Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed May 11 2022 Valid 12Z Sat May 14 2022 - 12Z Wed May 18 2022 ...Developing heat threat for California and the Southwest from the weekend into next week while persistent southern Plains heat intensifies and expands eastward... ...Record heat to continue over parts of New England into Saturday... ...Overview... A rapidly opening upper low over the East on Saturday (spreading some rain across the region) will provide the last vestige of the current blocky pattern over and near the contiguous U.S. Guidance is consistent in showing the upstream flow settling into a trough-ridge-trough configuration soon thereafter, with the eastern Pacific trough axis likely pushing into the West around the middle of next week as the eastern trough nears the East Coast. The combination of upper ridging that crosses the West, persistence of a mean ridge over the southern Plains, and with time rising heights to the east, should lead to an expanding area of unseasonably hot weather across the southern U.S. This heat will challenge daily record highs for multiple days at some locations. New England may see record heat extend into Saturday as well. A series of fronts pushed along by the developing central-eastern U.S. cyclonic flow and extending back into the Plains will produce episodes of showers and thunderstorms through the period. At the same time the mean trough eventually reaching the West Coast should send multiple waves/fronts into western North America, supporting periods of rain and perhaps very high elevation snow over the Northwest. ...Guidance Evaluation/Preferences... 12Z/18Z models and ensemble means (serving as the basis for the updated forecast) along with the new 00Z runs continue to agree for the overall pattern evolution but exhibit some spread/variability with details. Among the 12Z/18Z solutions the most notable issue involved the GFS, especially the 18Z run, straying faster and deeper than most other guidance with the shortwave reaching the Northwest around Sunday and the larger scale trough pushing into the West Tuesday-Wednesday. This in turn led to the 18Z GFS becoming faster with the eastern trough. The 12Z GFS was closer to consensus during the latter half of the period in particular. The 12Z CMC became slower/more amplified with the Pacific trough late in the period and has adjusted that feature favorably in the 00Z run. However the new CMC has become a lot more amplified with the eastern trough, which has been its tendency in a number of earlier runs. Multi-day trends have generally been toward slightly less amplitude with this trough but consecutive model runs are still oscillating, causing shifts in southward extent of the surface front reaching the East. Based on the guidance comparisons through 18Z, the updated forecast started with a blend of operational models early (GFS split between 12Z/18Z runs) followed by some incorporation of the 18Z GEFS/12Z ECMWF means along with the 12Z GFS/ECMWF/CMC (but low enough CMC weight to downplay its Pacific trough late in the period). ...Weather/Threat Highlights... The combination of initial southern Plains ridging plus the upper ridge crossing the West and reaching the Plains, along with eventually rising heights over the South will likely promote an expanding area of heat across much of the southern U.S. during the period. The primary focus of this heat during the weekend will be over the southern Plains (persisting from the short range time frame) and over the southern half or so of the West with highs 10-15F or so above normal. By Monday-Wednesday the gradual progression of the upper pattern should lead to slow intensification of the heat over the southern Plains as highs could reach up to 15-20F above normal, while plus 10-15F anomalies spread eastward across the South. The upper trough nearing the West Coast may offer slight cooling over California after Sunday and eventually trim the coverage of plus 10F anomalies farther inland. Potential for daily record highs will be most persistent through the period over parts of the southern Plains. Record highs are most likely over southern parts of the West Sunday into early next week and should steadily spread east of the southern Plains during the first half of next week. On the other hand, surface waves/fronts ahead of the approaching eastern Pacific upper trough will tend to keep the Pacific Northwest on the cool side with precipitation of varying intensity. Snow levels should be quite high into early next week and then may decline somewhat as the upper trough begins to move inland. Individual supporting shortwaves have low predictability so it will take additional time to resolve details of when/where precipitation could be locally heavier. Moisture ahead of a couple fronts could bring lingering moderate to heavy showers and thunderstorms to the Mississippi Valley region early in the weekend, with a consolidated front becoming more progressive as upper troughing heads toward and into the East. Some rainfall along the front could still be fairly intense but the greater progression may temper maximum totals over the East. Another trailing front could also support some showers/thunderstorms, while the trailing part of each front should stall over the Plains and could focus additional convection. Currently there are at least vague signals for locally enhanced rainfall potential within a broad area between the eastern Plains and Ohio Valley/central Appalachians south to the Lower Mississippi Valley. However shortwave/surface front details are not sufficiently agreeable to resolve any defined areas of relatively greater potential. Near the East Coast, record heat should continue into Saturday over New England. The rapidly opening upper low lifting through the East will produce some areas of rainfall farther south over the Mid-Atlantic and vicinity. Rausch 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