Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun May 16 2021 Valid 12Z Wed May 19 2021 - 12Z Sun May 23 2021 ...Heavy rain threat for the southern Plains to the western Gulf coast through midweek... ...Guidance Evaluation and Preferences... Models and ensembles continue to depict a heavy rain event likely to shift farther south toward the Texas Gulf Coast region on Thursday. There has been a consistent model signal for the heaviest rain to fall across eastern Texas before diminishing significantly on Friday. Thereafter, another rainfall axis will tend to set up closer toward the central and southern High Plains by next weekend as a deep upper trough approaches from the northwest. Models have again trended toward configuring this deep upper trough to be more positively-tilted as additional energy is sent downstream across the northern tier states and southern Canada. Meanwhile for the northern Rockies to possibly the nearby High Plains, there is increasing model support for sustaining a late-season wet snow event from the middle of next week to next weekend. In contrast to the cold upper trough in the West, there continues to be a trend toward modifying a cool high pressure system near the East Coast to a warm deep layer anticyclone by next week. The WPC medium-range forecast package was based on the consensus of the 00Z ECMWF/00Z EC mean, the 06Z GFS/GEFS mean, as well as some contribution from the 00Z CMC and CMC mean. Good WPC continuity was maintained. ...Weather Highlights/Hazards... Deep layered moisture from the Gulf of Mexico will likely sustain a heavy rain event with focus across eastern Texas Wednesday into Thursday before diminishing by Friday. Another axis of enhanced rainfall may then set up closer toward the central and southern High Plains next weekend as a cold front associated a deep trough moving through the western U.S. Upslope flow across the northern Rockies combining with upper-level divergence ahead of the upper trough will likely sustain a wet snow event into next weekend for the higher elevations and could extend into the High Plains especially during the night. Meanwhile, ridge amplification in the eastern U.S. will coincide with substantial warming this week from the Midwest to the especially the East where some record high minimum and maximum values will be possible as temperature anomalies build upwards to 10-20F above normal. Deep moisture and vorticity rotating around the building upper ridge over the East will likely bring some diurnally-driven downpours up into the northern Plains/upper Midwest, parts of New England, and initially the Florida Keys. Mid-later next week digging of amplified upper troughing over the West will support widespread moderate precipitation from the Pacific Northwest and Sierra through the north-central Intermountain West/Rockies, including some mountain enhanced snows. Ahead of the Pacific front that moves into the West, early week maximum temperatures 10-20F above normal from the Northwest U.S. through the northern Rockies/Plains will abruptly cool with pattern amplification and wavy cold frontal passage that may result in 10-20F below normal temperatures centered across the Intermountain West. Kong/Schichtel 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