Extended Forecast Discussion NWS Weather Prediction Center College Park MD 115 AM EDT Wed May 13 2020 Valid 12Z Sat May 16 2020 - 12Z Wed May 20 2020 ...Heavy rain possible for portions of Texas and the western Gulf Coast this weekend... ...Overview and Weather/Hazard Highlights... The flow pattern across the CONUS is forecast to become quite a bit more amplified next week, after starting out relatively zonal on day 3 (Sat). As the axis/dipole of a North Pacific/Arctic Rex Block shifts west of the Date Line, downstream flow from the northeast Pacific across North America will tend to amplify. A strong/anomalous trough/upper low across the northeast Pacific is forecast by the consensus of model/ensemble guidance to shift east, slowly moving onshore along the West Coast over the weekend and into early next week. As this occurs, a relatively strong upper-level ridge is forecast to develop across the central U.S. Warming temperatures are likely as heights rise across the central U.S. The addition of some downsloping flow will increase the potential for well above average temperatures across the north central U.S. by the early to mid next week, with highs forecast to reach 10 to 20 deg F above average. Along the northern periphery of the expanding ridge, shortwave energy embedded within stronger westerlies will cross the CONUS northern tier, with an associated frontal wave forecast to move from the Central Plains to the Great Lakes/Northeast Sun-Mon. Scattered to numerous showers and thunderstorms are forecast to accompany this area of low pressure, with areas of locally heavy rain possible. Meanwhile the trailing end of the shortwave appears likely to get cut off by the strengthening ridge, lingering across the lower Mississippi Valley/western Gulf Coast through early next week. Models/ensembles have shown a growing signal that this feature could focus slow-moving/persistent showers and thunderstorms across portions of eastern Texas over the weekend, with activity gradually shifting westward into south central Texas early next week. Latest guidance suggests that several inches of rain are possible for some areas, and flooding/runoff may pose a threat. Farther east, the trailing end of a stationary frontal boundary is forecast to linger in the vicinity of South Florida and the Bahamas this weekend. An area of low pressure is forecast to develop along the boundary over the Bahamas, and move northeastward out to sea. The National Hurricane Center is monitoring this area for potential development as a subtropical system by this weekend. Heights should begin to fall across the Pacific Northwest as early as Sun as the deep upper low/trough approaches. Stronger height falls should slowly spread inland through early next week as the system moves east, reaching the Great Basin by Tue night or early Wed. Increasing coverage of rain and high elevation snow are expected to accompany the system from California and the Pacific Northwest east into the Great Basin and northern Rockies, along with cooler temperatures (highs forecast to be 5 to 15 deg F below average). ...Guidance/Predictability Assessment... A blend of the 12Z ECMWF/GFS was used as a basis for the forecast during days 3-5 (Sat-Mon). More weight was placed toward the GFS relative to the ECMWF. The GFS forecast was closer to the overall ensemble consensus with the area of low pressure expected to develop off the Southeast coast. Elsewhere, the solutions were comparable with relatively minor timing/amplitude differences for various features noted. Weight placed on ECENS/GEFS ensemble means was increased gradually through time, with majority forecast weight on ensemble means during days 6-7 (Tue-Wed). By the middle of next week, models differed as to the evolution off the Eastern Seaboard, and any potential interaction between the area of low pressure drifting northward well off the Southeast coast, and northern stream shortwave energy. In general, guidance has trended toward a more amplified northern stream shortwave, but solutions still differ on the degree to which the shortwave interacts with any possible subtropical low. The 18Z GFS showed a much more extensive interaction, resulting in the development of a deep upper low off the Eastern Seaboard by the middle of next week. The preferred 12Z GFS showed a more limited interaction. The ECMWF, which continued to be much slower than the overall consensus with moving the southern low northward, showed no interaction. Along the West Coast, model consensus has improved with respect to the timing of the large Pacific trough/upper low forecast to move onshore, with solutions seeming to converge toward a central consensus. Ryan 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