Extended Forecast Discussion NWS Weather Prediction Center College Park MD 212 AM EDT Mon Jul 05 2021 Valid 12Z Thu Jul 08 2021 - 12Z Mon Jul 12 2021 ...Elsa is forecast to cross the eastern Carolinas on Thursday and may result in heavy rainfall near the Carolina Coast... ...Repeating heavy rain may persist over southern Texas into the weekend as low pressure lingers over northern Mexico... ...Overview... The western CONUS ridge/eastern mean trough pattern will amplify this weekend as a shortwave trough from the Pacific rounds the ridge before stalling over the Upper Mississippi Valley Saturday. The upper ridge over the Great Basin/Desert Southwest strengthens in the wake of the ridge riding trough and only intensifies the ongoing heat wave across The West. An initial shortwave trough pushing east from the Great Lakes to eastern Canada/New England by Friday will drag a weak cold front over the northeastern CONUS through Friday before it lifts back as a warm front ahead of the ridge riding shortwave trough over the weekend. Tropical Storm Elsa is forecast to push off the Carolinas on Thursday while an upper trough/low stalls over northern Mexico that may produce heavy rainfall over southern Texas (and northern Mexico) into or through the weekend. ...Guidance/Predictability Evaluation... The most recent guidance has come into much better agreement with the ridge riding shortwave trough that stalls near the Upper Midwest and with the timing and strength of Elsa. The 18Z and now 00Z GFS are stronger with the resultant low over the Upper Midwest with a more closed and thus slower system than the ECMWF, but overall the differences are not great and a dominant deterministic blend focusing on the GFS and ECMWF was able to be used through Day 6 when separation of the Upper Midwest low is great enough to warrant greater inclusion of the ensemble means. Meanwhile, the Elsa forecast for Thursday (as it moves across the eastern Carolinas to the sea) is in remarkable agreement among global guidance given all the variance over the past several days. ...Weather Highlights/Hazards... An upper low/trough that stalls over northern Mexico through next week will support a repeating rain threat over that region continuing for Thursday and perhaps well into the weekend, though the threat for extreme totals continues to diminish. However a persistent low level flow of Gulf moisture will feed into this slow moving disturbance, maintaining a flooding threat. Heavy rain would accompany Elsa as it tracks through the eastern Carolinas Thursday. Please continue to monitor products from the National Hurricane Center for the latest Elsa forecast track information. The initial front looks to be a rainmaker for the Northeast Thursday before the front shifts offshore. Confidence is increasing for Elsa associated rain on Friday to be off the Northeastern Seaboard. The ridge riding trough forecast to reach the Upper Midwest by Saturday will have access to Gulf-sourced moisture and should result in significant rainfall for the far northern Plains to the northern Great Lakes as the system becomes blocked/stalls. The heat wave over a significant portion of the West will continue at least into next week as the Great Basin/Southwest upper ridge restrengthens in the wake of the ridge riding trough. The Coast Ranges across the Great Basin to the northern Rockies should all see highs 10-15F above normal each day with locally higher readings possible. Several daily high temperature records are under threat across primarily the Great Basin each day with a slight western shift to California this weekend as the ridge axis drifts west. The only slight break over the Pacific Northwest looks to be Thursday when a weak frontal passage nudges highs down to near or slightly below 10F above normal. Meanwhile the strengthening and westward expansion of the upper ridge should produce a hotter trend with time over interior locations of California. The southern High Plains/Texas will likely see highs 5-12F below average for most of the week due to cloudiness and rain from the persistent upper level disturbance drifting over that region. The Midwest and Great Lakes will see below normal highs due to rainfall on Thursday and the over the weekend with the next system. The latter system's cold front may also bring a cooling trend into the central Plains by next weekend. Jackson 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