Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 AM EDT Mon Sep 12 2022 Valid 12Z Thu Sep 15 2022 - 12Z Mon Sep 19 2022 ...Overview... A somewhat messy upper level pattern is forecast for the latter part of the workweek with a myriad of small scale shortwaves embedded within the flow, but with a general pattern of troughing across the western to north-central U.S. and warm ridging from the south-central into east-central U.S.--with the exception of energy across the Southeast. This flow pattern will promote ample moisture originating from what was tropical cyclone Kay into northern parts of the Intermountain West and Rockies, leading to some locally enhanced rainfall from showers and thunderstorms Thursday-Friday. Farther east, rain focusing along a warm front in the Upper Midwest could also bring heavy rain to that region Thursday into Friday, with perhaps additional rounds of rain through early next week. Weak upper energy could 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. By the weekend into early next week, model guidance varies with the overall pattern, but with perhaps some agreement for the mean trough near the West Coast to deepen with time, possibly enough to increase coverage of precipitation over the Northwest. ...Guidance Evaluation/Predictability Assessment... Model guidance shows reasonably good consensus for the beginning of the medium range period Thursday across the CONUS, but model agreement quickly devolves especially upstream over the Pacific, where models are highly sensitive to differences that arise from Arctic and Pacific sources including the eventual track of Tropical Storm 15W and its transition to extratropical as it recurves. The GFS suite including the 12/18Z deterministic runs and most GEFS members have been most aggressive at closing off an upper low well offshore. This allows for ridging extending as far west as the the Great Basin by early next week. Meanwhile ECMWF deterministic and many ensemble members are considerably more progressive and track troughing across the north-central U.S. for a pretty much completely out of phase flow pattern by Sunday-Monday. The new 00Z GFS is farther east than its previous runs but still shows a closed low, while the new 00Z ECMWF is similar to its previous run. The 12Z CMC model suite actually seemed to be a good middle ground in between the EC and GFS, with periods of a closed low in the deterministic run in the eastern Pacific farther east than the GFS (thus closer to the EC troughing position), and the CMC ensemble mean actually seemed to serve as the best proxy for a middle ground forecast that was not too aggressive in one direction or another. Thus the WPC fronts/pressures forecast (in a somewhat unusual model blend) favored the CMC ensemble mean most heavily by the latter part of the period, with some EC mean as well. The 18Z GEFS mean was farther west of consensus like the deterministic GFS runs and thus not favored. Smaller scale energy in the Southeast and how it interacts with a wavy front is also a forecast consideration through the period as it affects the rainfall forecast across that region. Operational GFS runs continue 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, leading to more QPF onshore of the Carolinas and Georgia coast, while other guidance has little to no QPF there. The forecast preference remains drier than some GFS runs given better model agreement for drier conditions. ...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. By the medium range timeframe, this anomalous moisture should be in place across northern parts of the Great Basin and Rockies, leading to rain and some thunderstorms with potential for locally moderate to heavy rainfall. Flooding/flash flooding impacts of this rain are uncertain so this precludes a Slight Risk in the experimental Day 4 Excessive Rainfall Outlook at this time. Another area of focus for heavy rain will be parts of the Upper Midwest, where good moisture transport will be present in the vicinity of a warm front Thursday and perhaps into Friday. 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 a few model runs still sneak some rainfall onshore near the Atlantic coast. Southern Texas can expect showers and thunderstorms with the back end of the front. The central U.S. is expected to be quite warm during the medium range timeframe with periods of ridging aloft. Highs of 5-15F above average will be widespread across much of the Plains and Mississippi Valley, with the Central Plains perhaps seeing the greatest anomalies nearing 20F above normal with actual temperatures well into the 90s. The West meanwhile may see cooler than normal highs by 5-10F though lows could be around average, but details depend on uncertain troughing aloft. Over the East, expect near normal temperatures aside from some moderately below normal highs in the Northeast Thursday-Friday behind a reinforcing cold front. 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