Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Thu May 04 2023 Valid 12Z Sun May 07 2023 - 12Z Thu May 11 2023 ...Overview... The forecast flow pattern over North America next week looks to feature mean ridging over the central-eastern CONUS and mean troughing over the central-western CONUS. Upper-level impulses through southwesterly flow with an influx of Gulf moisture will favor broad and widespread shower/storm chances from the Plains to the Mississippi Valley and Southeast. Conditions will remain cool in the West under the influence of mean troughing, with daily precipitation chances for the northern tier of the region. ...Guidance/Predictability Assessment... Both the deterministic and ensemble guidance is in good agreement with the overall pattern over the CONUS at the start of the forecast period (12Z Sunday May 7), depicting mean longwave ridging over the central-eastern CONUS and mean longwave troughing over the central-western CONUS. Shortwave energy ejecting northeastward from the western trough over the Plains into the Upper Midwest that has been a mainstay in the recent guidance is still apparent, though has trended weaker, with the ridge amplifying a bit more to the east. This feature has and will continue to have downstream impacts on precipitation chances over the northern tier. Another consistent feature in the guidance is the development of a closed low over Atlantic Canada mid-forecast period. Flow over the East gradually becomes flatter/more zonal as the closed-low deepens and flattens out the ridge. The 12Z CMC is more aggressive with shortwave energy both in the southern stream and around the closed low to the north, amplifying troughing along the East Coast late period. To the west, several northern stream impulses dropping south from the northeastern Pacific lead to an energetic and less predictable pattern. One such impulse shifts southward towards the West Coast as a closed low early to mid-forecast period. The 12Z/18Z GFS and 12Z ECMWF show this feature continuing quickly eastward as an open wave in the mean flow. The 12Z CMC is more aggressive than the other guidance, with the closed low deepening over the West Coast mid- to late forecast period, becoming the dominant feature in the region. This solution diverges more fundamentally from the other guidance which retain a more steady pattern of additional impulses. The 12Z ECens and 18Z GEFS means complement their deterministic counterparts in suggesting an active pattern continuing to support mean troughing overall across the West, with more specific details in the timing of individual waves to be resolved over time. This sets up southwesterly flow over a good portion of the CONUS, supportive of broad precipitation chances seen in the QPF forecasts. One important caveat is the run-to-run consistency diminishes rapidly by the end of the period, with the phase of troughing over the west, and potentially into the central/eastern CONUS, out of sync between previous runs and the current runs of the guidance. However, the 00Z GFS closely followed the 18Z GFS/12Z ECMWF, especially with respect to the potential for an amplification of the trough/ridge pattern centered over the Intermountain West/Plains at the end of the forecast period, which increases confidence a bit in the current forecast package. The updated WPC forecast package used a general blend of the deterministic guidance for the early forecast period given good overall agreement on large-scale details. The contribution from the 12Z CMC was removed by the mid-forecast period when its solution begins to diverge from the other guidance noted over both the East and West Coasts. The blend also leans on the 18Z GFS over the 12Z in the mid- to late period as the 12Z is more aggressive on shortwave details hard to predict at this range. A contribution from the ensemble means is introduced mid- to late period to account for areas of lower confidence in finer-scale details, but retention of the 18Z GFS and 12 ECMWF, which generally fall in line with the means, provides a bit of added detail despite noted uncertainty with high run-to-run variability. ...Weather/Hazards Highlights... Southwesterly flow with a steady supply of Gulf moisture over the central/eastern CONUS will favor a stormy and wet pattern next week. Embedded shortwave energy aloft as well as general front progression will dictate the chances for locally heavier rainfall. While no Excessive Rainfall Outlooks have been introduced in the Day 4/5 timeframe, the environment would suggest there will likely eventually be at least Marginal Risk areas, if not higher. Model QPF has been rather nebulous but the focus looks to be on the Mississippi Valley, perhaps extending eastward to the Mid-Atlantic/Carolinas, and then west into the Plains mid- to late week. High temperatures will be running above average broadly from the Plains to the Southeast/Mid-Atlantic under the influence of mean ridging aloft, with some unseasonably warm highs 15+ degrees above average centered on the Central Plains/Middle Mississippi Valley. Seasonable temperatures and low precipitation chances are expected across the Northeast with high pressure in the vicinity through the forecast period. Some temperature fluctuation as well as breezy winds may effect New England dependent on the proximity of an upper-level closed low over Canada and subsequent cold front passage. Energetic upper-level pattern with mean troughing aloft will favor periodic precipitation chances in the West next week, especially for the Pacific Northwest/Great Basin/Northern Rockies. Some snow showers will be possible for higher mountain elevations, but seasonably rising snow levels will keep these chances isolated. Temperatures will remain cool and below average, especially across the West Coast states. Putnam 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