Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Thu Aug 25 2022 Valid 12Z Sun Aug 28 2022 - 12Z Thu Sep 01 2022 ...Overview... Most guidance over recent days has been advertising a transition from fairly flat mean flow aloft on Sunday to an amplified regime consisting of an eastern Pacific trough, western North America ridge, and eastern Canada/northeastern U.S. trough by next Tuesday-Thursday. There have been some typical embedded differences with specifics of one or more of the large scale features, while current Gulf of Alaska shortwave energy forecast to reach the Northwest as a compact upper low around Saturday (in the short range period) continues to be a particularly troublesome feature within or around the more agreeable mean pattern. The primary areas of focus for rainfall during the period should be with a cold front that pushes from the northern Rockies/Plains into the East/South as well as with diurnally favored showers and storms across the southern tier. The Northwest upper low may support some rainfall as well depending on its evolution/track. Warmest temperature anomalies should be over northern areas east of the Rockies early next week and over the Northwest Tuesday-Thursday. ...Guidance Evaluation/Predictability Assessment... The forecast of the compact upper low reaching the Northwest before the start of the period continues to be one of low confidence/predictability and high sensitivity to early differences in track. Among 12Z/18Z model runs, the potential location of this low ranged between northern California (12Z ECMWF) and the northern Rockies (18Z GFS and 12Z UKMET/CMC). From there guidance ultimately had it drifting into the Rockies or Plains. Enough ensemble members were on the slow side for the ensemble means to suggest the slow ECMWF scenario. Highlighting the uncertainty of the forecast, the new 00Z GFS has changed to a progressive northern tier track along the southern fringe of the westerlies (more like what consensus was doing multiple days ago). The 00Z UKMET/CMC drop the feature into the central High Plains by Tuesday. The ECMWF has trended well east of its 12Z run closer to the 00Z UKMET/CMC, lowering the probability of the western side of the spread. Another aspect of the forecast with low predictability due to fairly small scale and weak strength is an area of shortwave energy over the central U.S. as of Sunday and possibly weak upstream impulses. Thus far, agreement is poor with respect to what proportion progresses eastward versus lingering over the Plains. The most notable large scale issue in the 12Z/18Z guidance involved the GFS closing off an upper low within the Pacific trough by days 6-7 Wednesday-Thursday, ultimately leading to a flattening of the Canadian upper ridge seen in other models and all three of the ensemble means. The new 00Z GFS has trended much closer to consensus for the Pacific trough but still strays a bit for downstream details by day 7 Thursday. Outside of the questionable 12Z/18Z GFS, the differences for how quickly the leading edge of the Pacific trough reaches the Northwest are within typical guidance spread/error ranges. Based on 12Z/18Z guidance, the updated forecast emphasized operational runs for about the first half of the period along with some editing to reflect a more true compromise among the model runs for the western upper low. Then the forecast transitioned toward a half models/half means blend by the end of the period to balance detail uncertainty within the more confident large scale pattern. ...Weather/Hazards Highlights... The brief period of more progressive flow aloft over the West through the weekend should promote a lighter and more scattered trend for monsoon rainfall 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. The upper low emerging from the Northwest could produce some areas of rainfall between the Rockies and Plains but again with low confidence on details at this time. 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. A few places could challenge daily records for highs/warm lows. There is still some uncertainty due to a potential embedded shortwave/low but latest trends are taking this feature farther east. 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. The southern tier will see near to slightly below normal highs through the period, though California may trend a little above normal by midweek. Rausch 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