Extended Forecast Discussion NWS Weather Prediction Center College Park MD 503 PM EDT Mon Sep 12 2022 Valid 12Z Thu Sep 15 2022 - 12Z Mon Sep 19 2022 ...Overview... A trough-ridge-trough upper-level pattern is forecast to persist across the U.S. mainland through the medium-range period, with uncertain details on the various small scale shortwaves moving into the western U.S. One such shortwave is forecast to work in concert with another shortwave dipping across central Canada to organize an axis of heavy rain across the arrowheads of Minnesota and upper Michigan Thursday to Friday. Meanwhile, ample moisture originating from what was tropical cyclone Kay is forecast to interact with the western U.S. upper trough to provide locally enhanced rainfall from the Great Basin to the northern Rockies late this week. A meandering front will allow unsettled weather to continue along the Gulf Coast into Florida with an outside chance of tropical cyclogenesis off the southeast U.S. Meanwhile, heat is forecast to gradually build over the central Plains. ...Guidance Evaluation/Predictability Assessment... Model guidance shows reasonably good consensus to begin the medium range period Thursday with the pattern evolution mentioned above. Toward the end of the forecast period, model solutions rapidly diverge along the West Coast. The ECMWF in particular has abandoned yesterday's idea of closing off an upper low in favor of a more progressive trough moving through the Northwest by this weekend. In contrast, the GFS and CMC along with their ensemble means continue with a rather amplified pattern over the western U.S. this weekend into early next week. The poor model agreement, possibly the result of difficulties in resolving Typhoon Merbok in the central Pacific will likely continue to keep the forecast pattern uncertain over the western U.S. The ECMWF has been trending faster with the track of Merbok in better agreement with the GFS and the CMC which have been on the faster side of the guidance. The ECMWF becomes noticeably faster than its ensemble mean with the low pressure wave developing over the northern Plains by Day 7. Thus, the GFS/CMC consensus is favored for the CONUS. The WPC medium-range forecast package was based on 40% from the 00Z ECMWF/EC mean, 40% from the 06Z GFS/GEFS, and 20% from the 00Z CMC/CMC mean, transitioning to mostly the consensus of the ensemble means by Day 7, with less of the ECMWF being used relative to its ensemble mean. ...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 north of a warm front where a shortwave dipping southeast across south-central Canada is forecast to merge with a developing low pressure wave over the northern Plains Thursday and into Friday. Given that QPF amounts from model guidance have generally increased, a Slight Risk of excessive rainfall has been introduced for this area. 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 has generally trended offshore late this week, with the GFS indicating possibe tropical cyclogenesis off the coast of southeastern U.S. this weekend. 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. Kong/Tate/Rausch Hazards: - Heavy rain across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, the Northern Great Basin, the Upper Mississippi Valley, and the Great Lakes, Thu-Fri, Sep 15-Sep 16. - Excessive heat across portions of the Central Plains and the Southern Plains, Sat-Mon, Sep 17-Sep 19. 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