Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1151 AM EDT Fri Jul 20 2018 Valid 12Z Mon Jul 23 2018 - 12Z Fri Jul 27 2018 16Z Update: The models and ensemble means remain in good overall synoptic scale agreement on the pattern over the continental U.S. through the end of next week. The 12Z CMC indicated a greater degree of retrogression with the upper trough/closed low over the southeast U.S. compared to the model consensus, and also faster with an upper low near Greenland. There are modest differences regarding the evolving upper low over south-central Canada by the end of the week, and some of the ensemble means were incorporated for that portion of the forecast. There has also be a trend in the guidance over the past 24 hours regarding the upper low over the Gulf of Alaska for the middle of next week, and this is also resulting in some westward adjustments in the upper ridge axis over western Canada. A blend of the deterministic GFS/ECMWF/UKMET was used through Wednesday for pressures and fronts, and then some of the EC and GEFS means for Thursday and Friday. The previous discussion is appended below for reference. D. Hamrick __________________________________________ ...Heat Wave to Intensify Over the Southwest States Next Week... ...Heavy Rain / Flash Flooding Expected Over the Eastern U.S. Next Week... ...Overview and Guidance Evaluation/Preferences... It remains the case that the main mid-upper level troughs are of medium to long wavelength and fairly slow to evolve given the season. They are also relatively well defined, with one closed low center expected in the east/southeast next week, eventually replaced by another well defined upstream trough digging into the Great Lakes by Wed-Fri. A main convective storm track remains around a hot Southern Plains/Southwest ridge and into the aforementioned and dynamic/unstable main troughs. The lead mean trough position settled into the eastern/southeastern U.S. also has ample access to a tropical moisture plume that backs in off the Atlantic, setting the stage for a more widespread heavy rainfall threat. Underneath the ridge, the heat wave in the Southern Plains this weekend expands westward across the southwest quarter of the nation next week where some record hot temperatures are expected. In some places like northern Mexico and southern AZ/CA, where monsoon season is more commonly in full swing by now, these heights peak at 3.5 standard deviations above climatology. Standard deviations of +2.0 extend as far north as San Francisco and Salt Lake City. Much of this medium range forecast is of high confidence during this cycle, with strong agreement / clustering among the models and ensembles regarding these well defined and slowly evolving features. Most of the more noticeable model differences are across the very high latitudes, whereas the forecast over the CONUS shows less spread. Accordingly, the WPC medium range forecast suite was mainly derived from a composite blend of the latest GFS/ECMWF, GEFS and ECMWF ensemble means, the NBM, and WPC continuity. ...Weather Highlights/Threats... Significant hazards expected over broad areas of the country the next seven days. Heat stress will be a major story in the West. Forecast highs are in the upper 100s and teens (110-119+) in the desert areas (including Phoenix, Vegas) for several consecutive days. Low temperatures near or even above 80 deg for many areas will amplify the potentially hazardous impacts of the heat. The potential arrival of a cold front across the southern plains by next Tue-Wed could bring some slight relief there, but the heat in the Southwest looks to be a lengthy and potentially dangerous event. Flash flooding and perhaps even longer term, larger scale flooding (e.g., river flooding) will be possible in broad swaths of the eastern states. The closed low dropping through the OH/TN Valleys toward the Gulf Coast anchors a well defined trough, with precipitable water of 2.0 inches or greater forming an axis that overlaps with many mountainous areas in the Appalachians and New England. These areas are prone to flash flooding, and the combination of the synoptic support along with seasonably strong sun angle will likely yield pockets of instability sufficient to support an off and on heavy rainfall threat. Schichtel WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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