Extended Forecast Discussion NWS Weather Prediction Center College Park MD 407 PM EDT Thu Aug 25 2022 Valid 12Z Sun Aug 28 2022 - 12Z Thu Sep 01 2022 ...Overview... Most guidance continues to suggest a transition from fairly flat mean flow aloft on Sunday to an amplified regime consisting of an eastern Pacific trough, western North American ridge, and eastern Canada/northeastern U.S. trough by next Tuesday-Thursday. The details during this transition are a little less clear, with a shift towards a more progressive, in phase shortwave trough moving through the western CONUS before the noted further amplification to the east later in the period. The primary areas of focus for rainfall during the period will be with a cold front that pushes from the northern Rockies/Plains into the East/South Monday into Tuesday as well as with diurnally favored showers and storms along the Gulf. Warmest temperature anomalies are forecast over the Northwest Tuesday-Thursday as the ridge over the west amplifies. ...Guidance Evaluation/Predictability Assessment... The forecast of the shortwave trough reaching the Northwest at the start of the period continues to be one of lower confidence/predictability. The 00Z/06Z model guidance was much more progressive with a more compact/in phase shortwave trough moving from the Northwest and over the Northern Plains by Monday compared to previous forecasts. The mean guidance was a bit deeper and further west over the Pacific Coast than the deterministic models, but all of the 00Z/06Z guidance was in agreement with the more progressive movement. Fortunately the newer 12Z guidance maintained this faster trend. A closed cutoff low developed in the 00Z GEFS mean over northern California compared to the other guidance, and thus the initial WPC forecast for Sunday/Monday featured a general model blend of the deterministic guidance and ECMWF mean. Most of the forecast guidance shows an amplification of the pattern later into the period, with a trough deepening over eastern Canada/northeastern U.S. and a ridge amplifying over the West. Small scale differences in a potentially deeper, embedded trough axis over the central U.S. indicated most significantly in the deterministic CMC led to a trend away from the CMC and further emphasis on the ensemble means later in the period. The 06Z GFS was a notable outlier during the forecast period, with a closed, cutoff low developing over the south central U.S. towards the end of the period, so it was not included in the WPC forecast blend, favoring the deterministic 00Z GFS/ECMWF/CMC and ECMWF mean/GEFS mean. However, while the ECMWF continues to show more broad amplification of the ridge over the west/central U.S., the newer 12Z CMC and GFS have a similar solution to the 06Z GFS, and this trend will need to be monitored. Finally, there continued to be some indication of a subtle shortwave moving through the West at the end of the period indicated by the 00Z GFS/GEFS mean. Given the trend away from this solution from the prior 12Z guidance and the lack of a shortwave indicated by the ECMWF/CMC, opted to stick towards a solution without this scenario for now. A flattening of the ridge would have an impact on the notable warm temperature anomalies over the Northwest late in the period. ...Weather/Hazards Highlights... The brief period of more progressive flow aloft over the West through the weekend will lead to much lighter and isolated monsoon showers over the Southwest/Four Corners states into the Intermountain West. Some of the western U.S. deep moisture that is pushed eastward may contribute to the area of rainfall forecast to spread across portions of the central/eastern U.S. ahead of a High Plains surface trough and cold front progressing southeastward from the northern tier. There may be multiple areas of locally moderate to heavy rainfall within the broad moisture shield but with lower than average confidence in specifics. One potential area of focus could be over and just east of the central/south-central Plains where the cold front could interact with some Gulf moisture on Monday. A Slight Risk was considered for the experimental Day 5 ERO, but there was enough of a spatial difference between the heaviest totals amongst the guidance to hold off for now. Farther east, showers/thunderstorms will likely continue over the South and Florida Peninsula through the period but with any areas of enhanced totals likely to be fairly 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. The warmest temperature anomalies during the period should be over parts of the Northwest during Tuesday-Thursday as mean ridging aloft builds over the West, with some locations 10-15F above normal, and a shift towards the Northern High Plains by Thursday. A few places could challenge daily records for highs/warm lows. There is still a little uncertainty due to a potential embedded shortwave/low but the latest guidance has trended away from this solution. Expect highs of 5-10F above normal from the Northern Plains into the Northeast early in the week followed by a cooler trend after frontal passage. California may also trend a little above normal by midweek. Putnam/Rausch - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Mon-Tue, Aug 29-Aug 30. - Flooding occurring or imminent across portions of the Lower Mississippi Valley and the Southern Plains. 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