Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon May 01 2023 Valid 12Z Thu May 04 2023 - 12Z Mon May 08 2023 ...Progressively cool and wet pattern for the West Coast/Great Basin/Rockies this week... ...Overview... While a stubborn omega block upper-level pattern early this week will finally start to evolve late this week/weekend, mean troughing over the West will remain in place through the weekend leading to steadily increasing and widespread precipitation chances. Showers and storms also look to expand across the center of the country as increasing moisture and clashing frontal systems lead to daily rounds of convection. Temperatures and conditions should finally start to moderate this weekend over the Great Lakes/Northeast after a cool and wet several days. ...Guidance/Predictability Assessment... Model guidance shows that the stubborn omega block upper-level pattern that has been forecast to continue through at least mid-week begins to evolve by the weekend into early next week, with a lot of complex smaller-scale details that will likely remain harder to pin down, especially at this time range. The pattern begins to shift from the Great Lakes/Northeast upper-low, ridge, West Coast upper-low phasing that has been in place by the start of the current updated forecast period on Thursday. While general mean troughing/ridging/troughing looks to remain in place, especially with respect to the ridge and western trough, the two closed lows are forecast to weaken and shift eastward. The ridge axis is also expected to shift eastward, though the guidance differs on exactly how fast and how far this occurs during the current forecast period. The deterministic guidance shows general agreement in the early to mid-forecast period depicting the closed low over the Great Lakes/Northeast weakening and shifting eastward over the Atlantic, with a wave of upper-level energy shifting southward over the Upper Midwest and retrogressing westward as a closed high develops over Canada. The ridge over the Intermountain West/Plains shifts eastward over the Plains/Mississippi Valley as the closed low over the West Coast begins to weaken and shift northeastward over the Great Basin and eventually towards Canada as an open wave in the mean flow, with ramifications on precipitation coverage over the West during the period. The one notable difference between the deterministic guidance mid-period is greater amplification of the ridge and western trough in the 12Z CMC, and to some extent the 12Z UKMET, compared to both the most recent GFS and ECMWF as well as the ensemble means. The guidance shows mean ridging becoming dominant over the central/eastern CONUS, finally bringing an end to the omega block pattern centered over the CONUS, with mean troughing along the West Coast. A recent trend run-to-run in the ECMWF as well as the more recent 12Z/18Z runs of the GFS, especially the 12Z, is for one particular shortwave to move eastward over the Plains and into the Ohio Valley, breaking down the ridge. The 12Z CMC continues to show more highly amplified mean ridging/troughing without much of a signal for this feature. One last notable detail is a another wave of upper-level energy dropping south in the mean flow from the northeastern Pacific, potentially as a closed low as shown in the 12Z ECMWF and ECens mean, and to a lesser extent weaker but hinted at in the 12Z and 18Z GFS. The pattern remains more amplified in the 12Z CMC during this time frame. The updated 00Z GFS continued decent agreement with the 12Z ECMWF and the ensemble means in handling the potential shortwave over the Plains and the closed low upstream off the Pacific Coast. The updated WPC forecast package begins with a general model blend given overall agreement across the guidance on the large-scale for the early forecast period. Contributions from the 12Z CMC and UKMET are steadily reduced by mid-period as their handling of the evolution of subsequent troughing over the West Coast following the departure of an initial closed low differs from the other deterministic and ensemble mean guidance. Contributions from the 12Z ECens mean and 18Z GEFS mean are increased mid- to late period in replacement of the CMC/UKMET as already cumbersome smaller-scale details in a complex pattern evolution remain hard to resolve. The continued inclusion of the GFS/ECMWF deterministic guidance keeps more of the detail of the one shortwave moving eastward over the Plains which may have impacts on the surface frontal structure/organization in the late period. The resultant blend helps to retain some influence from this feature, which is even indicated by the ECens mean. ...Weather/Hazards Highlights... Precipitation chances will steadily increase from the West Coast through the Great Basin and Northern Rockies mid- to late week as a closed low over the west ejects northeastward over the Great Basin and eventually northward into Canada. The precipitation may be locally moderate to heavy at times, with some scattered thunderstorm chances. Snow levels will begin high but are forecast to fall through the latter part of the week as the upper-low/trough progresses inland and provides cool air aloft. The highest snow chances will be for the Sierra Thursday. 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 Thursday-Friday, but this will likely be driven more by local hydrological conditions harder to anticipate at this range. Moist, upslope flow settling in along portions of the northern/central High Plains also look to lead to some locally moderate to heavy rainfall late this week into the weekend. High temperatures in California and adjacent portions of the Great Basin/Southwest will be running as much as 10-15 degrees below average Thursday-Friday under the influence of the upper-level low. In contrast, anomalously warm temperatures that have been in place over the northern Great Basin/Rockies will again be running 15-20 degrees above average at least one more day on Thursday before the approaching upper-level wave and progressive frontal systems cool temperatures down through the weekend. To the east, 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 mid- to 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 a 'backdoor' cold front sinking southwestward as well as 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 over the Central Plains/Middle Mississippi Valley on Thursday, where a Day 4 "Marginal Risk" Excessive Rainfall Outlook is in place. This convection may trend downstream Friday into the Tennessee Valley, and additional heavy rainfall chances look possible over the weekend more broadly across the central/southeastern U.S., but confidence remains low. The cool/wet pattern this past week/weekend over the Great Lakes and Northeast under the influence of a stubborn upper-level low will finally start to moderate later this week as the low departs over the Atlantic. Periods of light to moderate precipitation should linger Wednesday across the Northeast before lightening Thursday-Friday as high pressure over Canada builds southward. Temperatures will remain below average by around 5-10 degrees Wednesday-Thursday before settling in right around average for early May by the weekend. 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