Extended Forecast Discussion NWS Weather Prediction Center College Park MD 444 PM 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 will slow down across Texas for a while, and in combination with shortwave energy aloft and copious moisture transport from the western Gulf of Mexico, should help to focus areas of heavy rain across much of central Texas into the weekend. Meanwhile, a 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 while persisting over western Canada. At the same time an upper low may retrograde across the Great Basin and Pacific Northwest, with some degree of mean ridging possibly building over the West in its wake. This pattern will promote much above normal temperatures over the Northwest and to a lesser extreme over other parts of the West. Downstream mean troughing that develops in response to the western Canada ridge will tend to favor cooler conditions and lower humidity over the East during the first half of next week. ...Guidance/Predictability Assessment... Comparing guidance from the 00Z/06Z cycle, the most prominent discrepancies involve the GFS runs in a couple respects. Continuing a trait that became more pronounced from around yesterday's 12Z run, the model is still slower than most other guidance (including the GEFS mean) with the southeastward progression of shortwave energy from the Great Lakes through Mid-Atlantic from late weekend into early next week. This leads the GFS to maintain a farther north surface wave/front over the East into Monday versus recent majority cluster trends that have been toward a more suppressed depiction. This conflict between the GFS and other guidance persists into the 12Z cycle. Then by the latter half of the period the GFS significantly breaks down the western Canada ridge in contrast to the GEFS mean and most other guidance holding onto a fairly strong ridge through day 7 Wednesday. The 06Z GFS was more extreme/quicker with its breakdown versus the 00Z run. The 12Z GFS still breaks down the Canadian ridge by day 7, and while the 12Z CMC brings a compact feature into western Canada it still maintains more ridging than the GFS by the end of the period. From day 4 Sunday onward, various low predictability small-medium scale differences aloft develop over the central U.S. with an influence on QPF details within an overall trend for lower coverage/amounts next week after the heavy rainfall expected over the southern Plains during the weekend. Forecast considerations led to starting the latest update with an operational model blend for the first half of the period but with slightly less than typical GFS weight. Then the blend replaced the GFS with the GEFS mean and overall steadily increased GEFS/ECMWF/CMC mean input while phasing out the operational 00Z CMC by day 7 Wednesday due to developing differences along the U.S.-Canadian border. This yielded a 75 percent means to 25 percent 00Z ECMWF weight by day 7. ...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. Expect some additional heavy rain going into Day 5 as well across parts of Texas and Oklahoma, and a Slight Risk is maintained from far southern Kansas to the Rio Grande to cover this potential, although the heaviest rainfall should be waning by this time. Guidance offers mixed messages for heavy rainfall potential over surrounding areas including the High Plains, due to differences in upper level details that have fairly low predictability several days out in 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. Some lighter rainfall may extend farther east/southeast as the weakening wave and leading front drop southward. Drier weather will arrive in time for Mother's Day across much of the north-central U.S., and coverage of showers and storms should decrease over central/eastern areas going into early next week. Meanwhile large scale easterly flow aloft to the south of the southwestern Canada ridge should bring 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 impulses, could produce scattered areas of rainfall. Most activity should be on the light side but localized higher totals could be possible given the moisture anomalies for the time of year. 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 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. While the best potential for highs up to 20-25F above normal will be Saturday-Monday (with some dependence on the upper low that could track over the region), low temperatures over the Northwest and some nearby areas could remain warm enough to challenge 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. Rausch/Hamrick Hazards: - Heavy rain across portions of the central and southern Plains, Sat-Sun, May 13-May 14. - Heavy rain across portions of the southern Plains, Sat-Tue, May 13-May 16. - Flooding possible across portions of the upper Mississippi Valley, the northern Plains, the northern Rockies, and the northern Great Basin. - Flooding occurring or imminent across portions of the mid- and upper Mississippi Valley, the northern Plains, the northern Rockies, and interior Oregon. - Flooding likely across portions of the northern and central Great Basin, the northern Plains, the central Rockies, the Great Lakes, and the Southwest. - Much above normal temperatures across the western portion of the Pacific Northwest, Sat-Mon, May 13-May 15. - Much above normal temperatures across the interior valleys of California, and the interior portion of the Pacific Northwest, Sat-Wed, May 13-May 17. 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