Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu May 12 2022 Valid 12Z Sun May 15 2022 - 12Z Thu May 19 2022 ...Heat threat for California and the Southwest from the weekend into next week while persistent southern Plains heat intensifies and expands eastward... ...Overview... The latest models and ensemble means continue to suggest that a trough-ridge-trough pattern aloft will make gradual eastward progress with time, such that the mean troughs should reach near the East and West Coasts around Wednesday. Troughing may begin to amplify somewhat over the West thereafter while the downstream ridge broadens as it crosses the Plains/Mississippi Valley. Initial western U.S. and southern Plains ridging followed by rising heights over the Southeast will lead to a broad area of unseasonably hot weather across the southern U.S. This heat will be most persistent over the southern Plains and vicinity while it should peak over western areas during Sunday-Tuesday and extend farther east from the Plains/Lower Mississippi Valley by around Tuesday-Thursday. High temperatures will likely challenge daily records for multiple days at some locations. A series shortwaves/fronts ejecting from the trough eventually arriving into the West will produce cool conditions with periods of rain and perhaps high elevation snow over the Northwest. Meanwhile fronts pushed along by the mean trough crossing the eastern half of the country and stalling back over the Plains will help to focus areas of showers and thunderstorms of varying intensity through the period. ...Guidance Evaluation/Preferences... For the most part, the latest guidance agreed fairly well for the mean pattern but with persistent differences for low-predictability shortwave details within the eastern U.S. trough as well as for shortwaves moving through and ejecting from the Pacific trough eventually pushing into the West. Day-to-day frontal position and thus specifics of convective focus will be sensitive to these uncertain medium to smaller scale aspects of the forecast. From the large scale perspective, the latest CMC runs have offered the greatest discrepancies versus remaining guidance as they are more amplified with the eastern trough and stray to the progressive extreme with western-central U.S. energy Tuesday-Thursday. The new 00Z run is not as extreme as its prior run in the East but still questionable. Meanwhile the 18Z GFS was a bit on the flatter side with the eastern trough, while in the latter half of the period the 18Z GFS was deeper with energy coming into the Northwest around Tuesday but the 12Z run ultimate strayed to the deep side with trailing amplification by next Thursday. Not surprising given some of the operational model differences that develop with the eastern Pacific/western U.S. trough, GEFS/ECens/CMCens member spread becomes noticeably more pronounced by Wednesday-Thursday. Guidance comparisons ultimately led to emphasizing the 12Z ECMWF/UKMET and 12Z/18Z GFS for the updated forecast during the first half of the period and then rapidly increased 18Z GEFS/12Z ECMWF mean input (reaching a total of 60 percent by day 7 Thursday) to emphasize the more agreeable large scale pattern while downplaying the uncertain details. ...Weather/Threat Highlights... Initial southern Plains/western ridging and then eventually rising heights farther east over the South will likely promote broad area of heat across much of the southern U.S. during the period. The greatest anomalies through the period should be over the southern Plains (persisting from the short range time frame) where some locations could see highs up to 15-20F above normal. The southern half or so of the West will tend to see highs up to 10-15F above normal from Sunday into at least the first part of next week followed by a cooling trend that reaches California initially and extends farther eastward by next Thursday as Pacific upper troughing arrives. Meanwhile by Tuesday or Wednesday through Thursday the coverage of highs 10-15F above normal should extend eastward through the Lower Mississippi Valley and into the Southeast. Potential for daily record highs will be most persistent through the period over parts of the southern Plains. Numerous record highs will be possible each day over areas with plus 10F and greater high temperature anomalies. In contrast, the series of surface waves/fronts ahead of the approaching/arriving eastern Pacific upper trough will keep the Pacific Northwest on the cool side with frequent episodes of precipitation. The trough should reach far enough eastward to bring highs down to 10-15F below normal over the Northwest/northern Rockies by next Wednesday-Thursday. Snow levels should be quite high into early next week and then begin to decline as the upper trough arrives. It will likely take into the short range time frame to resolve timing/intensity aspects of each precipitation episode due to the medium to smaller scale of the supporting shortwaves. Areas to the east of the Rockies should generally see two rainfall regimes during the period. From Sunday into Monday the best focus for showers and thunderstorms will be along a cold front pushing through the East. There may be one area of emphasis over and near the south-central Mississippi Valley on Sunday while additional areas of locally moderate to heavy rainfall may occur from the Great Lakes/Ohio Valley into New England. Frontal progression may temper amounts over these latter areas even if some of the rainfall is fairly intense while much of New England has dry soil conditions to mitigate effects of any locally heavy rainfall. Then from later Monday through Thursday, the stalling of the leading front and another one behind it over the Plains along with shortwaves aloft will provide focus for multiple areas of showers and thunderstorms/convective complexes from the northern-central Plains into the central Appalachians or vicinity. Some rainfall may be heavy, with some added time required to refine the details of coverage and timing of highest totals. 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