Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1202 PM EDT Mon Aug 12 2019 Valid 12Z Thu Aug 15 2019 - 12Z Mon Aug 19 2019 ...Overview and Guidance/Predictability Assessment... Upper pattern over the Lower 48 will be guided by strong ridging over southwestern Alaska and over the Desert Southwest, which favors troughing over western Canada into the Pacific Northwest. In the east, weak/modest troughing to start the period will transition to weak ridging in response to the digging western trough. For several cycles, the models/ensembles have struggled to agree on how the high latitude flow from Alaska and far northwestern Canada will evolve with quite disparate solutions seen. The cacophony of opinions in the 00Z guidance mostly was divided between the ECMWF and its ensemble members vs most everything else (GFS/Canadian and their ensembles) with respect to timing/track of the flow out of Alaska. However, trends in the preceding days favors something along the line of the 00Z ECMWF/GFS with a nearly closed low moving through Oregon Friday (06Z GFS and 00Z UKMET appeared much too weak and the 00Z Canadian seemed a bit too quick). From that point onward, only the ECMWF appeared to be the most reasonable solution as the 00Z GFS appeared too quick with the flow out of Alaska (though the ECMWF may be too slow). By next Sun/Mon, opted to rely most on the 00Z ECMWF ensemble mean that, despite its larger-than-typical shifts over the previous few runs, offered a solution that fit the favored pattern of renewed height falls into British Columbia that was at least supported by the Canadian ensembles as well. Confidence was lower than normal in the Pacific Northwest but otherwise near to above average over the central and eastern states, respectively. ...Weather Highlights/Threats... A warm front over the central/high plains may act as a focus out of the short range for locally heavy rainfall as a shortwave drops southeastward Thursday. This front will slowly lift northeastward and eventually dissipate as a baroclinic zone remains over much of the Northwest, enhanced as the shortwave or weak upper low move through on Friday. This will increase shower activity over WA/OR into ID/MT with over an inch possible in favored areas. Elsewhere, a lingering stationary boundary in the Southeast will provide broad-scale lift in a juicy environment for mostly afternoon-based showers/storms that may yield some locally heavy totals. Models may be suffering from an overzealous output of precipitation but overall setup will nonetheless be favorable for at least some rain for most areas. Temperatures in the Southwest and southern Plains will be hot and near record highs (and warm lows with many areas only in the upper 70s/80s) Thu/Fri. This may expand into the Ohio Valley/Great Lakes region next weekend as heights rise in the east. Heat indices will again rise in the low 100s for many areas in the South and over 110 in the Southwest as monsoonal rainfall will be virtually non-existent this weekend into Monday. Fracasso 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