Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun May 15 2022 Valid 12Z Wed May 18 2022 - 12Z Sun May 22 2022 ...Persistent record-breaking heat is likely in the south-central U.S. through this week with anomalous heat expanding into the East by next weekend... ...Overview... Upper troughing should shift out of the Northeast as the period begins on Wednesday, with broad ridging to zonal flow behind across the rest of the lower 48. The pattern is expected to amplify later in the week as upper troughing digs in the West, yielding cooler temperatures and lowering snow levels in a round of precipitation spreading across the Northwest. Stubborn ridging across the South will continue to promote unseasonably hot weather, with the most persistent and record-breaking heat centered in Texas and Louisiana. Warmer than normal temperatures are forecast to spread eastward as the ridge amplifies ahead of the trough in the West, and the heat should finally break by next weekend across the South. The best focus for showers and thunderstorms should be along a quasi-stationary boundary from the central Plains into the Ohio Valley, while some rounds of rain are also possible in the north-central U.S. and shower/thunderstorm chances will increase in Florida late next week. ...Guidance Evaluation/Preferences... Recent model guidance has come into better agreement early in the period with ejecting trough energy in the Northeast and also a weak shortwave through the northern tier Wednesday-Thursday. After this, models agree the pattern should become more amplified late period, but with varying degrees of seperation between another stronger shortwave across the northern Plains and also energy digging across the Intermountain West. Yesterdays 12z run of the CMC was the greatest outlier showing a faster and more conhesive trough through the West late week, while the ECMWF and GFS (and UKMET before it phases out after day 5) exhibited better seperation of streams. However, the 12z ECMWF was more bullish on digging the southern stream energy farther south into the Southwest, while subsequently buidling a stronger ridge across the East. Todays 00z ECMWF (available after forecast generation time) came in a little less amplified and more in line with the GFS and CMC, but there is still so much uncertainty in the details. WPCs forecast for tonight used a blend of the deterministic models early on trending quickly towards the ensemble means late period which resulted in a good middle ground approach. Some deterministic ECMWF and GFS were maintained through day 7 though just for some added system definition to the normally washed out means. ...Weather/Threat Highlights... Initial ridging across the southern Plains and then eventually rising heights farther east over the Deep South will promote a broad area of heat across much of the southern U.S. during the period. The greatest and most lasting anomalies through the period should be over Texas (persisting from the short range time frame) where some locations could see highs up to 15-25F above normal, with numerous daily record highs likely. Parts of the Southwest into the central Great Basin and southern/central Rockies can expect highs around 10F above average on Wednesday before a cooling trend as the deeper troughing presses inland. However, warm to hot temperatures could increase for the southeastern quadrant of the U.S. later next week, with potential for daily record highs, and warmer than average temperatures by 10-20F should expand in coverage across the Ohio Valley to Eastern Seaboard by next week. A warm front lifting through the Mid-Mississippi Valley and Ohio Valley will provide a focus for multiple areas of showers and storms/convective complexes during the middle to latter part of next week. The ingredients are in place for heavy rainfall, though the specifics of coverage and amounts are uncertain, but at least a localized flood threat is likely especially if heavy rain falls in areas that are sensitive to additional rainfall. Organized rainfall, heavy at times, is likely along a frontal boundary through the north-central U.S. later in the week with potential for at least localized flood and flash flood threats given very wet antecedent conditions across that region. Additionally, tropical moisture increasing across the Florida peninsula late next week could lead to a more rainy pattern for the Sunshine state. After a couple of initial fronts crossing the Northwest, the main well-defined front ahead of the deepening trough will bring unseasonably cool temperatures to the Northwest and expanding into the northern Rockies and High Plains by late week. This will also provide support for a notable round of precipitation, with lowering snow levels in the Cascades to Northern Rockies as the week progresses, with some chance of snow even into northern parts of the High Plains, though with very low confidence at this point. Santorelli/Tate 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