Extended Forecast Discussion NWS Weather Prediction Center College Park MD 535 PM EDT Wed May 13 2020 Valid 12Z Sat May 16 2020 - 12Z Wed May 20 2020 ...Heavy rain possible for portions of southeastern 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. with warming temperatures likely as heights rise. The addition of some downsloping flow will increase the potential for well above average temperatures (+10 to +20 anomalies and actual readings into the 80s and low 90s) across the north central U.S. by the early to middle part of next week. 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. By the end of the period, the upper trough moving through the Northeast may dig farther south than east in response to the blocky/amplified pattern, resulting in a near-offshore development of a surface low pressure. This may linger just off the coast, providing a cool onshore flow to coastal areas with wrap-around cloudiness and showers. 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... Through the 00Z/06Z guidance suite, a multi-model blend sufficed for the first few days of the forecast which mostly aligned with the ensemble consensus. Preferred the middle ground between the 00Z GFS/ECMWF for the area of low pressure expected to develop off the Florida 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 to account for timing/amplitude differences. By the middle of next week, models differed as to the evolution off the Eastern Seaboard with respect to the possible subtropical system (GFS may be too quick), northern stream shortwave across southeastern Canada, and the potential deeper/slower upper low slowly dropping through the Mid-Atlantic Tue. Ensemble trend has been for a sharper upper pattern across the CONUS which would bring this system closer to the coast in typical fashion for this time of year (maximum frequency of closed lows is late April into May for the Northeast). 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. Over Texas, ridging to the northwest of the upper low around Mon may crest to the north and essentially close off any escape route until Wed unless the system simply weakens in place along the Rio Grande. Fracasso/Ryan Hazards: - Heavy rain across portions of the Great Lakes into the central Appalachians, the eastern portion of the southern Plains, as well as from northern California to the Cascades and the Olympic Peninsula, Sat-Sun, May 16-May 17. - Heavy precipitation along portions of the Sierra Nevada, Mon-Tue, May 18-May 19. - Heavy rain across portions of Montana, Wed, May 20. - Heavy rain across portions of the central Appalachians into southern New England, Mon-Tue, May 18-May 19. - Flooding possible across portions of the central Plains, the Mid-Mississippi Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Great Plains into the Midwest, as well as the lower Mississippi Valley. - Much above normal temperatures across portions of the central Rockies eastward into the central and northern Plains, Mon-Wed, May 18-May 20. 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