Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue May 02 2023 Valid 12Z Fri May 05 2023 - 12Z Tue May 09 2023 ...Cool/wet pattern for the Pacific Northwest/Great Basin/Rockies into the weekend with localized flooding concerns... ...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 in the West under the influence of the upper-level troughing, with heavy rainfall on top of mountain snow melt leading to localized flooding concerns in the Great Basin/Northern Rockies. Moisture sourced from the Gulf and Pacific will bring the potential for multiple regions of locally heavy rain to the Plains/Mississippi Valley through the weekend and into next week depending on the evolution of shortwave energy aloft. ...Guidance/Predictability Assessment... Updated model guidance shows a transition in the upper-level pattern away from the stubborn omega block centered over the CONUS to mean ridging in the central-east CONUS and mean troughing over the central-west CONUS late this week and into the weekend. An initial wave of energy in the mean flow will be ejecting northward over the Great Basin/Northern Rockies into Canada as additional shortwave energy rounds the western trough, with the ridge building to the east. The building ridge replaces lingering troughing over the Northeast following the departure of a stubborn closed low over the region. The 12Z/18Z GFS and 12Z CMC/ECMWF are in generally good agreement on the large scale details through the early forecast period. The 12Z UKMET diverges from the other solutions rather quickly in the early to mid-forecast period with a deeper closed-low/trough lingering off the California coast, a bit out of phase with the other guidance. In addition, the 12Z UKMET is also more aggressive with shortwave energy rounding the trough over California and the Great Basin/Southwest. More significant differences appear rather quickly during the mid-forecast period. The first concern is the handling of a potential shortwave disrupting the ridge over the central/eastern CONUS. The 12Z ECMWF as well as the 12Z ECens mean are the most aggressive with this feature, which is apparent but to a much lesser extent in the 12Z/18Z GFS and 12Z CMC, and not present in the 18Z GEFS mean. The degree to which this disrupts the ridge in the late period is also a source of disagreement in the deterministic guidance, which makes the ensemble means likely more reliable in capturing the overall pattern at this point. This feature will have downstream impacts on the organization/progression of frontal systems over the Plains/Mississippi Valley. The handling of this feature is rather poor run-to-run in both the GFS and ECMWF so too much emphasis on any one solution would not be expected at this point. Updated guidance from the 00Z GFS was more aggressive with this shortwave, similar to the 12Z ECMWF, but the timing remains different. There is also significant disagreement with respect to the evolution of the pattern over the West late period between both the deterministic and ensemble guidance. The ECMWF has waffled run-to-run with depicting a closed low moving into the Pacific Northwest. The 18Z GFS is completely out of phase with the 12Z ECMWF, while the 12Z GFS, without the closed low, at least still retains mean troughing/ridging in generally the same phase. The 00Z GFS followed a similar evolution to the 18Z GFS, leading to even less confidence in the pattern late period. The amplification of mean troughing over the West is also lower in the 18Z GEFS mean compared to the 12Z ECens mean and the 12Z ECMWF/GFS/CMC. The updated blend has led to a bit weaker wave/more northerly track of the wave, but retains it compared to the 12Z and especially 18Z GFS, which accounts for trends in the updated guidance while not shifting too far away from the prior forecast package until there is more confidence in how this pattern evolves. The blend in the late forecast period is split between a contribution from the deterministic 12Z CMC/ECMWF/GFS as well as the 12Z ECens mean and 18Z GEFS mean given the noted low confidence in the pattern over the West. The contribution from the 12Z ECMWF was reduced given the emphasis on a closed low in the latest run. The combination of this specific guidance though at least has similar phasing if both small and large-scale details will still need to be resolved. ...Weather/Hazards Highlights... Precipitation chances will remain high and widespread for the Pacific Northwest/Great Basin/Northern Rockies late this week/weekend as an upper-level wave and multiple frontal systems impact the region. Rainfall may be locally heavy at times, with some scattered thunderstorm chances. The rainfall on top of ongoing snow melt may lead to some flooding concerns in terrain sensitive areas around the foothills of local mountain ranges as well as along local streams/rivers Friday, with a broad "Marginal Risk" Excessive Rainfall Outlook in place. Some snow showers will be possible at higher mountain elevations. Temperatures will remain a bit cool more broadly across the West, with highs generally running 5-10 degrees below average through the period. Moist, upslope flow settling in 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 Friday-Sunday. A "Marginal Risk" has been maintained for portions of the Northern/Central Plains Friday where more organized convection may produce some locally heavy rainfall totals. Further South, Gulf moisture flowing northward as well as additional moisture sourced from the Pacific will spread northeastward across the Central/Southern Plains and Middle/Lower Mississippi Valley into the Southeast late week leading to a broad region of showers and thunderstorms. The evolution of the pattern has remained rather nebulous the last few days as the guidance has varied on the timing and evolution of embedded shortwave energy aloft, lessening confidence on the specifics of regions of heavier rainfall and potential flash flooding concerns. However, the guidance has shown the highest confidence in widespread convection and high QPF amounts from the Middle Mississippi to Tennessee Valley Friday, with the potential for training appearing sufficient to support a "Marginal Risk" area. Additional heavy rainfall chances look possible over the weekend more broadly across the central/southeastern U.S., particularly the Lower Mississippi Valley, but confidence remains low given the guidance spread in how the surface pattern and supporting shortwave dynamics will evolve. High temperatures will be seasonably warm for early May from the Plains to the Southeast under the influence of mean ridging aloft, with some highs running 10-15 degrees above average in the Central Plains. 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. Precipitation chances will remain low 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, with amounts and coverage likely dictated by a weak wave aloft and surface frontal boundary influence that remains hard to pin down at this timeframe. 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