Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Tue Jun 15 2021 Valid 12Z Fri Jun 18 2021 - 12Z Tue Jun 22 2021 ...Record to dangerous heat continuing through the week across much of the Interior West... ...Extreme heat for the Southwest/California through the end of the week... ...Tropical system may bring heavy rainfall to Central Gulf Coast later this week and weekend... ...Overview... The anomalously strong upper ridge over the Four Corners region will continue the dangerous and record heat wave for much of the Southwest to Interior West into the end of the week while on Friday the East will be in the latter part of a brief period of drier/cooler air as an upper trough moves over the western Atlantic. The Four Corners ridge will then steadily weaken and drift southward while mean troughing sets up over the central U.S. As this trough deepens expect low pressure to track through and northeastward from the Plains Sunday onward. This low pressure and trailing front as well as a leading wavy stationary/warm front should produce increasing coverage and intensity of rain and thunderstorms. Meanwhile a potential tropical system could bring heavy rainfall hazards to portions of the central Gulf Coast and nearby areas from Friday through the weekend. The National Hurricane Center is now advertising a high likelihood for increased development and organization of that system over the next few days. ...Guidance/Predictability Evaluation... The full envelope of surface low tracks for the possible tropical system is still broader than desired. The average of GFS/ECMWF runs is still a bit faster than 24-36 hours ago, but the past two CMC runs have been much slower than their predecessors and the UKMET continues to be on the southwestern side of the spread. Over the past day the GEFS mean has gained some definition, suggesting a little improvement in clustering, while its track now more closely mimics the operational run that is somewhat on the eastern side of the envelope. Thus far the last couple ECMWF runs are on their own in maintaining as strong of a system as they show after landfall. After Sunday the progression of any remaining surface reflection will become increasingly dependent on the details of northern stream flow. Given the ongoing spread and varying trends over the past few days, the updated forecast made only modest nudges from continuity for strength and track. Away from the tropical system, there is decent agreement with the general evolution of the upper pattern but various differences with details. A blend approach resolves continued shortwave discrepancies for the southern Canada into northern tier U.S. shortwave that pushes a front through the northeastern quadrant of the lower 48 with the western part stalling over the Plains/Rockies. Convective outflows still provide some uncertainty over precise frontal position. Then there are typical differences with how incoming energy amplifies into the developing central U.S. mean trough aloft Sunday-Tuesday. Recent GFS/ECMWF runs have trended deeper with the core of this trough, leading to a deeper and more northeastward surface low that will interact with the leading central/eastern U.S. front. Guidance comparisons for the Gulf system as well as flow to the north ultimately led to a starting blend incorporating the 18Z-12Z GFS/18Z GEFS mean and 12Z ECMWF/ECMWF mean through the period. More weight went to the operational runs early and slightly more to the means late, while awaiting more continuity to gain confidence in the operational ECMWF/GFS solutions at that time frame. Also the blend removed the 12Z GFS by day 7 Tuesday as its northern tier trough began to differ more from consensus. ...Weather Highlights/Hazards... Dangerous and record breaking heat wave for much of the western U.S. will continue through this week with some of the most extreme temperatures over the Southwest. Expect daily temperatures 15F to 20F above normal with afternoon readings well into the 100s and 110s for the Southwest, California Central Valley, and portions of the Interior West. To the north readings will be in the 90s. The combination of extreme daily temperatures and duration, as well as being rather early in the season for such an event, will make this a dangerous heat wave for sensitive groups. Excessive heat warnings/watches are in place for many areas of the Southwest and the California Central Valley. Temperatures over most of California and the Southeast will gradually moderate with time into early next week but areas from the northern Great Basin into the Northwest should continue to see highs 10-20F above normal into next week--but any daily records should be fairly isolated. Some areas from the central/south-central Plains into the Midwest should see highs up to 10-15F above normal on Friday before trending cooler with frontal passage. The central U.S. should see a broadening area of modestly below normal readings early next week under an amplifying upper trough. The East will start out on the cool side Friday morning and then rebound to or a bit above normal, though clouds/rain reaching parts of the South may lead to below-normal highs during the weekend. A potential tropical system is forecast to drift northward from the Bay of Campeche later this week and approach the central Gulf Coast by Friday into this weekend. The National Hurricane Center is monitoring for increased organization and development but regardless, the system will be capable of producing heavy rainfall over portions of the Central Gulf Coast and this could spread inland over the Southeast this weekend. Farther north, a leading front crossing the northeast quadrant of the lower 48 followed by a system expected to track through the Plains/Upper Great Lakes may serve to focus multiple areas of locally heavy rain within a broad area from Plains/Midwest into the Ohio Valley/Great Lakes. Details in convective complexes and potential interactions from the tropical system to the south lower forecast confidence quite a bit so it may take a few days to resolve any potential heavy rain threat there. Rausch/Taylor 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml