Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1206 PM EDT Sat Aug 17 2019 Valid 12Z Tue Aug 20 2019 - 12Z Sat Aug 24 2019 ...Overview and Guidance/Predictability Assessment... Recent global guidance consensus is for phasing low pressure to rapidly develop Monday night into Tuesday in the northeast Pacific and shift into southeast AK Tuesday night. The only runs to not agree with this is the 00Z CMC and 12Z/16 GFS which are considered outliers. The 00Z UKMET is slower with this system and is a bit of an outlier despite developing a similar deep low. This system is key to the overall patter as it will promote amplification of the downstream ridge which will extend over the western Canadian Prairies and northern U.S. Rockies. Downstream of this ridge is a deep trough and shifts along the south side of Hudson Bay Tuesday into Thursday before shifting east to Labrador through Friday night. There is good agreement among global guidance on the upper trough south of this low that pushes east across the Great Lakes Wednesday to across the Northeast by Friday. An associated cold front reaches the East Coast Thursday and stalls over the southeast Friday and Saturday. The trailing part of the front will likely stall over the south-central Great Plains Thursday before lifting back east/northeast as a warm front Friday into Saturday. An atmospheric River south of the northeast Pacific low will diminish as it reaches the Pacific Northwest ahead of a cold front Wednesday. To the south of the main band of westerlies, expect initially strong Rockies/Plains ridging to weaken gradually/become more zonal after midweek. The main differences in deterministic guidance (besides the northeast Pacific low mentioned above) are with the timing of the eastern Canadian low as it traverses Labrador Day 6/7, so ensemble means were preferred for Friday/Saturday. Shortwave details are noted around these main lows, though the overall pattern is maintained well through Day 5. A very limited CMC/UKMET general deterministic blend that greatly favors the GFS/ECMWF was chosen for Days 3-5 with similar ensemble means warranting greater inclusion for Days 6/7. Ensemble spread by late in the period and the low amplitude of expected mean flow yield low confidence in shortwave details across the Northwest/northern Plains and eastern Canada by Fri-Sat. ...Weather Highlights/Threats... There may be pockets of locally heavy rainfall in the eastern U.S. with the cold front moving from the northern Plains through the East Tuesday to Thursday, but frontal progression should keep totals from being too extreme. However the central Plains look to have more potential for areas of enhanced rainfall due to the trailing end of the front stalling over the High Plains and then returning as a warm front. Eventual deceleration of the front over southern latitudes of the East may lead to more rainfall over parts of the southern Mid-Atlantic/Southeast as well. Expect diurnal convection with generally localized heavy amounts over the central/eastern Gulf Coast region. The front reaching the Pacific Northwest Wed-Thu may looks to only bring brief rainfall to northern areas. The cold front pushing through the central/eastern states will bring a noticeable cooling trend, with some highs up to 10F or so below normal over the northern Plains Tuesday to Thursday and for the northeast on Friday. The hot weather ahead of the front will persist over central U.S. into Tue and over the East into Wed. A majority of the central-eastern U.S. should see temperatures within a few degrees of normal by Sat. Expect above normal readings to prevail over most of the western U.S. through the period. The greatest anomalies (with some record highs possible over the Southwest) looks to be on Wed. Jackson 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