Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Fri Apr 28 2023 Valid 12Z Mon May 01 2023 - 12Z Fri May 05 2023 ...Northeast heavy rain threat this weekend lingers into Monday as a deep upper low settles over/east of the Great Lakes... ...Much cooler than average temperatures in the east-central U.S., much above average in the Intermountain West/Rockies... ...Overview and Guidance/Predictability Assessment... The forecast for the CONUS at the start of the medium-range period (12Z May 1) begins with a developing omega block pattern shown across the model guidance. An upper-level low will be deepening over the Great Lakes as a second upper-level low deepens to the west along the West Coast, with a sharp ridge over the Intermountain West in between. Despite the complex pattern, both the deterministic and ensemble guidance is in good agreement on not only the overall pattern but the location and phasing of the upper-level low/ridge/low. The deterministic GFS and ECMWF also both show good run-to-run consistency. Only more subtle differences in the strength of each low are apparent. Both the deterministic and ensemble guidance remain in generally good agreement into the middle of the forecast period, with the eastern upper-level low drifting to the north/east and the western upper-level low drifting south along the coast, and an even greater, negatively-tilted amplification of the ridge in between. Expected smaller-scale differences in embedded shortwave energy as well as the strength of the upper-level lows does becomes a bit more apparent. The one more notable change is less run-to-run consistency in both the deterministic GFS and ECMWF, with prior runs showing the movement of the upper-level low becoming more nebulous overall and disagreement on the amount of shortwave energy. Predictability decreases quite a bit in the mid- to late forecast period as the evolution of both upper-level lows differs significantly between the guidance. In the east, most recent runs of the deterministic and ensemble guidance show the upper-level low starting to move faster towards the northeast with a separate piece of energy lingering over the Great Lakes. The details in the strength and location of both the low and residual troughing to the west are expectedly less predictable given the increasing lead time. There is an indication of this evolution in the ensemble means. However, the 18Z GFS shifted away from this solution, with a stronger, more compact upper-level low lingering over New England. A blocking ridge looks to remain in place, shifting towards the central CONUS, as an upper-level trough remains in place along the West Coast. However, the guidance also differs quite significantly here in how long a closed-low remains (GFS) or if the low weakens as the trough becomes less amplified (ECMWF). The deterministic GFS also shows another closed low developing just to the west over the eastern Pacific following the weakening of the original closed low. The ECMWF and CMC do not show this development, and the ensemble means are expectedly less detailed and depict overall mean troughing remaining in place. Run-to-run consistency of the GFS and ECMWF are notably low as well, especially with regards to the weakening of the closed low over the West, which is a more recent trend. Given the agreement across the guidance, a general model blend of the deterministic 12Z GFS/ECMWF/CMC/UKMET is used for the early to mid- forecast period, which also remains consistent with the prior WPC forecast blend. Contributions from the 18Z GEFS and 12Z ECens are steadily increased during the mid- to late period as the UKMET forecast ends and noted uncertainty with respect to the deterministic guidance increases. A reliance on the means later in the period captures the general phasing/pattern of the mean troughing/ridging/troughing over the CONUS with more specific details to be resolved later. The 12Z GFS was used over the 18Z GFS given the noted differences in the evolution of the eastern upper-low compared to the agreement in evolution seen between the 12Z GFS/ECMWF/CMC. ...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. Locally heavy rainfall will likely last at least one more day across New England as anomalously high moisture streams northward ahead of a frontal system pushing off the East Coast Monday. A day 4 "Marginal Risk" Excessive Rainfall Outlook has been maintained as much of the rainfall Monday is expected to fall in a relatively short time frame during the morning-early afternoon hours. Light to moderate wrap-around precipitation will linger as far west as the Great Lakes Monday-Tuesday, and some snow may mix in for the U.P. of Michigan Monday. Precipitation amounts overall have come up early to mid-week as the closed-low is forecast to linger longer over the region. High temperatures will also remain well-below average in the Great Lakes region by about 10-20 degrees through at least mid-week 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 eastern U.S., trending further 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 mid-week, with positive anomalies of 15-20 degrees focused on the northern Great Basin/Rockies/High Plains. A lingering frontal system along the Central/Southern Rockies will be the focus for some light shower/storm chances for the Southern/Central High Plains, particularly on Wednesday. Precipitation chances will increase for the West Coast and eventually the northern Rockies/Great Basin mid- to late week as another 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, though the QPF forecast has trended upward. Warmer air in place with the ridge to the east will likely keep most precipitation as rain for all but the higher mountain elevations, with snow chances focused on the Sierra. 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