Extended Forecast Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Tue May 09 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 and gradually weaken as the upper system opens up. Expect this system to produce widespread rain across the Dakotas and eastern portions of Wyoming and Montana, with moisture also extending toward the East Coast along a leading front. The cold front extending south from the initial Plains system will likely 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 should develop across much of central Texas going into the weekend. Meanwhile, an upper ridge starting to build over the northwestern U.S. and into British Columbia and Alberta on Friday will continue to strengthen through the weekend and may remain centered over western Canada into the first part of next week. The ridge will promote several days of much above normal temperatures over the Northwest, while ultimately leading to increased downstream upper troughing across eastern North America and a return to cooler conditions with lower humidity for the beginning of next week. ...Guidance/Predictability Assessment... A 00Z/06Z operational model composite provided a good representation of the overall pattern evolution through the first half of the period that covers Friday into the weekend. The building Northwest U.S. into southwestern Canada upper ridge should close off a high over Canada by early Sunday, while the upper low drifting into the northern half of the Plains should open up with some northern stream energy dropping down from Canada possibly interacting with low's remaining energy. There is decent agreement on a weak southern stream impulse/upper low reaching the southern High Plains by early Saturday but a lot of spread develops for one or more upper lows that may retrograde over parts of the West, underneath the upper high closing off to the north. 12Z models add to the uncertainty with some notable changes in specifics relative to the 00Z/06Z runs. Elsewhere typical detail uncertainties by the latter half of the period that goes through next Tuesday favored adding 06Z GEFS/00Z ECMWF mean input to the operational runs. The primary adjustment in the latest guidance clustering was for a farther south progression of a cold front into the northern tier from southern Canada early next week. Also of note, adding the 12Z runs have actually increased the spread for the ultimate surface pattern from the Plains into the East. The 12Z UKMET/CMC/ECMWF have become noticeably more suppressed with the Plains/Midwest surface wave versus the prior cycle, while the 12Z GFS differences aloft (slower to develop troughing over the Ohio Valley/Mid-Atlantic) lead to a much slower progression than other guidance, including the 00Z/06Z GFS or latest GEFS means. ...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 remains in place for the Day 4 period (12Z Friday-12Z Saturday) 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 (12Z Saturday-12Z Sunday) 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, in the previous issuance and the latest update plans to maintain this area. There will likely be some changes to location/coverage of this risk area in future forecast updates as guidance refines important details. Elsewhere across the continental U.S., widespread rain of varying intensity will likely be ongoing across the Dakotas on Friday in association with the upper low tracking over or near that region. The associated surface wave and front extending to its east should then focus an area of rain from the Midwest into portions of the East, but with less confidence on coverage and totals. Meanwhile, one or more upper lows/shortwaves tracking underneath the southwestern Canada upper high may help to produce scattered areas of rainfall over the southern three-fourths of the West during the weekend into next week. The most extreme heat relative to normal will be over the Pacific Northwest, with Saturday through Monday having the best potential to see highs reaching 15-25F above normal. Such anomalies may yield highs reaching into the 90s for some inland valley locations and could challenge daily record highs. Record warm lows are also likely during this event and could extend into Tuesday when daytime highs could start to moderate slightly over western Washington and Oregon. On the other hand, widespread rain and cloudy conditions will promote multiple days of below normal highs over the southern High Plains with some locations seeing highs 10-20F below normal--especially during Saturday-Monday. Morning lows will be near average though. Much of the eastern half of the country will see temperatures 5-15F above normal during Friday-Saturday but then a cooler trend will push in from the west and north, confining most of the above normal readings to the Southeast thereafter, perhaps aside from a rebound over the Midwest and vicinity ahead of a front dropping south from Canada early next week. Rausch/Hamrick Hazards: - Heavy rain across portions of the Northern Plains, Fri, May 12. - Heavy rain across portions of the Central and Southern Plains, Fri-Sun, May 12-May 14. - Flooding possible across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding occurring or imminent across portions of the Upper Mississippi Valley, the Middle Mississippi Valley, the Northern Plains, the Northern Rockies, and the Northern Great Basin. - Flooding likely across portions of the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, the Northern Great Basin, the Great Lakes, and the Southwest. - High winds across portions of the Central and Northern High Plains, Fri, May 12. - Much above normal temperatures across portions of the Pacific Northwest and California, Sat-Mon, May 13-May 15. 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