Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 AM EDT Wed May 03 2023 Valid 12Z Sat May 06 2023 - 12Z Wed May 10 2023 ...Overview... The upper level pattern is forecast to evolve from the stubborn omega block centered over the CONUS this past week/weekend and towards mean ridging building over the central/eastern CONUS and mean troughing over the central/western CONUS. Conditions will be cool and wet through the weekend in the West under the influence of the upper-level troughing. Ejecting shortwave energy over the central/eastern U.S. will trigger multiple areas of showers and thunderstorms from the Plains to the Mississippi Valley and Southeast, with abundant Gulf moisture available for the potential of locally heavy rainfall. ...Guidance/Predictability Assessment... Most of the guidance shows shortwave energy lingering west of departing upper trough over the East Coast as well as a shortwave lifting north through the northern Great Basin/Rockies in the mean flow to the West will both depart into Canada as a ridge over the east begins to amplify a bit. The 12Z GFS, and to a lesser extent the 18Z GFS, show this energy actually shifting southward and disrupting the amplification of the ridging over the East Coast, which would have a potential impact on precipitation chances in the region. The trend has been for a closed low to eventually develop over Atlantic Canada, with impacts for the weather in New England and the Interior Northeast. The updated 12Z UKMET has fallen in line with the other guidance with respect to the handling of troughing over the West after being a bit out of phase. In the mid-forecast period, all of the deterministic guidance depicts shortwave energy traversing the larger trough ejecting over the Plains and disrupting the ridge to the east. However, run-to-run consistency on the timing and location of this feature remains poor in both the more recent GFS and ECMWF runs. Despite the smaller-scale nature of this feature, it has remained a consistent presence in the forecast for days, again present in the most recent 00Z GFS. This complicates the forecast as there is confidence in this overall scenario, which will have downstream impacts on the surface frontal evolution and shower/storm chances, but the specifics have changed day to day. The handling of the pattern evolution towards the end of the period remains very poor across the guidance. In general, the guidance suggests the trough over the West will shift eastward with time. However, there is little consistency run-to-run in both the ECMWF and GFS in handling the overall phasing of the eastern ridge/western trough progression, let alone more smaller-scale impulses traversing the flow. At least some of the updated runs show some similarity. The 12Z CMC diverged away from the weak consensus between the other guidance with a closed low developing over the Southwest. Adding to low confidence, the 12Z ECens and 18Z GEFS means show a flatter flow, but at least keep the flow southwesterly from the Southwest to the Midwest which would be expected in some fashion with the initial placement and expected movement of the trough. A more detailed review shows that the deterministic guidance tends to have weak ridging over the West Coast/Intermountain West, while the ensemble guidance has flat flow to slight troughing over the region. The updated 00Z GFS was more progressive with the potential for ridging, shifting it over the Plains with a trough to the west, similar to the early runs of the ensemble means. The updated WPC forecast package begins with a general blend of the deterministic guidance. An emphasis was given to the ECMWF over the 12Z/18Z GFS given the differences between the GFS and the other guidance over the East Coast. Reliance on the deterministic guidance continues into the mid-forecast period to keep more detail in the evolution of the shortwave over the Plains. The contribution from the 12Z CMC is removed mid- to late forecast period given the diverging solution over the West. A contribution from the 12Z ECens and 18Z GEFS means was added for the late forecast period given increasing forecast divergence in the deterministic solutions. A flatter flow is a resultant compromise over the West where there is disagreement between the deterministic and ensemble guidance, as well as the deterministic guidance run-to-run, and confidence in any other solution would be very low at this point. ...Weather/Hazards Highlights... The overall pattern looks wet for the central to southeastern CONUS through the period as southwesterly flow sets in, with two areas of potentially greater focus of showers/storms. Moist, upslope flow along portions of the Northern/Central High Plains ahead of a potential better organized surface frontal system passing through the Plains/Upper Midwest look to bring multiple days of rainfall this weekend into early next week. Further South, Gulf moisture will spread northward across the Central/Southern Plains and Middle/Lower Mississippi Valley into the Southeast leading to a broad region of showers and thunderstorms. In both cases, the evolution of the pattern has remained rather nebulous the last few days as the guidance has varied on the presence and timing of embedded shortwave energy aloft, lessening confidence on the specifics of regions of heavier rainfall and potential flash flooding concerns. Currently, the greatest chance of locally heavy rainfall is on Sunday over portions of the Upper Missouri Valley and Sunday/Tuesday for the Lower Mississippi Valley. High temperatures will be seasonably warm for early May from the Plains to the Southeast/Mid-Atlantic under the influence of mean ridging aloft, with some highs running 15+ degrees above average in the Central Plains this weekend. The persistent cool/wet pattern over the Great Lakes and Northeast due to a stubborn upper-level low this past week/weekend will finally moderate this upcoming weekend, with high temperatures returning to near to slightly above average levels into early next week. Temperatures may eventually trend cooler depending on the evolution of a potential upper-level closed low to the northeast. Precipitation chances will remain low through the weekend under the influence of a broad area of high pressure in place. Shower and thunderstorm chances will increase for the Carolinas south into Florida this weekend due to a lingering frontal boundary and potential downstream MCS activity. Moderate to locally heavy rainfall will taper off through the weekend for the Pacific Northwest/Great Basin/Northern Rockies as an upper-level wave lifts north into Canada. Additional shortwave impulses/surface frontal systems pushing through the West will keep some precipitation chances, particularly across the northern tier. Some snow showers will be possible at higher mountain elevations. Temperatures will remain a bit cool more broadly across the West through the weekend, with highs running 5-10 degrees below average through the period, before conditions look to moderate a bit later in the period as the upper-level pattern aloft trends more neutral. 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