Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 PM EST Sun Nov 3 2024 Valid 12Z Wed Nov 06 2024 - 12Z Sun Nov 10 2024 ...Watching possible tropical development into the Gulf of Mexico by the medium range period... ...Overview... Latest guidance maintains the recent theme of a strong western Atlantic upper ridge extending into parts of the East while strongly digging western U.S. trough energy likely closes off a low near the Four Corners around midweek, followed by a slow eastward drift and some acceleration northeastward over the Plains by the weekend. Expect the upper low to produce an area of potentially significant snowfall over parts of the central-southern Rockies around midweek and then develop moderate to locally heavy rain over the Plains later in the week, with moisture continuing to progress during the weekend. Around the periphery of the Atlantic upper ridge, guidance still shows possible Caribbean into Gulf of Mexico tropical development (being monitored by the National Hurricane Center) with a lot of guidance spread for where the surface system itself may track, as well as for the path of moisture to its north/east. The Northwest should see a wetter trend by next weekend with the approach/arrival of a couple frontal systems. ...Guidance/Predictability Assessment... Models continue to show pretty good agreement on the evolution/track of the Four Corners upper low. The 06z GFS was quite a bit faster than consensus, but the 12z run has trended slower/more westward and much more in line with consensus and the WPC forecast blend. There remains some question on its strength and whether it becomes more of an open wave or a closed low as the system tracks northeastward into the Upper Midwest next weekend. There is still a lot of spread among the dynamical and ML models regarding the combination of possible leading moisture reaching the Southeast/Florida and possible Caribbean into Gulf of Mexico development. The CMC has tended to be on the fast/eastern side of the dynamical model spread for the possible system but some ML guidance is somewhat on the faster side as well with an envelope spanning the central-eastern Gulf. The ECMWF continues to be more suppressed and west with the track into the western Gulf. The manual blended forecast is close to a GFS/ECMWF consensus. As for other details, there is some model spread for amplitude/timing of the Pacific shortwave trough expected to progress into western North America during the weekend. There is also some spread for the details of eastern Canada/northeastern U.S. troughing late week into the weekend as upstream ridging progresses across Canada. The updated forecast starts with an operational model blend emphasizing the 00z ECMWF with modest 00z UKMET and 06z GFS for the first half of the period. Added more of the ensemble means later in the period to reflect the growing uncertainty. Overall, this maintained good agreement with the previous WPC forecast. ...Weather/Hazards Highlights... Guidance continues to have difficulty in resolving the details of moisture and shortwave energy rounding the western Atlantic deep- layer ridge and potentially bringing areas of significant rainfall to the Southeast/Florida, with possible Caribbean into Gulf of Mexico tropical development providing another potential rainfall influence. Over the course of the Days 4/5 Excessive Rainfall Outlook period (Wednesday through Thursday night) there is a fair clustering of dynamical/ML guidance suggesting an area of enhanced focus over or near Georgia/South Carolina with some guidance a little farther south into parts of Florida. These solutions all lie within a general area of anomalous moisture and some instability, supporting a broad Marginal Risk area in both ERO days as the best representation of the situation at this time. Rainfall details continue to be uncertain beyond early Friday, again depending on the ultimate evolution of possible tropical development. The current forecast path of the upper low tracking through the Four Corners region would bring the best potential for significant snowfall to portions of the Colorado and New Mexico Rockies in the Wednesday-Thursday time frame. Gradual improvement in guidance clustering for upper low specifics over the past day has raised the probabilities for some of this snow to extend 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 Marginal Risk area depicted over the southern-central Plains in the Day 5 ERO represents the most agreeable region of focus in the guidance during the first part of this event. Expect the precipitation shield to progress eastward/northeastward into the weekend, generally with lighter amounts but with uncertainty over how much interaction there could be with Gulf moisture and the possible surface system. Thus potential still exists for some areas of heavier rain into the weekend. The Northwest should be dry around midweek and then see increasing rain/high elevation snow by late week and the weekend, as a leading dying front approaches Friday and then stronger systems approach/arrive during the weekend. For the most part, eastern U.S. temperatures should be above normal through the period due to persistent upper ridging over the western Atlantic and extending into parts of the East. Wednesday into early Thursday should be particularly warm with broad coverage of plus 15-25F anomalies, challenging daily records for highs/warm lows. By late week into the weekend expect highs to moderate somewhat (5-15F above normal) while some areas over the southern half or so of the East could still see morning lows 15-20F above normal. Canadian flow could bring temperatures closer to normal over New England. Meanwhile, as the Four Corners upper low forms and then eventually ejects into the Plains, much of the West will see well below normal highs on Wednesday followed by persistence of very chilly highs over/near the central-southern Rockies through Friday (some locations possibly 20-30F below normal for a day or two) while the Interior West moderates. Then the upper low's departure should support a warmer trend over the central-southern Rockies during the weekend. Santorelli/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