front between the Northern Plains and Upper Great Lakes during the mid-late part of the week. Behind this area of rainfall, confidence in details across the northern half of the Plains and vicinity decreases considerably. The Pacific Northwest may start to see an increase in precipitation toward the end of the week or weekend but again confidence is very low in the specifics at this time. A quasi-stationary front and perhaps a wave or two 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. This wavy front and 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 forecast is consistent in showing multiple days of above normal temperatures over most of the Plains except for far northern areas. Most days should feature plus 5-15F anomalies for highs which equates to 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 Interior West may return closer to normal by Friday and the weekend while below normal readings become more confined to the Pacific Northwest Coast where upper troughing may gradually deepen. Overnight/morning lows should remain near to above average over most of the West through the period. Over the East, expect near normal temperatures aside from some moderately below normal highs in the Northeast Thursday-Friday behind a reinforcing cold front. 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