Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Sun Apr 30 2023 Valid 12Z Wed May 03 2023 - 12Z Sun May 07 2023 ...Much cooler than average temperatures in the east-central U.S. and much above average temperatures in the Intermountain West/Rockies may gradually moderate... ...Progressively cool and wet pattern for the West Coast/Great Basin/Rockies this week... ...Overview... An omega block upper-level pattern will be in place this week during the medium range period, as upper lows are centered atop the California coast and the Lower Great Lakes at the start of period Wednesday, and similar areas reload with additional energy into at least next weekend. Amplified ridging should be located in between the two over the Intermountain West/Plains, with an upper high likely eventually breaking off into central Canada. Cool and wet conditions are likely under the influence of the lows, while warmer than average temperatures are forecast for the Great Basin and across the northern/central parts of the Rockies and Plains. Both could gradually moderate closer to normal by next weekend. ...Guidance/Predictability Assessment... Updated model guidance continues to show that the stubborn upper-level omega block pattern centered over the CONUS looks to remain in place through the coming week likely into the weekend. Both the deterministic and ensemble guidance show a closed low roughly centered between the Great Lakes/Northeast, a highly amplified ridge over the eastern Intermountain West/central Plains that continues into western Canada, and a second closed, cutoff low along the California coast at the start of the forecast period (Wed May 1). The deterministic GFS/ECMWF show good run-to-run consistency in maintaining this pattern/general placement. Most of the deterministic and ensemble guidance also seems to be in generally good agreement on the evolution of these three main features through the early forecast period. The eastern low moves to the northeast while mean troughing remains over the East Coast and the western cutoff low moves to the northeast over the West, weakening as an open wave at some point, as mean troughing also remains in place in the eastern Pacific. The handling of more important details becomes more cumbersome and complex by the middle of the forecast period. One notable region of disagreement is with the evolution of shortwave energy dropping southward from Canada in the mean flow of the eastern trough, and gradually shifting westward around a building upper high over Canada. Models are at least more agreeable now that this energy (possibly becoming a small closed low) should disrupt the mean ridge a bit and undercut an upper high developing in central Canada, but not completely eliminate the high like some previous models. But timing/placement differences remain with this feature. Another area of disagreement is with the evolution of the pattern upstream of the western cutoff low. All the guidance shows another surge of energy/shortwave dropping southward in the mean flow. The 00Z CMC seemed to be an outlier in dropping more potent energy farther south across the eastern Pacific compared to consensus by Friday and beyond, so this was not favored. Interactions between these troughs lower confidence in the details of the pattern through next weekend. The updated WPC forecast package begins with a blend of the deterministic 00Z ECMWF, 00Z/06Z GFS, and 00Z UKMET, excluding the CMC because of its issues developing in the West. The GEFS and EC ensemble means were introduced and increased through the mid- to late forecast period amid increasing spread and uncertainty. ...Weather/Hazards Highlights... The cool/wet pattern over the Great Lakes and Northeast early this week will continue through midweek under the influence of a stubborn upper-level low/trough pattern. 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 cool and below average across the region Wednesday-Thursday under the influence of the closed low, with some high temperatures over the Lower Great Lakes/Upper Ohio Valley as much as 10-20 degrees below average. Temperatures should moderate somewhat by the weekend as the closed low departs to the northeast. Upstream, a strong upper ridge will slowly progress from the Interior West into the Rockies and eventually the 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 on Wednesday. The Pacific moisture along with additional Gulf moisture flowing northward clashing with a cold front pushing southward look to increase shower and thunderstorm chances more broadly across Central/Southern Plains and Middle/Lower Mississippi Valley into the Southeast mid- to late week. While the specifics remain somewhat uncertain, confidence in placement and timing of convection causing heavy rainfall is tending to increase through Thursday/Day 5. A Marginal Risk has been maintained in the Excessive Rainfall Outlook for that time, with some broadening to the west and south based on model trends and the placement of the best instability gradient. There may also be a nonzero threat of flash flooding with convection farther south into Texas on Thursday, but with less model agreement there at this point. Precipitation chances will increase for the West Coast and the northern Rockies/Great Basin mid- to late week as the western upper-level low drops southward along the coast, moves inland, and becomes an open wave. Again, the presence and timing of embedded upper-level shortwaves and the amount of moisture directed inland from the Pacific will dictate specifics on amounts, but rounds of precipitation may be widespread at times. Around midweek, precipitation is likely to be focused over California given onshore flow ahead of a low in place off the coast. Coastal regions of California may see some moderate to locally heavy rainfall rates which, in conjunction with the ongoing high streamflows from winter snowmelt, may lead to a low-end flooding concern. A Marginal Risk remains in place for coastal California for Wednesday when there could be a bit of instability for some moderate rainfall rates. The Pacific Northwest and the Intermountain West to Rockies should see expansive precipitation chances into late week. Snow levels may begin relatively 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. Higher elevations of the Sierra Nevada should see snow while peaks farther north and east may also see snow late week. There could be isolated issues for flooding in mid-range elevations in the foothills of the Sierra and extending north into the Pacific Northwest and Northern Great Basin around Thursday-Friday, but this remains questionable. 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 10-20 degrees below average. 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