Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Thu May 11 2023 Valid 12Z Sun May 14 2023 - 12Z Thu May 18 2023 ***Flash flooding potential starts to abate Sunday across Texas, and an early season heatwave affects the Pacific Northwest*** ...Synoptic Scale Overview... An anomalous building northwestern U.S./southwestern Canada upper ridge will likely close off a high over southwestern Canada by Sunday and then possibly open up again to a ridge axis while persisting over western Canada through at least the middle of next week. At the same time a weak upper low may retrograde across the Great Basin and Pacific Northwest, with some degree of upper ridging possibly building over the Western U.S. in its wake. This pattern will result in an early season heatwave across the Northwest and to a lesser extent over other parts of the West. A synoptic scale upper trough extending from Hudson Bay Canada to the East Coast develops in response to the western Canada ridge with multiple shortwave passages, and this will tend to favor cooler conditions and lower humidity over the East during the first half of next week. ...Guidance/Predictability Assessment... The 00Z model guidance agrees well on most aspects of the synoptic scale pattern going into Sunday and Monday, with the GFS only slightly slower with the embedded upper low across the Intermountain West, so a multi-deterministic model blend works well for a starting point in the forecast process. Looking ahead to Tuesday, the GFS weakens the western upper ridge a bit more than the other guidance, but it is otherwise close to the consensus across the eastern U.S. with the amplifying trough. By the end of the period Thursday, the ECMWF becomes stronger with an upper low dropping south across central U.S., the CMC considerably weaker with the western U.S. upper ridge and loses ensemble support, and the GFS is a little slower with the arrival of the next shortwave trough reaching the East Coast. Taking these factors into account, the forecast blend was primarily derived from about half of the ECMWF/GFS and half of the ensemble means for Wednesday and Thursday. ...Weather/Hazards Highlights... Although the heaviest rainfall is likely to be prior to Sunday morning, there will likely be some lingering areas of moderate to heavy rain going into Day 4 Sunday across central parts of Texas and into Oklahoma, and a Slight Risk is planned from the OK/TX border to the Rio Grande to cover this potential. The areal extent of the Slight Risk is smaller now given a more confined signal in the guidance, and even though the rainfall will be tapering off, the ground will likely be quite saturated from rainfall in the Day 3 period. Guidance offers mixed signals for heavy rainfall potential over surrounding areas including the western High Plains and into Oklahoma/Kansas, owing to differences in upper level details that have fairly low predictability several days out in time. Meanwhile large scale easterly flow aloft to the south of the southwestern Canada ridge should advect increasing moisture across portions of the West during the period. The upper low forecast to track through the Great Basin and Pacific Northwest during the weekend and early next week, as well as other potential smaller scale impulses, could produce scattered areas of mainly light to moderate showers. Going into the middle of the week, a drier weather pattern is likely from the central/northern Plains to the East Coast, but there will still be enough moisture near a weakening cold front over the Gulf Coast region to generated scattered showers and storms from Texas to the Southeast states. The other story making weather headlines will be the early season heatwave centered over the Pacific Northwest starting this weekend and persisting into next week as the anomalous upper high becomes anchored in place over southern British Columbia. The latest temperature forecast calls for highs reaching the middle 80s to middle 90s for many of the lower elevations in Washington and Oregon, which would set daily high temperature records. Overnight lows will also be quite warm with readings only falling into the 60s for many of these same areas. The warm overnight lows over the Northwest and some nearby areas could also challenge some daily records through at least midweek. It will also be quite warm for the Central Valley of California with highs well into the 90s there as well. Elsewhere across the nation, the heat and humidity across the central and eastern U.S. should decrease some going into early next week as a weak frontal boundary crosses the southern tier states, and a stronger cold front passes through the Great Lakes and Northeast bringing a more refreshing airmass. Cooler weather owing to cloud cover and rainfall will likely be the case across much of the southern Plains with highs running 5-15 degrees below average most days. Hamrick 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