Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Sep 13 2021 Valid 12Z Thu Sep 16 2021 - 12Z Mon Sep 20 2021 ...Heavy Rain From Tropical Storm Nicholas May Continue Into Thursday... ...Overview and Guidance Evaluation/Preferences... The 15Z National Hurricane Center advisory for Tropical Storm Nicholas shows the system weakening to a remnant low over the Lower Mississippi Valley by the start of the period early Thursday and dissipating soon thereafter. Potential exists for associated moisture to maintain a threat for some heavy rainfall over parts of the southern tier into Thursday. Check the latest NHC products for additional information on Nicholas. Models and means continue to show amplification of the large scale pattern from late this week to the start of next week, leading to a more pronounced contrast in weather conditions. An upper trough reaching the West Coast will bring a period of well below normal high temperatures to the Northwest along with enhanced rainfall (and high elevation snow in the northern Cascades). On the other hand rising heights aloft downstream with ridging becoming more prominent over the East by next Sunday-Monday will promote an expanding area of well above normal temperatures from the High Plains into the Mid-Atlantic/Northeast. Flow between the trough and developing ridge will move along frontal systems between the northern half of the Plains and New England. Meanwhile guidance continues to signal potential for an area of low pressure to develop and track northward off the East Coast. How much if any rainfall that reaches the East Coast will be very sensitive to the system's track--which at the moment appears to be just far enough east to keep most of the moisture offshore except perhaps for far eastern North Carolina and the southwest corner of New England. Over the past day guidance has trended much closer together for the details of the upper trough and leading surface front extending into the northern U.S. from a strong system initially over southern Canada. This developing consensus has been perhaps two-thirds from the past couple ECMWF runs trending noticeably slower/northward with the front due to a slower/less amplified depiction of the upper trough and one-third from the GFS trending faster. Forecasts of the West Coast trough diverge after Saturday due in part to timing differences over the Aleutians/North Pacific. Latest GFS runs have been on the fast/amplified side of the spread while CMC runs have been slowest. GEFS means have been moderately progressive but not quite to the extent of the GFS while the 00Z ECMWF mean is slower than its operational run, just a bit faster than the CMC. Guidance has been shuffling around for timing over recent days so an intermediate approach seems best. Consensus track of western Atlantic low pressure has held up fairly well over the past 24 hours. The 12Z CMC adjusted closer to recent GFS/ECMWF runs after previously being farther westward. Based on guidance comparisons the forecast blend started with a composite of 06Z/00Z models with more GFS/ECMWF weight versus CMC/UKMET early in the period, followed by a trend toward two-thirds ensemble means with the remainder more 00Z ECMWF than 06Z GFS. ...Weather/Hazard Highlights... It appears fairly likely that moisture from Nicholas may produce at least a localized heavy rain threat along and inland from the central/east-central Gulf Coast into Thursday. There is still considerable uncertainty over how long Nicholas could maintain its definition, and specifics of weak Gulf flow that could still interact with a potential convergence boundary near the coast even if Nicholas dissipates. Continue to monitor the latest NHC advisory products. Some of this tropical/Gulf moisture may enhance rainfall over parts of the eastern U.S. but again with low confidence in the details. Northern tier fronts may focus areas of rain with locations over the Upper Mississippi Valley and Great Lakes tending to see the best possibility of some locally moderate to heavy activity. With greater confidence, expect areas of moderate to locally heavy rainfall to reach the Pacific Northwest Friday into the weekend with some snow possible in the highest elevations of the Washington Cascades. The majority of rainfall with the possible system off the East Coast should remain offshore but areas from the Mid-Atlantic into New England should continue to monitor forecasts closely due to the western periphery of the moisture shield being very sensitive to exact track. The cooling trend over the Northwest will bring highs down to 10-15F below normal from Friday onward and with gradual expansion into the northern Rockies. At the same time the coverage of well above normal temperatures from the High Plains eastward will expand by the weekend, with plus 10-15F anomalies late this week and possibly some plus 15-20F anomalies by Saturday-Monday. For the five-day period the warmest max temp anomalies should be from the central High Plains into the Great Lakes while warmest min temp anomalies should align from the central/north-central Plains into Mid-Atlantic/Northeast. Rausch 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