Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Thu May 11 2023 Valid 12Z Sun May 14 2023 - 12Z Thu May 18 2023 ...Flash flooding potential across Texas to decrease gradually from Sunday onward... ...Early season heat wave to affect the Pacific Northwest... ...Overview... An anomalously strong upper ridge/high over southwestern Canada as of Sunday should ultimately weaken somewhat and open up but fairly strong ridging will likely persist over western Canada through most of next week. Meanwhile an upper low retrograding from the Great Basin into the Pacific Northwest may settle over/shear out near the Canadian border as it becomes embedded within the larger scale ridge evolving over western North America. This pattern will result in an early season heat wave across the Northwest and to a lesser extent over other parts of the West. The western Canada ridge will promote mean troughing aloft over eastern North America with individual shortwaves supporting cold fronts that drop into the northern/eastern parts of the country, with variable temperatures averaging close to normal. Some rainfall may linger over the southern Plains and Rockies for a number of days from Sunday onward but in a more scattered/less intense fashion than expected in the short range time frame (through Saturday). ...Guidance/Predictability Assessment... Guidance generally agrees well for the overall pattern evolution but there are still the typical embedded detail differences. GFS runs through the 12Z cycle continue to be on the flat side of guidance with shortwave energy forecast to drop through the Great Lakes and Mid-Atlantic early in the week, causing the leading wavy front pushing into the southeast quadrant of the lower 48 to be slower/north versus consensus. The next front reaching the Northeast by Tuesday has exhibited decent agreement/continuity recently, though latest guidance has trended toward a stronger wave developing along the front somewhere over/offshore New England or the Canadian Maritimes late Tuesday-Wednesday. Another front may reach the northern tier by day 7 Thursday as shortwave energy drops through central Canada. Among the 00Z/06Z solutions, the 06Z GFS is most amplified with this shortwave and farthest south with the front, favoring minimal influence of its specifics in the forecast by day 7. The 12Z GFS is less amplified but an eastern extreme aloft. Farther west, the 00Z/06Z GFS are a little deeper and slower than the majority cluster for the Great Basin/Northwest upper low (12Z GFS trending favorably). Latest GFS runs have finally reverted back to consensus that maintains a fairly strong western Canada ridge through the end of the period, after having a day or so of runs trying to break it down. Now the 12Z CMC is the suspicious model, bringing a northeast Pacific upper trough/low into the southwestern Canada mean ridge position. Finally, there are still some subtle but important detail differences in flow aloft over the southern Plains/Rockies with low predictability several days out in time. These differences affect the distribution of QPF, with the GFS runs tending to be somewhat drier than most other guidance over the southern half of the High Plains. A 00Z/06Z operational model composite with slightly less than typical GFS weight accounted for the array of guidance during the first half of the period. Increasing detail spread favored incorporating the 06Z GEFS/00Z ECMWF means along with the models by days 6-7 Wednesday-Thursday, with the means reaching half total weight by the end of the period. ...Weather/Hazards Highlights... Although the heaviest rainfall is likely to be prior to Sunday morning, still expect some lingering areas of moderate to heavy rain going into Day 4 Sunday across central parts of Texas and into Oklahoma. This rainfall combined with wet conditions from rain over previous days support having the Day 4 Excessive Rainfall Outlook maintain a Slight Risk area from portions of the Rio Grande to the vicinity of the southwestern corner of Oklahoma, with the afternoon issuance reflecting a slight northwestward expansion based on latest model/ensemble guidance. Guidance continues to show very 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, favoring persistence of a Marginal Risk across those regions. For Day 5 Tuesday the afternoon update plans to introduce a Slight Risk area over southern parts of central Texas, given sufficient guidance signals for locally heavy rainfall over very wet ground. This is within a broader southern Plains Marginal Risk area that encompasses the relative majority of still diverse guidance for where heaviest pockets of rainfall may be. Meanwhile large scale easterly flow aloft to the south of the initial southwestern Canada ridge should advect some of the central U.S. 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 rainfall with typical diurnal enhancement. Most amounts should be light to moderate but guidance is showing some pockets of anomalously high moisture for the time of year (at least 2-4 standard deviations above average), so there could be potential for heavier activity on a localized basis. 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 generate scattered showers and storms from Texas to the Southeast states. The cold front dropping into the Northeast by Tuesday and another one reaching the northern tier around Thursday may produce scattered areas of rainfall. Another story making weather headlines will be the early season heat wave centered over the Pacific Northwest starting this weekend and persisting into next week as large scale upper ridging prevails over western North America (though with a weakening embedded upper low settling over the region). 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 would be 20-25F above normal and set daily high temperature records. Morning lows will also be quite warm with readings only falling into the 60s for many of these same areas. In contrast to the highs that may moderate below potential record levels after Monday, morning lows could keep challenging daily records for most of next week--with a few monthly records possible as well. 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, heat and humidity over 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 with a day or so of moderately below normal highs. Cloud cover and rainfall will likely keep highs 5-15F below normal over the southern Plains for multiple days, with very gradual moderation over the course of next week. Rausch/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