Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sat Apr 29 2023 Valid 12Z Tue May 02 2023 - 12Z Sat May 06 2023 ...Much cooler than average temperatures in the east-central U.S., much above average in the Intermountain West/Rockies... ...Overview... An omega block upper-level pattern will be in place next week during the medium range period, as upper lows (both around 3 standard deviations below normal for 500mb heights) take hold across the West Coast and the Great Lakes, with amplified ridging in between over the Intermountain West. Cool and wet conditions are likely under the influence of the lows, while warmer than average temperatures are forecast for the Great Basin across the northern/central parts of the Rockies and Plains. ...Guidance/Predictability Assessment... The model guidance continues to show a highly amplified omega block upper-level pattern over the CONUS already in place at the start of the current forecast and continuing through the full medium-range period. The pattern is relatively consistent in a larger sense, with an upper-level low over the Great Lakes/Northeast, an upper-level low along the West Coast, and a ridge shifting from the Intermountain West towards the western Plains and eventually amplifying northward into Canada with time. The initial evolution of the pattern is also consistent across the deterministic and ensemble guidance in the early to mid-forecast period with the upper-level low initially centered more over the Great Lakes weakening and shifting towards the East Coast and a second surge of energy shifting southward as troughing develops in the flow to the west over the Upper-Midwest. Upstream, the upper-low over the West Coast weakens and moves northeastward inland over the Southwest/Great Basin as an open wave. The 12Z CMC came back into agreement on the evolution of the upper-low over the West after showing energy coming in from the north splitting into two separate lows. The more notable differences arise with the 18Z GFS holding on to a stronger upper-level low over the Northeast and notably weaker troughing extending back towards the Upper Midwest. Updated guidance at 00Z showed the GFS had trended back in line with the 12Z GFS and the other guidance with respect to the developing troughing west over the Upper Midwest. Divergence in the deterministic guidance notably increases in the mid- to late forecast period as the 12Z CMC is a bit more aggressive with another closed low developing over the Great Lakes associated with the initial embedded energy over the Upper Midwest, and more amplified troughing developing towards the Central/Southern Plains in the late forecast period. The open wave shifting over the Intermountain West also weakens much quicker compared to the other guidance, with mean ridging shifting back west towards the Intermountain West compared to the other guidance. The 12Z GFS shows a more detailed breakdown of the upper wave moving over the Intermountain West with lingering energy shifting eastward over the Plains, and embedded details overall amongst the deterministic guidance become quite cumbersome. The 12Z ECMWF showed the best agreement between the location and strength of the larger-scale features to the ensemble means with fewer smaller-scale differences. The updated WPC forecast began with a general blend of the deterministic guidance given the agreement on the overall details between the models, including the 12Z CMC. A contribution from both the 12Z and 18Z GFS is included given the previously noted periods of agreement and difference with the other guidance that even out through the forecast period. The contribution from the 12Z CMC is reduced mid-period as the solution diverges more notably from the other deterministic and ensemble guidance, and the 12Z UKMET is reduced as well given the limited forecast range, with an introduction of the 12Z ECens and 18Z GEFS means in replacement. The means are steadily increased towards the end of the period as the small-scale differences notably become more nebulous. The contributions from the deterministic 12/18Z GFS are reduced faster than the 12Z ECMWF given the noted differences in the two runs throughout the period and the better agreement with the ECMWF to the ensemble means. ...Weather/Hazards Highlights... The cool/wet pattern over the Great Lakes and Northeast this weekend will continue through next week under the influence of a stubborn upper-level closed low. Periods of light to moderate precipitation should continue through at least Wednesday. Chances should trend downward late week and into the first part of the weekend as high pressure over Canada builds southward. High temperatures will remain cool and below average across the region under the influence of the closed low, with some temperatures over the Great Lakes/Upper Ohio Valley as much as 15-20 degrees below average Tuesday and Wednesday. Upstream, a strong upper ridge will progress from California and the Interior West into the Rockies and High Plains. Much above average high temperatures should continue through at least Thursday, with positive anomalies of 15-20 degrees focused on the northern Great Basin/Rockies/High Plains. A monsoon-like pattern appears to develop underneath the upper ridge as well with moisture sourced from the Pacific flowing into the Southern Plains, bringing some shower/storm chances to the Southern/Central High Plains, particularly on Tuesday. The Pacific moisture along with additional Gulf moisture flowing northward clashing with a frontal system pushing southward look to increase shower and thunderstorm chances broadly from the Mississippi Valley to the East Coast later next week. Specifics have remained a bit nebulous and uncertain in terms of location and amounts, but the trend has been towards the potential of heavier and widespread rainfall. Precipitation chances will increase for the West Coast and the northern Rockies/Great Basin throughout the week as the western upper-level low drops southward along the coast and then moves inland as an open wave. Again, the presence and timing of embedded upper-level shortwaves and the amount of moisture directed inland from the Pacific will likely dictate specifics on amounts, but rounds of precipitation may be widespread at times. Confidence is high enough that onshore flow ahead of a low in place off the California coast will focus at least one of these rounds on southern California Wednesday, with a Day 5 "Marginal" Excessive Rainfall Outlook introduced. As in the East, the trend has been for the potential of more expansive and heavier rainfall across the region later next week here as well. Warmer air in place with the ridge to the east should keep most precipitation as rain for all but the higher mountain elevations, with snow chances focused on the Sierra. In contrast to other locations in the West, high temperatures in California and portions of the southern Great Basin/Southwest will be running as much as 10-15 degrees below average. 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