Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM 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 the 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 the Lower Mississippi Valley. Warmer than normal temperatures are forecast to spread into the East as the ridge amplifies ahead of the trough in the West, but the heat should finally break by next weekend across parts of the South. Initially the best focus for showers and thunderstorms around midweek will be over the Ohio Valley along a quasi-stationary front, before precipitation spreads into the north-central U.S. around Thursday and eastward late week. Flash flooding and severe weather are both threats with this system. Shower/thunderstorm chances will also increase in Florida late this week given increasing tropical moisture. ...Guidance Evaluation/Preferences... Model guidance remains in good agreement for generally zonal flow with a couple of embedded shortwaves across the lower 48 to start the medium range period Wednesday, before amplification later this week as troughing builds in the West creating ridging downstream. The process for building this trough stems from potent energy initially just offshore of British Columbia spilling into the Northwest late Wednesday into Thursday and progressing eastward. However, some notable differences remain regarding the splitting of this energy into multiple features or more consolidation, as well as variation with an embedded upper low being closed at different points/times. Recent model guidance including some of the 00Z runs and the newer 12Z runs have trended toward showing a closed low around Friday, separate from but eventually interacting with a farther north Hudson Bay upper low. Namely, the 00Z/12Z ECMWF and UKMET indicated a closed low, while the 12Z GFS and CMC trended toward this as well after more open earlier runs. 12Z runs are thus showing better agreement for the surface low position as well in the Upper Midwest day 5/Friday, so hopefully frontal positions and QPF are following suit. The southern end of the trough's progression remains somewhat uncertain as does the eastern side by around Saturday, with the latter issue dependent on interactions with that northern stream low. There has been a gradual trend toward a slower trough and thus frontal progression in the northeastern U.S. compared to previous forecasts. The 00Z CMC appeared to be on the faster side. The updated WPC forecast used a blend of the 00Z/06Z deterministic guidance early in the period, transitioning toward a near even blend of the GFS/ECMWF/GEFS mean/EC mean by the latter part of the period. ...Weather/Threat Highlights... Initial ridging across the southern Plains and 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, which should help moderate temperatures in Texas by the weekend. However, warm to hot temperatures are also forecast for the southeastern quadrant of the U.S. 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 later in the 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 the 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. Then organized rainfall, heavy at times, is likely along a frontal boundary through the north-central U.S. around Thursday with potential for at least localized flood and flash flood threats given wet antecedent conditions across that region. Storms that could be severe are forecast to shift from the Upper Midwest into parts of the Ohio and Mississippi Valleys along this front Thursday into Friday. Additionally, tropical moisture increasing across the Florida peninsula late this 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. Tate/Santorelli 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