Extended Forecast Discussion NWS Weather Prediction Center College Park MD 351 PM EDT Wed May 12 2021 Valid 12Z Sat May 15 2021 - 12Z Wed May 19 2021 ...Heavy rainfall possible over parts of the Southern Plains into the Mississippi Valley early to middle of next week... 1830 UTC Update... Not much has changed with the new 00z/06z guidance. The 00z ECMWF has backed off a bit on such strong ridging over the East the second half of the period, which brings it a little more in line with other pieces of guidance. With that, there were really only minor changes made to the previous forecast and so the thoughts and details outlined in the discussion below still stand. The WPC progs for this cycle favored a pure deterministic blend (ECMWF/GFS/CMC/UKMET) for days 3-5, with a majority blend towards the ensemble means by day 7. Minority weighting of the ECMWF was carried throughout the entire period just to lend some deterministic details to the forecast. Santorelli Previous discussion below issued at 0700 UTC... ...Overview... Thanks to recent trends in GFS runs toward the ongoing majority consensus, today's guidance shows better agreement with the large scale evolution during the period. Within a mean trough aloft forecast to be established over the West, a leading trough/upper low progressing from California through the Southwest or Great Basin and then ejecting into the Plains should be followed by an upstream trough that should reach the West by next Tuesday. Meanwhile the combination of an initial Rockies/Plains ridge and a faster moving southern Canada/northern tier U.S. ridge will likely build into the East toward the middle of next week. The mean evolution aloft along with a surface pattern that should promote a multi-day feed of Gulf moisture will favor the best potential for heavy rainfall over portions of the Plains and Mississippi Valley. Weak shortwave energy filtering through the generally rising heights aloft over the East may extend some areas of meaningful rainfall into parts of the East. Also expect some precipitation over portions of the Rockies while the second Pacific trough should bring some rain and high elevation snow into the Northwest. Double-digit temperature anomalies should be mostly of the warm variety and are most likely from the Northwest into the northern High Plains. ...Guidance Evaluation and Preferences... Over the past day the GFS has made significant progress in adjusting its Pacific through North America pattern faster toward other guidance. So instead of developing a Great Lakes closed low from northern tier/southern Canada shortwave energy, the GFS is weaker and waits until New England and the Canadian Maritimes to show amplification--a more plausible scenario with latest CMC runs and GEFS/ECMWF means also suggesting this possibility. However there are still issues with this energy and possibly other weak low-predictability shortwaves over the East, creating a lot of latitude spread for rainfall/convective systems that may extend eastward from the Midwest. Preference is for an intermediate solution, between the more amplified GFS/GEFS and least amplified (with strongest ridging) 12Z ECMWF. Farther west, guidance is still waffling a bit with specifics of the trough/upper low initially over California but the consensus timing appears to be fairly consistent through at least Monday followed by typical divergence with some dependence on upstream flow. As for the second western trough, a blend approach represents the mean pattern well while downplaying the more extreme details such as the 12Z ECMWF developing more southeastward amplitude than most other guidance by next Wednesday. Late the period the new 00Z ECMWF compares better to the guidance average in both respects. Based on guidance comparisons the starting blend incorporated a composite of operational guidance for the weekend and then trended toward 40-70 percent total input of the 18Z GEFS/12Z ECMWF means by days 6-7 Tuesday-Wednesday as confidence in various model specifics declined. ...Weather Highlights/Hazards... Early in the period expect weak shortwave energy and a wavy front to focus one area of rainfall over parts of the Central Plains and Midwest while improving Gulf inflow will begin to increase rainfall over the Southern Plains. Some of the activity over the Midwest could eventually reach into the Ohio Valley/eastern states as the focusing front and shortwave energy extend farther eastward, though currently there is some uncertainty over the precise latitude of this axis. Confidence is somewhat greater for a multi-day threat of heavy rainfall over the Southern Plains and parts of the Mississippi Valley through early-mid week as the low level flow of Gulf moisture should persist and the initial western U.S. upper system emerges into the Plains. Parts of this area have seen much above normal precipitation over the past several weeks, which may increase flooding concerns. Across the Rockies/High Plains, there may be some enhancement of precipitation due to potential for some easterly upslope flow along with the system emerging from the West. In most cases any precipitation to the west of the Rockies should be fairly light and scattered. Activity reaching the Northwest late in the period may be somewhat better organized but should still be on the lighter side of the spectrum. Higher elevations may see some snow. The upper trough about to depart from the East Coast may produce areas of mostly light mid-late day rainfall from the northern Mid-Atlantic into the Northeast during the weekend. One or more days with highs of 10-15F or so above normal will extend from the Northwest U.S. into the northern High Plains with an upper ridge moving over the area and warm flow ahead of a frontal system moving into the West. In contrast the system crossing southern parts of the West should bring moderately below normal highs to areas from the far southwestern U.S. into southern High Plains. The Northwest should see highs drop to below normal levels by next Tuesday-Wednesday behind the late-period front. Meanwhile moderately cool temperatures over the eastern half of the country during the weekend (though up to minus 10F or so anomalies for morning lows over the southern Mid-Atlantic/Southeast on Saturday) will moderate to yield near-normal highs and somewhat above normal lows Monday-Wednesday. Rausch 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 Hazards: - Heavy rain across portions of the Central Plains, the Great Lakes, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley, Sat-Mon, May 15-May 17. - Heavy rain across portions of the Southern Plains, Sat-Sun, May 15-May 16 and Tue-Wed, May 18-May 19. - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Mon-Wed, May 17-May 19. - Flooding possible across portions of the Northern Great Basin. - Flooding occurring or imminent across portions of the Lower Mississippi Valley and the Southern Plains. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml