Extended Forecast Discussion NWS Weather Prediction Center College Park MD 105 AM EDT Thu May 14 2020 Valid 12Z Sun May 17 2020 - 12Z Thu May 21 2020 ...Heavy rain possible for portions of southeastern Texas through the weekend... ...Overview and Weather/Hazard Highlights... The flow pattern across the CONUS is forecast to become quite a bit more amplified over the next week, 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 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. 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). 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. Trailing shortwave energy appears likely to get cut off by the strengthening ridge, lingering across the lower Mississippi Valley/northern Gulf of Mexico through early next week. This feature could focus slow-moving/persistent showers and thunderstorms across portions of eastern Texas through the weekend, with activity gradually diminishing in coverage 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, an area of low pressure is forecast to develop along a weakening frontal 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 Eastern Seaboard through the middle of next week, providing a cool onshore flow to coastal areas with wrap-around cloudiness and showers. ...Guidance/Predictability Assessment... A blend of the 12Z ECMWF/18Z GFS was used heavily during days 3-5 (Sun-Tue). The GFS was a bit closer to the overall guidance consensus with respect to the developing area of low pressure of the Southeast U.S. coast, and thus was weighted a bit more in the forecast relative to the ECMWF. By day 5 and onward, models showed differences with respect to the evolution of the amplifying shortwave along the Eastern Seaboard. Overall, there has been a trend over the past day toward a slower/deeper solution, with development of a closed upper low along the East Coast early next week now appearing more likely. This trend was evident across a number of models/ensembles - and makes conceptual sense given the increasingly amplified/blocked large scale flow across North America, thus confidence in this trend is at least moderate. With the amplified trough moving into the western U.S., overall model/ensemble consensus is above average, with spread having reduced substantially over the past couple days at the large scale. Some differences persist, however, with respect to smaller scale shortwaves traversing the broader trough - thus, the use of more ensembles by the middle of next was was also further warranted. Ryan/Fracasso 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