Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Nov 4 2024 Valid 12Z Thu Nov 07 2024 - 12Z Mon Nov 11 2024 ...Watching Potential Tropical Cyclone Eighteen as it tracks into the Gulf of Mexico toward the Central Gulf Coast... ...Four Corners upper low to bring significant Rockies snow and areas of heavy rain to the Plains... ...Overview... Models and ensembles agree fairly well with the large scale pattern evolution, with northeastward ejection of a Four Corners upper low while upper ridging prevails over portions of the western Atlantic, Gulf of Mexico, and eastern U.S. The upper level reflection of Potential Tropical Cyclone Eighteen, forecast to reach the Gulf by the start of the period early Thursday, should become embedded within the larger scale ridge. Meanwhile the West Coast will see steadily decreasing heights aloft as a sequence of shortwaves dampen initial ridging, with the strongest one likely to reach the coast around next Monday. The Four Corners upper low will produce significant snow over the southern half of the Rockies and areas of heavy rain over the Plains, while the uncertain track of PTC Eighteen keeps confidence lower for how much of its moisture may reach into the lower 48. Trends aloft near the West Coast will increase precipitation coverage and intensity over the Pacific Northwest into northern California by the weekend into early next week. The Four Corners states will see chilly temperatures late this week while much of the East should see persistently above normal temperatures through the period. ...Guidance/Predictability Assessment... The updated forecast started with a blend of 12Z operational models for the first half of the period with somewhat more ECMWF weight than other models, with some subsequent editing to accommodate the National Hurricane Center track of PTC Eighteen. Then the blend incorporated some of the 18Z GEFS/12Z ECens means with an eventual shift to slightly more 12Z/18Z GFS input relative to the 12Z ECMWF by Day 7 Monday. Dynamical and model guidance continues to offer very diverse tracks for PTC Eighteen. ECMWF runs continue to be fairly far on the extreme western side of the envelope, tracking the surface reflection into the western Gulf, while the new 00Z UKMET has trended in that direction as well. Remaining dynamical and machine learning (ML) models favor a track over the central or eastern Gulf into the Gulf Coast. While some GFS runs have leaded a tad on the faster side over the Four Corners/Plains, overall the latest dynamical model runs offer fairly good clustering for the upper low of interest and the ML models seem to agree somewhat better with the dynamical guidance today after running a little on the fast side in recent days. By next Monday most guidance suggests a little slower timing over southeastern Canada versus the 12Z ECMWF with a surface pattern more similar to GFS runs and a number of other models/ensemble means. There is a fair amount of spread for specifics of shortwave energy reaching western North America around Saturday, with the UKMET running on the fast side and the 18Z GFS less so, while most ML models are still a little offshore early in the day (with the 00Z GFS nudging a little slower than the 18Z run). The next trough looks to be more vigorous and ML models generally suggest that the 12Z ECMWF and its prior runs may not be amplified enough. The new 00Z ECMWF has trended a little deeper, supporting a consensus of remaining guidance. ...Weather/Hazards Highlights... Over the Southeast/northern Florida as of Thursday, guidance continues to offer some signal of lingering heavy rainfall potential from anomalous moisture and shortwave energy rounding the western Atlantic deep- layer ridge, to the north of Potential Tropical Cyclone Eighteen that should reach the southern Gulf of Mexico. Clustering for specifics remains fairly poor, favoring maintenance of the existing Marginal Risk area in the Day 4 Excessive Rainfall Outlook. By Friday the guidance signal becomes more diffuse over the Southeast, while most rainfall directly associated with PTC Eighteen should still be over the Gulf, favoring no area in the Day 5 outlook. After early Saturday expect some increased rainfall along/inland from the central Gulf Coast based on the 0300 UTC NHC advisory track for PTC Eighteen, with some potential interaction with a front approaching from the west/north. The upper low forecast to track across Arizona/New Mexico and then depart to the northeast would bring the best potential for significant snowfall to portions of the Colorado and New Mexico Rockies into late this week, with some of this snow likely extending a little eastward into the High Plains. Just to the east, this system should produce some areas of locally heavy rainfall over the southern/central Plains Thursday-Friday. The Day 4 ERO made only minor adjustments to the existing Marginal Risk area depicted over the southern-central Plains while the new Day 5 outlook expands the Marginal Risk area. The combination of guidance totals and damp soils from observed/short-term rainfall suggest potential for an embedded Slight Risk area, but would like to see less spread in the guidance to help refine such an area. Continued progression of the upper low and surface system/fronts will spread rainfall of varying intensity into the East by the weekend. Aforementioned moisture from PTC Eighteen may provide some enhancement depending on the system's track. The Northwest will see a steadily wetter pattern with high elevation snow from Friday onward due to a succession of systems, with the one arriving by Monday likely to bring the highest totals of the period along with farther southward extent into northern California. The Four Corners states and nearby areas will be the focus for below normal temperatures late this week as the upper low drifts across the region. Some locations may see highs at least 20-25F below normal on Thursday and only a few degrees warmer on Friday. The weekend will bring a noticeable moderating trend as the upper low departs, though the southern Rockies may not quite recover back to normal by early next week. The remainder of the West should trend to near normal readings by the weekend into early next week. Except for northern New England likely staying close to normal most days, the eastern U.S. should see above normal temperatures through the period due to the persistent mean ridge aloft over the western Atlantic and parts of the Gulf, plus extending into the East through early Sunday. Thursday morning should be particularly warm with lows 20-25F above average from parts of the Mid-Atlantic into the Southeast. Otherwise, with day-to-day variability expect some coverage on most days of plus 15-20F anomalies for morning lows and plus 10-15F anomalies for daytime highs. Above normal temperatures should extend across the Upper Midwest and northern Plains as well. 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw