Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Fri Apr 28 2023 Valid 12Z Mon May 01 2023 - 12Z Fri May 05 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 normal temperatures are forecast for the Great Basin across the northern/central parts of the Rockies and Plains. ...Guidance/Predictability Assessment... Most of the recent model guidance remains in good agreement with the slow-moving amplified pattern at least for the early part of the forecast. The main outlier observed at the beginning of the timeframe Monday was the 00Z CMC with the West Coast situation. The CMC indicated two disparate upper lows forming from the energy streaming southward into the West Coast, unlike other guidance and the CMC ensemble mean. The new 12Z CMC coming in with one upper low increases confidence that the 00Z was an outlier. Thus the updated WPC forecast utilized a blend of the 00Z ECMWF and UKMET and 00Z/06Z GFS runs for early in the period. This model blend also served well with the ridge to the east and the upper low even farther east over the Great Lakes, with the blending process serving to smooth out smaller-scale differences. Surface lows also look to meander over/near the Great Lakes through around midweek before shifting east. The 06Z GFS in particular held the surface low farther west through Wednesday compared to consensus, which is why GFS components were split between the 00Z/06Z cycles for this model blend. The incoming 12Z cycle overall seems to show better agreement with this surface low position. Through mid- to late week, predictability decreases quite a bit as the evolution of both upper-level lows differs significantly between the guidance. In the East, there is a general consensus for the upper low to shift from the Great Lakes to the Northeast. But then guidance diverges with the potency of additional energy diving southeastward from north-central Canada and its evolution/interaction with the original upper low. Several ECMWF runs have been most persistent in showing strong energy coming in and pushing the first low out into the Atlantic while it takes hold atop the Great Lakes once again (potentially with some component of initial energy combining with it. The latest 12Z model runs have trended more like this, including the GFS, unlike its previous runs that held on to the upper low over the Northeast. Meanwhile in the West, models show some differences with bringing energy and the closed low eastward into the Southwest and beyond, which could interact with another closed low forming in the eastern Pacific in some uncertain way. This affects precipitation distribution among other sensible weather elements across the western to central U.S. and will take additional time to get resolved. Given these differences, the WPC forecast introduced and gradually increased GEFS and EC ensemble mean components into the blend to about half through the later part of the period, which captured the general phasing/pattern of the mean troughing/ridging/troughing over the CONUS with more specific details to be resolved later. ...Weather/Hazards Highlights... The cool/wet pattern over the Great Lakes and Northeast this weekend will continue into next week under the influence of a stubborn upper-level closed low. Heavy rain is now expected to mainly have exited New England by the beginning of the Day 4 period 12Z Monday, so the Marginal Risk of excessive rainfall from the previous forecast cycle was able to be dropped. But multiple rounds of light to moderate wraparound precipitation will linger as far west as the Great Lakes through the first half of the week, including some accumulating snow for the U.P. of Michigan Monday. High temperatures will also remain well below average in the Great Lakes region by about 10-20 degrees through at least midweek under the influence of the closed low. The presence and timing of embedded upper-level shortwaves in the flow as the broader upper-level low evolves look to lead to some additional precipitation chances late week for the central and eastern U.S., trending farther southward in the Midwest and Southeast, but specifics remain very uncertain at this time. 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 midweek, 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, bringing some shower/storm chances to the Southern/Central High Plains, particularly on Tuesday-Wednesday. 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. 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. 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. Tate/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