188 FXUS02 KWBC 191603 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1202 PM EDT Thu Jul 19 2018 Valid 12Z Sun Jul 22 2018 - 12Z Thu Jul 26 2018 ...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... 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 early next week, eventually replaced by another well defined upstream trough digging into the Great Lakes by Wed/Thu. So the main storm track is up and over the Southern Plains Ridge, and this mean trough in the northern/eastern U.S. has ample access to a tropical moisture plume that backs in off the Atlantic into the eastern quarter of the nation. Meanwhile beneath the ridge, the heat wave in the Southern Plains this weekend then expands westward as the ridge becomes repositioned, with the 594 decameter 500-mb height covering a very broad expanse by Tuesday. 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 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. We therefore began with a simple GFS/ECMWF blend, but began to work in more of the GEFS and ECMWF Ensemble Mean. The operational 00z and 06z GFS were the first to be dropped from the blend as they were quite aggressive in digging the closed low southwestward along the Gulf Coast D4-5. The operational ECMWF saw more ensemble support in this regard, and perhaps the GFS solutions is being driven more by convective feedback onto the synoptics. By Days 5-7 we were using roughly a 40/40/20 blend of GEFS mean / EC Mean / Operational ECMWF. ...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 100 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. Burke/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 $$