Extended Forecast Discussion NWS Weather Prediction Center College Park MD 352 PM EDT Wed Aug 07 2019 Valid 12Z Sat Aug 10 2019 - 12Z Wed Aug 14 2019 ...Overview and Guidance/Predictability Assessment... For the period as a whole there is reasonable agreement on a persistent ridge aloft over/near the southern Plains and progressive flow across the northern tier states. There is also good consensus regarding an upper trough/low crossing the Northwest during the weekend. Beyond those aspects, models and ensembles are exhibiting considerable spread and/or run-to-run changes for details across the eastern two-thirds of Canada (affecting frontal systems over the lower 48), ultimate path of ejecting Northwest U.S. energy, and evolution of eastern Pacific/West Coast flow by the latter half of the period. One significant change in the guidance is toward the idea of central Canada upper low/trough energy on Sat feeding more aggressively into the longer-term eastern North America mean trough. 00-06Z GFS runs are a bit on the fast side with leading height falls but overall there is a pronounced trend in model/ensemble guidance toward more energy feeding into the mean trough than depicted in previous cycles. This change results in a much faster southeastward progression of a cold front that reaches the extreme northern Plains by early day 3 Sat, along with significant adjustments for the leading front over the central-eastern states. Meanwhile guidance diverges for the ultimate path of the energy crossing the Northwest during the weekend. Best clustering of models/means (albeit only a faint signal in the means) show continued eastward northern tier U.S. progression within the westerlies, in contrast to the 00Z ECMWF that pulls it up into southern Canada and merges it with other nearby energy. The 12Z/06 ECMWF was closer to consensus for this shortwave. This feature should produce some degree of surface waviness but confidence is low regarding the specifics. By the latter half of the period the forecast also becomes increasingly uncertain over parts of the eastern Pacific and western U.S., downstream from ridging that rebuilds just south/southwest of mainland Alaska. The fact that by day 7 Wed the 00Z GFS (flat flow) and 00Z ECMWF (amplified trough) near the West Coast are the exact opposite of yesterday's 12Z runs provide reason enough for low confidence. Teleconnections relative to the strongest height anomaly centers in the D+8 multi-day means support some degree of mean troughing near the Pacific Northwest but possibly not as amplified as the latest ECMWF. Thus by later in the period prefer to tilt the forecast mostly toward the 00Z NAEFS/ECMWF means, as the CMC mean has an intermediate degree of troughing versus the GEFS mean's flat flow. Note that the flat flow scenario cannot be completely ruled out as it may end up being a transient state before mean troughing redevelops soon after day 7. ...Weather Highlights/Threats... The upper trough/low ejecting from the West Coast will likely produce episodes of showers and thunderstorms with areas of locally heavy rainfall during the period--initially over the Northwest/northern Rockies and then into the Plains and possibly parts of the East. Beyond the Plains confidence is low regarding location/intensity of rainfall given guidance spread and run-to-run variability. Locally heavy convection may also focus along a wavy front initially extending from the Southeast U.S. into the central Plains as well as another front pushing southeast from the northern Plains. Monsoonal convection over the Four Corners states may decrease in coverage somewhat over the course of the period. For the five-day period the best signal for highest rainfall totals exists over parts of the northern/central Plains but potential exists for locally significant totals at other locations within the extreme Northwest and eastern half of the country. The upper ridge centered over the southern Plains will keep temperatures 5-10F above normal over the southern states through at least early next week. Some gradual moderation is possible by the middle of next week. Greater likelihood for approaching/reaching daily record values will be for warm lows. An area of well below normal highs (up to minus 10-15F anomalies) will accompany the upper trough passing through the Northwest and then into the northern Plains from Sat into the first half of next week. 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 Hazards: - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Northern Rockies, and Interior Washington, Sat-Sun, Aug 10-Aug 11. - Heavy rain across portions of the Northern Plains, Sun-Mon, Aug 11-Aug 12. - Heavy rain across portions of the Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Mon-Tue, Aug 12-Aug 13. - Flooding possible across portions of the Central Plains and the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Northern Plains. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Sat-Tue, Aug 10-Aug 13. 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