Extended Forecast Discussion NWS Weather Prediction Center College Park MD 455 PM 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 warmth 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 lower 48. 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 over the Southeast, 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... Model guidance remains consistent with the overall pattern during the medium range period, which begins Saturday with a closed upper low in south-central Canada and the opening low/trough centered to its southeast, while ridging builds into the interior West and southern Plains. These features all show good agreement in the 00Z/06Z and newer 12Z models. Somewhat more notable differences arise as troughing develops across the east-central U.S. by Monday from combining features. GFS runs remain flattest/less deep with the northern stream part of the trough while CMC runs are deeper and slower than consensus and almost every individual ensemble member (though the newer 12Z run perhaps in better alignment than the 00Z through Monday, but beyond Monday the 12Z CMC closes off an upper low over the Northeast unlike other guidance). Preferred a middle ground solution like the 00Z ECMWF along with the UKMET for the depth/extent of the eastern trough on Monday, and the ECMWF and GEFS and EC ensemble means Tuesday and Wednesday. This blend of model guidance (00Z/06Z deterministic model composite led by the EC early on, phasing in the GEFS/EC means and phasing out the CMC by day 5) also worked well for ridging building west of the eastern trough, as well as troughing upstream of the ridging. An upper low offshore of British Columbia could send several shortwaves through the West with overall low amplitude troughing coming in, but with differences in the low predictability details. The 00Z CMC once again appeared too amplified compared to consensus. ...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 into the Lower Mississippi Valley and Southeast 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 disagreements in shortwave/surface front details make it difficult to resolve any defined areas of relatively greater potential. At this point signals point to enhancement across parts of the Lower Mississippi Valley/Mid-South on Saturday, which could cause localized flooding especially over areas of training convection and/or wet soils. 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. Tate/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 Hazards: - Flooding possible across portions of the Upper Mississippi Valley and the Northern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Upper Mississippi Valley and the Northern Plains. - Excessive heat across portions of California and the Southwest, Sun-Tue, May 15-May 17. - Excessive heat across portions of the Central Plains and the Southern Plains, Sat-Wed, May 14-May 18. - Much above normal temperatures across portions of the Lower Great Lakes, and the Northeast, Sat, May 14. 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