Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 PM EDT Sun Sep 12 2021 Valid 12Z Wed Sep 15 2021 - 12Z Sun Sep 19 2021 ...Tropical Storm Nicholas Likely to Bring Heavy Rain to Southern/Eastern Texas and Louisiana through the middle of the week... ...Overview and Guidance Evaluation/Preferences... The wave reaching the southwestern Gulf of Mexico has developed sufficiently for the National Hurricane Center to start advisories for Tropical Storm Nicholas. Based on the 15Z advisory Nicholas should track generally northward in the short term, close to the Texas coast by Monday-Tuesday, and reach southeastern Texas as a tropical depression as of the start of the medium range period early Wednesday. Expect Nicholas to bring a threat of heavy rainfall to portions of southeastern Texas and Louisiana through at least Wednesday and possibly beyond depending on how long the system persists. GFS/ECMWF runs continue to show the best definition and northward extent (though with the GFS straying to the east side of the latest spread and the 00Z ECMWF slower than the NHC forecast) but additional differences in other guidance still keep confidence for specifics lower than desired. Farther north the models and ensembles agree on the general evolution toward a deepening mean trough aloft along the West Coast late this week into the weekend while western Atlantic/eastern U.S. ridging (or at least well above normal heights) deflects the westerlies across the northern tier U.S. and southern/eastern Canada. Especially after early Thursday model differences become increasingly pronounced for the amplitude and timing of shortwave energy progressing across central/eastern Canada into the northern U.S., affecting the front expected to push eastward from the northern Plains. ECMWF runs continue to be on the fast/amplified side with the upper trough and farthest east with the cold front while among remaining guidance suggests recent GFS runs could be too slow with height falls and the front. Latest model/mean intermediate solution does slow the front a little and provides reasonable leeway depending on how future runs may cluster/trend. The new 12Z ECMWF has tempered the amplitude of its upper shortwave. Behind this system, a blend/mean approach still looks good for the West Coast trough as both the GFS and ECMWF have varied among faster or slower timing over recent days and medium to smaller scale shortwave uncertainties remain. The last system to note is an area of low pressure expected to form north of the Bahamas and track northward, off the East Coast. The GFS had been the strongest with possible tropical development of this system but it has been trending weaker over the past day. Recent GFS/ECMWF trends have also been a bit more offshore. Other guidance differs significantly for definition and path. A fair degree of uncertainty persists for specifics of flow aloft reaching the East Coast from late week into the weekend but the majority scenario has enough westerly flow to push the system and its rainfall away from most if not all of the Northeast. This system requires close monitoring as minor changes could have significant implications for any rain that may fall across an already saturated coastal Mid-Atlantic/Northeast. ...Weather/Hazard Highlights... Regardless of the exact strength of Nicholas at any particular time, the system will bring a threat for heavy rain over portions of southern/eastern Texas and Louisiana through at least the middle of next week. Potential remains for some locations to see 10-15 inches while some guidance is suggesting that progression will be slow enough to result in even higher totals. Also Nicholas could maintain its definition long enough to support a heavy rain threat beyond midweek--which would also lead to higher totals. There is still significant uncertainty in the details so continue to monitor latest forecasts closely, as the heavy rainfall may produce flash and urban flooding, particularly for more sensitive locations... urban regions and those areas that have already seen above normal rainfall over the past couple of weeks. Mostly lighter rainfall will accompany frontal passages through the central and northeastern parts of the CONUS. Some potential exists for isolated/localized pockets of heavier amounts from the Ohio Valley/eastern Great Lakes into the Northeast with a leading front on Wednesday. Confidence is lower in how much rain may occur with the next front in the series given model spread for timing and waviness. A slower and more wavy front could produce heavier rainfall over the Upper Mississippi Valley/Great Lakes while rainfall intensity would be less with a faster front. Meanwhile moderate to locally heavy rainfall may reach the Pacific Northwest late this week into the weekend with some snow possible in the highest elevations of the Washington Cascades. Recent trends have shown decreasing East Coast rainfall as the possible western Atlantic system tracks a little farther offshore late in the week. Evolution of the large scale pattern will favor cooler temperatures settling over the West Coast states, with some highs 10-15F below normal over the Northwest by the end of the week/next weekend, along with steadily increasing coverage of plus 10F and greater anomalies for max/min readings over the Plains and east-central U.S. Locations to the south of an initial northeastern U.S. front will also see well above normal temperatures from midweek onward. Any clouds/moisture with the system forecast to track over the western Atlantic will be a wild card with respect to high temperatures near the East Coast. Rausch/Santorelli 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