Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1152 AM EDT Thu Aug 08 2019 Valid 12Z Sun Aug 11 2019 - 12Z Thu Aug 15 2019 ...16Z Update... No major changes were needed to the previous forecast. The 06Z GFS and GEFS seemed to come into better agreement with the model consensus with a couple of features. First, while there remains some issue with hints of ridging in the Northeast U.S. on Sun as described in the previous discussion, the 06Z GFS places the main upper low in central Canada farther north than its older runs, which is in better alignment. More significantly, by Wed/Thu the 06Z GFS and GEFS mean develop more troughing than previous runs in the eastern Pacific approaching the Northwest U.S., aligning more with the last several runs of the ECMWF as well as the CMC. Given the ridging centered in the Gulf of Alaska/northeastern Pacific, it seems prudent to have a higher degree of troughing downstream over the Northwest U.S. (as teleconnections suggest). So this was a favorable trend in the GFS/GEFS. Overall did lean away from the operational GFS early on in the period for continuity and because of the ridging issue described above, but was able to utilize the 06Z GEFS mean later on. The 00Z ECMWF and ensemble mean as well as the 00Z CMC and UKMET were incorporated in the forecast as well. For more details and discussion of sensible weather, please see the previous discussion. Tate ...Previous Discussion... ...Overview and Guidance/Predictability Assessment... Models continue to struggle with the zone of relatively progressive mid/upper-level flow across southern Canada and the northern U.S. during the medium range. The GFS continues to be nearly out of phase with the flow from central Canada to the Northeast U.S. even at the beginning of the forecast period (Day 3/Sun). The 12Z ECMWF/UKMET/CMC showed upper troughing across New England Sun morning while the GFS continues to have the trough weaker and offshore with ridging across New England. Meanwhile, timing differences with a deep upper low crossing near southern Hudson Bay at the same time also result in 500-hPa heights that are nearly out of phase between the GFS and the other mentioned guidance. Given that the GEFS ensemble mean continued to look at least somewhat more like the non-GFS solutions in these areas, decided to steer away from the GFS similar to last night. This shortwave should move a cold front east across the Midwest/Great Lakes Sun-Mon and toward the Eastern Seaboard by Tue, with a trailing wavy frontal boundary extending back into the Mid-Mississippi Valley and the Southern/Central Plains. Farther west, model consensus is much improved with a shortwave across the Pacific Northwest on Sun, which moves east skirting the northern Rockies on Mon. Models begin to diverge with respect to this system by Tue as it starts to interact with the stronger westerly flow across Canada, resulting in increased timing differences as the shortwave begins to shear eastward. Given these considerations, the forecast was initially based heavily on a blend of the 12Z ECMWF/CMC during days 3-4 (Sun-Mon). During days 5-7 (Tue-Thu), a blend of the ECMWF and ECENS/GEFS ensemble means was used, with weighting of ensemble means gradually increasing through time. ...Weather Highlights/Threats... Multiple areas of showers and thunderstorms are expected to focus across the Northern/Central Plains and east into portions of the Midwest Sun-Mon as the polar cold front moves southeast across these areas and the Pacific shortwave moves east across the Rockies. Models show some signal for potentially heavy rainfall across the Northern Plains, especially on Sunday, although given model differences confidence in the specifics of heavy rain location is moderate at best. By Tue-Thu, expect showers and thunderstorms to spread eastward in association with a wave of low pressure expected to move along a front from the Ohio Valley to the Mid-Atlantic. As the trailing frontal boundary moves into the Southeast, extending back into the Southern Plains by next Wed-Thu, expect showers and storms to increase in coverage across that region as well. Locally heavy rainfall will be possible in any of these areas. Below normal temperatures are expected across the northern Great Basin and northern Rockies as the upper trough moves overhead Sun-Mon, with high temperatures forecast to be 5 to 15 deg below normal. These cool temperatures may also spread east into the Northern Plains and portions of the Upper Midwest by Mon-Tue. Farther south, the Southern Plains will remain hot through much of the period beneath the upper ridge, with the heat spreading east toward the Southeast as well. Highs are forecast to be 5 to 10 deg above normal through much of the forecast period across these areas. Ryan 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