Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Thu Aug 20 2020 Valid 12Z Sun Aug 23 2020 - 12Z Thu Aug 27 2020 ...The heat wave continues for the Desert Southwest through next week... ...Above normal temperatures for much of the rest of The West and northern High Plains into next week... ...Overview and Weather/Hazard Highlights... A strong upper high center currently over AZ is expected to drift northeast to near the Four Corners by Sunday morning and stall there, remain just as strong into Tuesday before some gradual weakening. An ejecting Gulf of Alaska low pushing east across the Canadian Rockies Saturday and subsequent reinforcing shortwave troughs over a similar path through next week with a somewhat active pressure pattern for the Northwest CONUS. The high and broad downstream ridging will maintain well above normal temperatures for much of The West into the Rockies and north-central parts of the Plains eastward to the Northeast atop another upper high in the western Atlantic. Best potential for record highs with readings up to 10-12F above normal on Monday is over and in the lee of the south-central Rockies of Colorado/New Mexico and back toward southeast AZ from the Great Basin/Southwest into the lee of the Rockies and western High Plains. Expect broader coverage of plus 10-15F anomalies and potential daily record warm values for morning lows Monday through Thursday from coastal California, across the Desert Southwest/Great Basin and over the south-central Rockies/onto the adjacent high plains. There should be isolated to scattered showers/storms over The West each day, aided by shortwaves flowing around the upper high center. The most prominent focus for rainfall will be across Florida and along the Gulf Coast from two approaching and potentially land falling tropical cyclones. T.D. 13 is forecast by NHC to move west-northwest across the Bahamas Sunday/Sunday night before crossing anywhere from Cuba to the Florida peninsula as a potential hurricane early next week. T.D. 14 is forecast to lift northwest across the western Gulf early next week after crossing the Yucatan Peninsula this weekend. There is still considerable track and intensity uncertainty with these systems among global guidance. Please see the latest forecast from NHC for further information. Low amplitude upper flow across southern Canada and northern U.S. will support multiple waves/fronts across the northern states and episodes of showers and thunderstorms between the Northern Plains and Northeast. Most features will be fairly transient so rainfall totals should generally be on the moderate side, though locally more intense activity could be possible at times. Locations to the north of a front on a particular day may see below normal temperatures while above normal readings (up to plus 5-10F anomalies) are possible over the north-central latitudes that will tend to be in the warm sector ahead/south of the fronts. ...Guidance/Predictability Assessment... The medium range forecast was based on a blend of the 00Z ECMWF/CMC/UKMET and 06Z GFS for Days 3 to 5 due to good agreement of the upper high position and shortwave low progression through the Pacific Northwest and northern tier of the CONUS. The forecast basis then transitioned to the 00Z ECMWF/ECENS 06Z GFS/GEFS for Days 6/7 to smooth out uncertainty with shortwave activity over the northern tier in that time frame. The greatest uncertainty is with the track/strength of T.D. 13. The low amplitude of northern tier flow aloft continues to provide challenges in resolving northern U.S.-southern Canada wave/front specifics. No single solution was preferred CONUS-wide, but rather aspects of each were deemed more or less plausible. Blending the several solutions formed a good starting point for the pressures forecast. For T.D. 13, there was good agreement with the 06Z GFS/00Z CMC (which persists with the 12Z GFS/CMC). The 00Z ECMWF does not feature much with this system in around the CONUS, while the 00Z UKMET was farther west with the track, adding to track and intensity uncertainty. For T.D. 14 the 12Z GFS has come more in line with the 00Z ECMWF for timing into the western Gulf Coast early next week, but the 12Z CMC is shunted farther south, so uncertainty reigns with that system as well. Off the West Coast, there remains uncertainty with timing/strength of the southern part of the upper trough lifting into central/northern California Monday night or Tuesday. Jackson Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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