Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Mon Jul 16 2018 Valid 12Z Thu Jul 19 2018 - 12Z Mon Jul 23 2018 ...Overview... Latest guidance continues to provide above average confidence that the large scale flow pattern aloft will become more amplified during the period. A ridge positioned over the Great Basin/Four Corners as of early Thu will expand and settle over the southern Rockies/Plains by Sat-Mon. Meanwhile multiple pieces of energy will carve out a trough over the eastern half of the country with a possible embedded low center most likely to track into the Great Lakes. Farther upstream a shortwave originating from the Gulf of Alaska will skirt the Pacific Northwest by late in the week, briefly lowering heights across that region before continuing eastward across the Northern Tier Sun onward. A look at hemispheric teleconnections shows support for this pattern evolution across North America, further increasing confidence in the impending pattern change. An anomalous and persistent Rex block across eastern Asia favors upper ridging across the southwestern and south central U.S., with a relatively strong vortex across the Arctic, and an equatorward extension in the form of favored troughing across Hudson Bay and the Great Lakes. Given the persistent nature of the aforementioned Rex block, would expect that the upcoming pattern shift across North America will not be a transient one. ...Guidance Evaluation and Preferences... During the early/middle part of the forecast period a blend of mostly operational guidance represented consensus well and in most cases provided good continuity. The one notable change is that the majority of guidance is showing somewhat more of a signal for a defined wave along the East Coast boundary that starts lifting up as a warm front by late Fri-Sat--and with a slower northward progression. Also of note, operational 00Z-06Z GFS runs were weighted less from the start of the period than would typically be the case with some inclusion of the 06Z GEFS preferred as a substitute. GFS runs were faster than consensus (including the GEFS mean) with timing of the wavy eastern front and also differed from other solutions for some flow details aloft between Hudson Bay and the upper Great Lakes. In addition guidance trends have been gravitating much closer to what ECMWF/CMC-based guidance had advertised a couple days ago. It remains to be seen whether the upper low within the trough will track as far south as depicted in recent parallel FV3 GFS runs though. Elsewhere, the 00Z ECMWF leaned toward the extreme side of the solution envelope for troughing over the Northwest by Fri-Sat but was still within the typical range of error at that time frame. Increasing detail spread/variability among operational runs and agreeable solutions among the GEFS/ECMWF ensemble means recommended trending toward a fairly even model/ensemble mean blend by late in the forecast. Timing/details of energy crossing southern Canada and the northern U.S. will likely provide ongoing uncertainty--with some slower trends evident over the past couple days. The same will be case for specifics of the eastern U.S. trough including the degree of influence from upstream flow. ...Weather Highlights/Threats... The evolving pattern will bring the potential for one or more periods of significant rainfall to locations from the northern Plains/MS Valley through much of the East. This activity will be beneficial for some areas that have received very little rain over recent weeks (pending activity that is possible in the short range time frame) while it may be more problematic over other areas. A cold front likely becoming stationary from the Southeast to the southern/central Plains as of the start of the period will provide one focus for scattered to numerous showers and thunderstorms--some with heavy rainfall--from late in the week onward. This boundary will eventually lift northward as a warm front. Farther north evolving trough energy aloft and the associated surface low pressure system drifting from the northern Plains into the Great Lakes will generate convection with locally heavy rain. The amplifying upper trough over the East may also promote transport of deeper moisture northward across the eastern U.S., with widespread convection/areas of heavy rain possibly reaching portions of the Mid-Atlantic and Northeast by the weekend. Finally, expect monsoonal moisture to result in scattered afternoon/evening convection from the southern Great Basin to the southern/central Rockies through much of the week and into the weekend. Some locally enhanced amounts are possible but activity should generally be lighter and/or less organized than in some recent episodes. The expanding ridge aloft from the Southwest into southern Plains will be accompanied by multiple days of hot conditions from late week onward. The focus for highest max temperature anomalies will likely be over Texas and vicinity with many 100+F highs and some locations reaching 10-15F above normal. Readings may reach or exceed record high/warm low values at a number of locations. The southern 2/3 of the West should see rather warm min temperatures most days but highs should be less extreme relative to normal. Expect some below normal readings over the East at the start of the period (especially morning lows Thu), and below normal highs over areas that see clouds/rainfall from the Midwest into the East during the period as well as the northern Rockies/Plains by early next week. Rausch/Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 http://www.wpc.ncep.noaa.gov/heat_index.shtml