Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 PM EDT Thu Aug 26 2021 Valid 12Z Sun Aug 29 2021 - 12Z Thu Sep 02 2021 ...Heavy rain threat for areas along and north of the central Gulf Coast for early next week with tropical system... 18Z Update: The newly designated Tropical Depression Nine is becoming more defined in the extended forecast guidance, with the 15Z NHC forecast track closest to a compromise of the 12Z GEFS mean and 12Z UKMET. The CMC is the fastest solution throughout the period, and the GFS is the second fastest through Tuesday before becoming more in line with the model consensus. The 00Z ECENS is on the southwest edge of the guidance, and this will likely adjust to the northeast with its 12Z run, similar to other guidance over the past 24 hours. As a result of these changes, the axis of heaviest rainfall has trended a little more to the east with this forecast cycle, and additional adjustments to the track are likely with future forecast updates. Elsewhere across the continental U.S., the guidance is generally in good agreement on the synoptic scale, except for some timing differences noted in the CMC, particularly the 00Z run that was becoming out of phase across the Great Lakes region. It is also worth noting that the eventual track of Tropical Storm Nora could have implications on rainfall across the Desert Southwest by the end of the forecast period late next week, with a chance of heavy rain moving northward across this region. The updated forecast was primarily derived from a blend of the GFS/ECMWF/UKMET/some CMC through Tuesday, and then primarily the GFS/ECMWF and their respective means for Wednesday and Thursday. The previous discussion follows below. /Hamrick ...Overview... By mid-late next week the ongoing pattern of progressive flow aloft extending from the northeastern Pacific across the northern tier U.S./southern Canada shows signs of slowing down as mean troughing becomes more established over western Canada into the northwestern U.S. as well as over eastern Canada into the northeastern states, with a ridge building in-between. A leading shortwave will push a Midwest/Plains front into the East while the upper trough pushing into western North America will bring a cold front into the Northwest. At lower latitudes, guidance still exhibits some spread and trends but offers increasing confidence that a Caribbean wave will ultimately generate a tropical system that will track northward over the Gulf of Mexico and into the southern U.S. as it finds a weakness within the mean ridge across the southern half of the country. This system would produce hazards including heavy rainfall and strong winds. The heavy rainfall threat may extend northward depending on system track and possible interaction with the front that stalls after reaching the eastern U.S. In addition the National Hurricane Center is issuing advisories on eastern Pacific Tropical Depression Fourteen-E which should eventually strengthen to a hurricane for a time and track close to Baja California. Some moisture from Fourteen-E could eventually reach the Southwest. ...Guidance Evaluation and Preferences... For the possible tropical system tracking over the Gulf of Mexico, trends are less dramatic than seen over the past couple days but the 18Z GFS provided yet another adjustment on the faster/eastward side of the spread versus other guidance. The new 00Z GFS backed off a bit on its speed versus the 18Z run while the 00Z CMC is close to the GFS for most of the period (and east of the 12Z run). The 00Z UKMET agrees with the model consensus for a Louisiana landfall and is close to the 12Z ECMWF/00Z GEFS mean but is slower than the new GFS/CMC. The new 00Z ECMWF continues the faster/eastward theme. While specifics remain uncertain, these latest model runs are increasing confidence in tropical development and a track somewhere toward the central Gulf Coast. Based on guidance available at the time of forecast preparation, the 12Z ECMWF/CMC/GFS provided the composite template in principle along with a compromise with continuity. A blend of those models compared rather well to the ensemble means for the significant northern stream features through the period so even by day 7 Thursday the forecast needed to employ only a modest weight of the means to smooth the proverbial rough edges. There has been a noticeable slower trend with the leading side of the incoming western trough aloft which has a corresponding effect on surface features eventually reaching the northern Plains. Finally, guidance is not well clustered for Fourteen-E so confidence is well below average for how much moisture from this system may eventually reach the southwestern U.S. and where. The 12Z ECMWF/ECMWF mean were the closest solutions to the track from the 03Z National Hurricane Center advisory. ...Sensible Weather/Hazard Highlights... The latest guidance is adding to confidence in the general forecast of tropical development and potential impacts, most likely along and near the central Gulf Coast from late weekend into next week. However there is still a fair degree of uncertainty for exact track and timing (with somewhat of an eastward trend still evident) since guidance has not sufficiently stabilized yet for those aspects of the system. Current consensus would extend the heavy rain threat northward at least into parts of the Lower Mississippi Valley/Tennessee Valley. How this moisture may interact with a front that stalls over the East will require monitoring as well. Before the tropical system's arrival this front will be accompanied by areas of rain and thunderstorms as it progresses from the Midwest/Plains into the East during the first half of the week. The trailing part of the front over the Plains may provide a focus for one or more episodes of rainfall over portions of the Midwest/northern tier states. A wavy stationary/warm front over the Northeast may focus some rainfall Sunday-Monday. With time rainfall should gradually increase in coverage and intensity over the Southwest and vicinity as the upper pattern becomes more favorable for drawing some moisture from western Mexico and the eastern Pacific. A significant wild card will be the exact track of Tropical Depression Fourteen-E and how its moisture may ultimately be incorporated into the flow. Locations from the Great Lakes into the southern Mid-Atlantic will be quite warm to hot at the start of the week, ahead of the cold front emerging from the Plains/Midwest. Expect somewhat greater coverage of plus 10-15F anomalies for morning lows relative to daytime highs, with daily records also more likely for warm lows. Then eastern U.S. temperatures will moderate over the rest of the week. California and the Great Basin will see moderately above normal highs Sunday-Monday. Then a majority of the West will see a cooling trend to below normal highs as an upper trough reaches the Northwest and clouds/rainfall expand over the Southwest. The central High Plains will see highs 5-10F or so above normal Monday-Wednesday. 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, 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