Extended Forecast Discussion NWS Weather Prediction Center College Park MD 425 PM EDT Fri Sep 09 2022 Valid 12Z Mon Sep 12 2022 - 12Z Fri Sep 16 2022 ...Overview... Most guidance agrees fairly well with the large scale pattern through the period. Expect a closed upper low and occluded surface low to track over the Great Lakes during the first part of the week, sweeping a leading cold front off the East Coast by midweek. A trailing upper ridge should progress at least as far east as the Mississippi Valley, bringing multiple days of very warm temperatures to the southern two-thirds of the Plains. Mean troughing composed of multiple shortwaves will likely set up near the West Coast and promote cooler weather over the West. The western U.S. flow pattern should also direct moisture originating from Kay across the Intermountain West and parts of the Rockies, leading to some locally enhanced rainfall from showers and thunderstorms. Finally there is some signal for an upper low/weakness to develop over the southeastern U.S. late in the week and interact with a wavy stalled surface front. At this time confidence is low regarding coverage and magnitude of associated rainfall that may affect parts of the extreme Southeast at that time. ...Guidance Evaluation/Predictability Assessment... Latest guidance appears to confirm the recent acceleration in the ejection of the Midwest/Great Lakes upper low early in the week, in response to more amplified southern Canada flow. The 00Z CMC/UKMET were still slower than the latest GFS/ECMWF runs through 06Z but the new 12Z CMC has joined the GFS/ECMWF cluster and the 12Z UKMET has in fact trended even faster than the current majority. After this upper low departs there is a developing signal for some trailing energy to settle somewhere over the Southeast later in the week. However up to this point there has been significant spread and variability in the shape and location of the energy--anywhere between the Lower Mississippi Valley and Southeast Coast. The best average of models and means (either explicitly or implied) would have this feature centered over or a little south of the far southern Appalachians by early next Friday. Some individual solutions (such as the GFS) have the interaction of this feature and wavy surface front to the south bringing heavy rainfall into parts of the far Southeast/Carolinas while the ECMWF and ensemble means are notably suppressed. Overall prefer an intermediate solution given the detail uncertainty, leading to increased rainfall potential but for now not to the extent of the heaviest solutions. Any adjustments over the West fall more into the minor detail realm. Lingering upper energy from Kay should lift northeastward ahead of a shortwave pushing into the Northwest by Tuesday, with the combination of these features reaching the High Plains/southern Canada by midweek. Another weak feature should drift into the West mid-late week while a northern stream trough may push into British Columbia and possibly the Northwest on Friday. Some new 12Z guidance is adding some doubt in the consensus though, with the 12Z CMC/ECMWF each straying significantly out of sync over the Pacific and/or southern Canada in very different ways by Friday. The update to the WPC fronts/pressures forecast emphasized the 06Z GFS/00Z ECMWF with lesser input of the CMC/UKMET early in the period. Detail uncertainties led to increasing GEFS/ECMWF ensemble mean weights to about half total by the end of the period. This approach led to fairly good continuity with most aspects of the forecast. The main change reflects the developing signal for a better defined late-week upper feature over the Southeast and associated QPF increase. Still, preference was to temper the heaviest NBM totals a bit along the Gulf/Southeast coasts to allow for a smoother continuity transition. ...Weather/Hazards Highlights... While Kay is currently a tropical storm in the eastern Pacific off the coast of northern Baja California per the 1500Z National Hurricane Center advisory, NHC forecasts show Kay becoming a post-tropical cyclone and then dissipating during the medium range period. Regardless of Kay's status, increased tropical moisture looks to flow into much of the West with deep layer flow from the southwest. Rain and some thunderstorms are possible in the Desert Southwest Monday, with a northward trend to the forecast rainfall into the central/northern Great Basin and Rockies as next week progresses. Most guidance shows precipitable water values of at least 2-3 standard deviations above normal within this moisture shield. Thus expect some locally moderate to heavy rainfall with refinement in the details likely to come in the shorter range time frame. Farther east, the cyclone moving across the Upper Midwest/Great Lakes and its attendant fronts will produce rain and thunderstorms across the northeastern quadrant of the U.S. and across the Eastern Seaboard Monday-Tuesday before the cold front clears precipitation chances out on Wednesday. Isolated flash floods are possible in both the West and East in areas where rain is particularly heavy. At the moment, the best confidence in a concentrated area of heavy rainfall is with a band north of the Great Lakes upper low. The experimental Day 4 Excessive Rainfall Outlook has introduced a Slight Risk area for that activity. The southern end of the eastern 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. Confidence is low in details as this evolution has really just started to become more apparent in the guidance over the past day. Continue to monitor forecasts as slight adjustments in specifics could significantly influence rainfall coverage and amounts over some locations. The most notable temperature anomalies next week will be over the Plains. Northern areas will see highs 10-15F above normal on Monday followed by a cooler trend closer to normal. Then above normal readings will settle over the central-southern Plains for the rest of the week with some plus 10-15F anomalies for highs and/or morning lows. 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. Meanwhile the east-central U.S. should be cooler than normal behind the cold front Monday-Tuesday but moderate to near normal for the latter half of the week. 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 Hazards: - Heavy rain across portions of the Southeast and the Mid-Atlantic, Fri, Sep 16. - Heavy rain across portions of the Southeast, Thu, Sep 15. - Heavy rain across portions of the Great Lakes, Mon, Sep 12. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. 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