Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Wed May 10 2023 Valid 12Z Sat May 13 2023 - 12Z Wed May 17 2023 ***Flash flooding a growing concern across portions of Texas this weekend with a Moderate Risk of excessive rainfall on Saturday, and an early season heatwave across the Pacific Northwest*** ...Synoptic Scale Overview... The low pressure system over the Midwest on Saturday, associated with a decaying upper level low, will continue weakening going through the weekend, with a swath of moderate to heavy rain across portions of the Upper Midwest. The trailing cold front from this storm system is expected to slow down considerably across Texas, and in combination with shortwave energy aloft and copious moisture transport from the western Gulf of Mexico, areas of heavy rain are becoming likely across much of central Texas going into the weekend. Meanwhile, the upper ridge continues to build across the northwestern U.S. and into British Columbia and Alberta, with much warmer temperatures becoming widespread. In response to the building ridge will be a downstream trough that develops further from the Great Lakes to the Northeast and Mid-Atlantic region with a return to cooler conditions and lower humidity for the beginning of next week. ...Guidance/Predictability Assessment... The 00Z model guidance suite has a good overall synoptic scale depiction of the upper level and surface pattern for the weekend, with above average confidence for most areas so a multi-deterministic model blend suffices as a starting point in the forecast process. The models agree well on a strong upper ridge building into southwestern Canada, Washington and Oregon, and the upper low over the Dakotas evolving into an open wave that merges with the westerlies. There is also better model consensus on the axis and magnitude of the heavy QPF expected over Texas this weekend, and the GFS has a better signal for a weak upper low over western Texas on Saturday. Looking ahead to Monday, there are some timing differences with the shortwave building across the eastern U.S. and also for the second reinforcing shortwave dropping south from central Canada on Tuesday. To account for the increasing model differences for the beginning of next week, use of the ensemble means increased to about 50% for the end of the forecast period. ...Weather/Hazards Highlights... A high impact heavy rain event is looking increasingly likely across much of central and southern Texas going into the upcoming weekend, in addition to the rain already expected during the short range period. A favorable combination of a stalling front, multiple weak shortwave impulses passing aloft, sustained deep moisture flux from the western Gulf of Mexico, and appreciable low level instability south of the front will set the stage for multiple mesoscale convective systems with training thunderstorms and high rainfall rates. This unsettled pattern going into the Day 4 period Saturday will likely result in 2-5 inches of rainfall with locally higher totals possible across much of central and southern Texas. Since this heavy rainfall on Day 4 will be falling on already highly saturated grounds from the previous days, a Moderate Risk remains in effect across portions of south-central Texas, including the greater San Antonio area. There will likely be some changes to this risk area in future forecast updates once the model signal becomes more refined. There will likely be some additional heavy rain going into Day 5 as well across parts of Texas and Oklahoma, and a Slight Risk is planned from far southern Kansas to the Rio Grande to cover this potential, although the heaviest rainfall should be waning by this time. Farther north across the Upper Midwest, some lingering thunderstorm complexes are a good possibility going into Saturday in association with the weakening upper trough/surface low. Drier weather arrives in time for Mother's Day across much of the north-central U.S., and then more of the central/eastern U.S. gets less coverage of showers and storms going into early next week. The other story that will be making weather headlines will be the early season heatwave centered over the Pacific Northwest starting this weekend and persisting into early 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 easily set daily high temperature records. Overnight lows will also be quite mild with readings only falling into the 60s for many of these same areas. 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. is expected to 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