Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Sep 21 2019 Valid 12Z Tue Sep 24 2019 - 12Z Sat Sep 28 2019 ...Heavy rainfall threat over the Southwest decreases after Tue... ...Areas of heavy rain possible over parts of the central U.S. during the period... ...Precipitation expanding over the West late week/weekend... ...Overview... Guidance continues to show a brief period of fairly flat mean flow followed by a rapid transition to a very amplified West Coast trough/eastern ridge by next weekend. As this pattern becomes established expect a pronounced trend to much below normal highs over the West with increasing coverage of precipitation including higher elevation snow. Meanwhile the eastern half of the country should see an expanding area of much above normal temperatures. Between these two air masses, portions of the Plains/Mississippi Valley will be in the best position to see one or more episodes of locally heavy rainfall. Guidance continues to have difficulty in resolving specifics of an upper low reaching the Southwest by early Tue and timing of northeastward ejection ahead of the amplifying West Coast trough. This feature and leading moisture from the tropical East Pacific may support some enhanced rainfall near the start of the period and eventually over the central part of the country. ...Guidance Evaluation/Preferences... The early part of the updated forecast started with highest weight on the 12Z ECMWF with supporting contributions from the 12Z GFS/UKMET/CMC. As the amplified trough-ridge pattern becomes established the forecast transitioned to greater emphasis on the past two ECMWF runs (to downplay detail uncertainties) and ECMWF mean with the remainder composed of the 12Z CMC and 18Z GEFS mean. Guidance still varies for some aspects of the upper trough/low affecting New England Tue-Wed. At the very least the 18Z GFS had the lowest probability solution with a progressive open shortwave versus slower upper low. The new 00Z GFS improved upon that 18Z run but is still on the faster side of the envelope. Farther west a blend provided a reasonable starting point for troughing that reaches central Canada and the northern Plains by Wed. One trend of note is that consensus has adjusted farther south with the associated central-eastern U.S. front. Meanwhile guidance continues to vary among each other and run to run with the Southwest upper low. The manual forecast incorporated the slower tendency of most 12Z models but confidence is still lower than desired as some of the new 00Z runs have jumped faster. Either way the amplifying western trough will at some point promote rapid ejection of the upper low. By late Wed or Thu through the rest of the week there are still some pronounced differences for individual pieces of energy arriving into the Northwest as the upper trough begins to take shape. In varied ways recent GFS runs have differed from consensus enough to downplay in the forecast blend after day 5 Thu, and in addition lower heights into the Plains somewhat more than consensus late in the period. ECMWF/GEFS means have been fairly stable and similar with the trough into day 7 Sat while the ECMWF runs back to the 12Z/19 cycle have been stable with the overall trough and Pacific Northwest upper low. What minor trends existed through the 12Z cycle seemed to favor hedging a little closer to the coast and the new 00Z CMC has adjusted to this scenario as well. Fast southwesterly flow downstream will present shortwave challenges (including the ejecting Southwest upper low) so it will likely take some time to resolve specifics of central U.S. waves/fronts. ...Weather Highlights/Threats... The upper trough amplifying along or near the West Coast late this week will lead to increasing coverage of precipitation over portions of the West. Best potential for high elevation snow should be over the extreme northern Rockies with lesser probabilities over the Northwest and perhaps even into the Sierra Nevada by next weekend. Medium/smaller scale detail uncertainties aloft temper confidence in where heaviest precip will occur but potential exists for some enhancement over favored terrain from the Pacific Northwest/northern California into the northern Rockies/Great Basin. Farther south expect the compact upper low initially over the Southwest to support some localized rainfall enhancement early in the period. Locations along and near an axis from the central Plains into Upper Mississippi Valley will see the best potential for one or more episodes of heavy rainfall during the period. Once the amplified upper trough-ridge pattern gets established, teleconnections suggest the best emphasis should be over the Upper Mississippi Valley. One convective episode may accompany a cold front approaching the region early in the period along with a leading advancing warm front. The cold front will stall to the south and then return back to the north with one or more waves, providing a focus for additional convection. One or more shortwaves aloft and eastern Pacific tropical moisture may contribute as well. The cooling trend over the West will start over northern areas with the Northwest seeing highs 10F or more below normal by Thu. Minus 10-20F (and locally greater) anomalies for highs should cover an increasing proportion of the West by Fri-Sat. Clouds/precip will keep min temps from being nearly as extreme. At the same time temperatures 10-20F above normal will become more common over the eastern half of the country by late week and the weekend. Some daily record highs appear increasingly likely over and near the Southeast from Thu onward. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml