Extended Forecast Discussion...amended NWS Weather Prediction Center College Park MD 339 AM EDT Tue May 9 2023 Valid 12Z Fri May 12 2023 - 12Z Tue May 16 2023 ***Flash flooding a growing concern across portions of Texas Friday and into the weekend with a Moderate Risk of excessive rainfall on Saturday, and an early season heatwave across the Pacific Northwest*** ...Synoptic Scale Overview... A slow moving low pressure system situated over the central/northern Plains on Friday, associated with a upper level low, will track across the Midwest states while weakening going into the weekend, with widespread rain across the Dakotas and eastern portions of Wyoming and Montana. 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 end of the week and into 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 CMC and GFS have a better signal for a weak upper low over western Texas on Saturday. Looking ahead to Monday, there are some amplitude differences with the shortwave building across the eastern U.S. and also for the next shortwave dropping south from central Canada on Tuesday where the CMC is strongest with that feature. The 12Z ECMWF was quickest in breaking down the northwestern upper ridge, but the 00Z ECMWF maintains it longer. To account for the increasing model differences by days 6 and 7, 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 Friday and continuing through much of the upcoming weekend. 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. A broad Slight Risk area of excessive rainfall is currently warranted for the Day 4 period across mainly central Texas to the Rio Grande with a strong model signal for 3-6 inch totals during this time period. The unsettled pattern remains unabated going into the Day 5 period Saturday with an additional 3-6 inches likely with locally higher totals possible. Since this heavy rainfall on Day 5 will be falling on already highly saturated grounds from the previous days, a coordinated Moderate Risk was introduced 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. Elsewhere across the continental U.S., widespread light to moderate rain will likely be ongoing across the Dakotas on Friday in association with the upper low crossing that region, with amounts generally in the half to one inch range. This then reaches the Upper Midwest on Saturday and shrinks some in coverage, and then drier weather arrives in time for Mother's Day across much of the north-central U.S. and scattered showers over the northeastern U.S. In terms of temperatures, highs will likely be running 10 to 15 degrees above mid May averages from the Upper Midwest to the Great Lakes and Northeast states to close out the work week, and then trending cooler going into the weekend as an upper-low over Canada builds southward and multiple cold fronts likely pass through the region. Anomalous early season heat will build across the Pacific Northwest by Friday and especially into the weekend with highs reaching 15-25+ degrees above average across much of Washington and Oregon. The potential exists for highs to reach into the 90s for some of the inland valley locations and may challenge some daily record highs. The heat wave will likely persist going into early next week as the upper ridge remains strong. Lows may be a little less extreme but could still challenge a few daily records for warm lows by the weekend. Highs will likely be slightly below average across much of New Mexico, Texas, and Oklahoma owing to widespread rain and cloudy conditions, but overnight lows should be near average. Warm and humid conditions will likely continue for much of the southeastern U.S. going into the weekend. 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