Extended Forecast Discussion NWS Weather Prediction Center College Park MD 226 AM EDT Wed Aug 24 2022 Valid 12Z Sat Aug 27 2022 - 12Z Wed Aug 31 2022 ...Overview... A rather nondiscript and slowly progressive multi-stream upper flow pattern laden with numerous embedded impulses over the lower 48 is expected to consolidate and significantly amplify after the weekend in response to the sharp digging of an amplified Gulf of Alaska upper trough. The amplified flow will then highlight a ridge over western North America and a trough over eastern portions of the continent. ...Guidance Evaluation/Predictability Assessment... The GFS/ECMWF/UKMET/Canadian models offer only broadly similar mid-larger scale pattern evolution over much of the medium range forecast period as greater than normal system differences and run to run continuity issues remain prevalent. These are mainly associated with the depiction of a slew of upper flow embedded shortwave impulses and associated surface system reflections. Accordingly, the WPC product suite was primarily derived from a composite blend of the more steady and compatible 18 UTC GEFS/12 UTC ECMWF ensemble means along with the 01 UTC National Blend of Models (NBM). WPC continuity is well maintained in the manner. Model and ensemble main QPF patterns seem fairly organized for August and in sync, so manually targeted changes to NBM QPF was confined to the expansion of light monsoonal rainfall coverage from the Southwest into the Rockies given lingering moisture from the recent wet pattern and secondly to accelerate frontal convection over the central U.S. next week more in line with the slightly more progressive 12 UTC ECMWF/ECMWF ensembles given recent trends, driving height falls/density current potential and overall guidance convective propagation bias. ...Weather/Hazards Highlights... The brief transition to more progressive flow aloft over the West during the weekend should promote a lighter and more scattered trend for monsoon rainfall over the Southwest/Four Corners states. Some of the western U.S. deep moisture that is pushed eastward may contribute to the expanding area of rainfall forecast to spread across the Plains and eventually into the Mississippi Valley/Great Lakes. A leading warm front followed by a surface trough and cold front, as well as northern tier upper dynamics, will all provide varying degrees of focus for what could be multiple areas of locally moderate to heavy rainfall within the broad moisture shield. Current forecasts show one area of rainfall focus around Oklahoma/Missouri/Arkansas where the front may be less progressive than the northern part of the front and as enhanced Gulf moisture returns inland across the lower Mississippi Valley. Showers/thunderstorms will likely continue over the South and Florida Peninsula through the period but with any areas of enhanced totals likely to be more localized and uncertain on a day-to-day basis, including some model variability with whether the bulk of rainfall will be offshore or onshore in any given period. A weakening front reaching the Gulf/Southeast coasts may still provide some focus late this week into the weekend. A front over the East this weekend may produce areas of rainfall with varying intensity. While seasonal or even moderated late summer temperatures will still feel warm for many, the greatest threat of anomalous heat in this pattern may emerge early-mid next week over the interior Northwest with expected flow pattern amplification set to build an upper ridge over the region leading to maximum temperatures upwards to 10-15 degrees above normal values possible. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml