Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Sun Jul 19 2020 Valid 12Z Wed Jul 22 2020 - 12Z Sun Jul 26 2020 ...Overview and Weather/Hazard Highlights... The latest model and ensemble runs continue to show amplification of the large scale pattern over North America after midweek, with a strengthening upper ridge over central/east-central portions of the U.S. and Canada along with mean troughs becoming established over the Pacific Northwest coast and New England/Canadian Maritime provinces. Multiple areas of rain/thunderstorms will be possible around the periphery of strongest upper ridging likely to be centered near or just south of the Middle Mississippi Valley. One feature helping to focus rainfall will be a surface wave tracking from the Great Lakes into southeastern Canada/New England Wed-Thu along with a leading warm front and trailing cold front whose western portion should stall over the Plains. Expect the wave/fronts to promote scattered to numerous showers and thunderstorms, with locally heavy rain possible between the Great Lakes/Ohio Valley and East Coast. The cold front will also bring at least a brief interruption of the intense heat across the Mid-Atlantic after mid-90s F highs persist into Wed-Thu. Guidance suggests potential for a more significant convective event during the latter half of the week over the Northern Plains and Upper Midwest, possibly into the Upper Great Lakes, with ejecting western U.S. shortwave energy aloft interacting with a stationary/warm front. Some areas of heavy rainfall will be possible and the Storm Prediction Center is monitoring this area for severe potential. Consult upcoming SPC outlooks as important details become more clear. Some showers and storms may extend farther east/southeast into the remainder of the Great Lakes plus Ohio Valley/Northeast into the weekend, though possibly becoming somewhat more scattered/disorganized in nature. Farther south a broad surface trough and associated energy aloft will likely track westward across the Gulf of Mexico Wed-Fri, increasing the potential for scattered to numerous showers and thunderstorms across central/western portions of the Gulf Coast. Meanwhile the Southwest/southern Rockies seasonal monsoon should begin to ramp up as the central U.S. ridge aloft intensifies late in the week with convection becoming heavier and more widespread across the Four Corners states. The most prominent area of above normal temperatures during the period will spread from the northern Rockies/High Plains into the upper half of the Mississippi Valley between midweek and early next weekend. Some of this heat may extend into the Great Lakes/Ohio Valley/Mid-Atlantic next weekend depending on uncertain details at the surface and aloft. Anticipate highs up to 5-10F above normal with some areas across the northern tier seeing morning lows 10-15F above normal on one or more days. A majority of the western U.S. will likely see near to below normal highs for most of the period due to the upper trough/leading surface front reaching the Northwest and clouds/rainfall over the Four Corners states. ...Guidance/Predictability Assessment... As in recent cycles there is good agreement for the overall pattern but detail discrepancies persist. GFS runs continue to be on the deep/slow side with the Great Lakes through southeast Canada wave and supporting dynamics mid-late week, and keep the Northeast mean trough aloft on the amplified side of the spread into the weekend via upstream energy. As a result the surface front on the southern periphery of the upper trough generally extends farther south than in most other guidance. Even among non-GFS solutions there is still a fair amount of spread for exact strength/timing. The new 12Z GFS is still deep with the leading system but at least by next weekend it is a bit weaker with portions of the Northeast upper trough and is quicker to erode surface high pressure that reaches the Mid-Atlantic/southern New England--closer in concept to other guidance. Confidence is fairly low with specifics of surface waves/frontal position from the Northern Plains into Great Lakes during the late week-weekend time frame due to low predictability of shortwaves ejecting from the western U.S. This favors a blend approach until better agreement emerges for one or more defined features. For the upper trough settling over the Pacific Northwest, consensus shows leading energy reaching the area by Thu and then a compact Alaskan upper low dropping into British Columbia by Fri. Some spread develops for progression of this feature into the weekend but there is decent agreement that the Pacific Northwest trough should weaken somewhat on Sun as the upper low continues onward over western Canada and pushes a cold front into the northern High Plains. The early part of the forecast emphasized operational model guidance but with GFS input split between the 06Z GEFS mean and operational run due to the lower confidence evolution of the Great Lakes/Northeast system. Uncertainty over upstream shortwaves led to a gradual increase of total ensemble weight to 35/55/70 percent for days 5/6/7 respectively. This solution maintained the consensus large scale pattern evolution and provided reasonable continuity in most respects, though the latest blend did yield some adjustment to frontal position east of the Plains on day 7 Sun. Rausch/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