Extended Forecast Discussion NWS Weather Prediction Center College Park MD 422 PM EDT Wed May 17 2023 Valid 12Z Sat May 20 2023 - 12Z Wed May 24 2023 ...Warmer than average temperatures will persist across the Northwest through this weekend... ...Overview... Strong upper ridging across the West will continue to cause warm to hot temperatures there through the weekend, before an upper trough finally shifts into the Northwest next week and shifts warmer than normal temperatures into the north-central CONUS. A monsoonal pattern will be in place across the West as anomalously high moisture underneath the ridge promotes scattered showers and thunderstorms. Farther east, a cold front will push a round of rain through the East this weekend, and then additional frontal systems may spark showers and thunderstorms across Florida in particular through much of next week. Temperatures will be near to above normal over most of the Lower 48 except for portions of Texas where daily rain chances are highest. ...Guidance/Predictability Assessment... 12Z guidance shows reasonable clustering in the day 5-6 period as northeastern Pacific troughing moves into the Pac NW and Great Lakes-Northeast troughing relaxes. Prior to this, the ensembles were uncharacteristically in noticeable disagreement on the speed/depth of a Great Lakes small upper low on Saturday -- the GFS remained quickest while the ECMWF and some runs of the Canadian/UKMET were slower. The GFS/GEFS has had a multi-day trend of a slower progression while the ECMWF has been rather steadfast, and again relied on the slower non-GFS consensus to start the forecast. In the West, better agreement overall favored a multi-model consensus. By next Wednesday, models diverge in how to handle any shortwave or upper low over northwestern Canada and how that digs into/reinforces the Pac NW trough, so the preference trended toward an ensemble mean blend as any of the deterministic solutions have been too inconsistent. Thus the WPC forecast was based on a blend favoring the 00Z ECMWF/Canadian days 3-4 before bringing in the GFS/GEFS as uncertainty increased. Trended toward a majority ensemble mean blend by next Wed noting that the shape of the western trough will very likely change. ...Weather/Hazards Highlights... A cold front is forecast to track through the East this weekend and produce widespread rain along and ahead of it, though amounts may be light to modest. A leading system over the Mid-Atlantic may sneak northward into southeastern New England with a robust push of moisture and a chance for some needed but perhaps locally heavy rain on Saturday. Flash flooding is not expected to be a big threat due to the ongoing dryness in the region and progressive nature. The back end of the front stalling near the central Rio Grande Valley could focus moisture and instability there that could produce some heavy rain rates atop moist soils from recent rainfall. A Marginal Risk was maintained for parts of southwestern Texas for this forecast cycle for Day 4/Saturday. Meanwhile, scattered showers and thunderstorms are also possible in the West at times this weekend into next week especially in the afternoons/evenings with a common diurnal cycle, as a monsoon-like pattern sets up with anomalously high moisture remaining over a large portion of the western U.S., first under the upper ridge axis and then as troughing edges into the Northwest. Chances for showers and thunderstorms may increase in the Plains by Tuesday-Wednesday. Meanwhile multiple frontal systems could move through the southeastern U.S. including Florida, which could provide focus for ample moisture and enhance daily diurnally-driven showers and thunderstorms. Coastal areas of the Pacific Northwest can expect temperatures to remain warmer than normal by around 10-15 degrees on Saturday, before moderating to around normal next week as upper troughing and a cold front or two move in. Interior areas like the Great Basin are likely to see above average temperatures by 15-25F underneath the upper ridge axis, with highs in the 90s that could be record-setting on Saturday. Low temperatures in the 50s and 60s could also break records for warm minimum temperatures across widespread areas of the West over the weekend. Then upper troughing coming into the Northwest should shift warmer than average temperatures eastward into the north-central U.S. through Monday-Wednesday, moderating a bit with anomalies around 10-15F above normal. Meanwhile farther south, post-frontal cooler air and periods of rain and clouds could lead to slightly below average temperatures for parts of the Plains and especially West Texas at times through early next week. Fracasso/Tate 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 Southern Plains, Sat, May 20. - Heavy rain across portions of the Southeast, Mon-Wed, May 22-May 24. - Flooding possible across portions of the Central Rockies, the Pacific Northwest, the Northern Rockies, the Northern Great Basin, and the Northern Plains. - Flooding occurring or imminent across portions of the Upper Mississippi Valley, the Middle Mississippi Valley, the Southern Plains, the Northern Rockies, and the Northern Great Basin. - Flooding likely across portions of the Central Rockies, the Central Great Basin, the Northern Plains, the Northern Great Basin, the Southern Rockies, the Upper Mississippi Valley, and the Southwest. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat-Mon, May 20-May 22. - Much above normal temperatures across portions of the Northern Plains and the Northern Rockies, Sat-Tue, May 20-May 23. - Much above normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Mon-Tue, May 22-May 23. - Heavy precipitation across portions of mainland Alaska and the Aleutians, Mon-Tue, May 22-May 23. - Flooding possible across portions of mainland Alaska, Sat-Wed, May 20-May 24. 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