Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu Oct 3 2024 Valid 12Z Sun Oct 06 2024 - 12Z Thu Oct 10 2024 ...Anomalous heat will persist across the Southwest into early next week... ...Chances for heavy rainfall across the Florida Peninsula will increase into next week... ...Overview... Latest models and ensembles still show the upper pattern settling into a slower-evolving pattern dominated by large scale features. Expect an upper ridge covering the western-central U.S. to produce a broad area of much above normal temperatures over that part of the country while a trough settling over the East (anchored by an upper low that may track over or near New England) will bring a cooler trend. The leading edge of a Pacific trough will produce gradual height falls over the Pacific Northwest and bring some precipitation to the northern parts of the region. Ahead of the dynamics that will ultimately establish the eastern upper trough, a front crossing the Great Lakes and Northeast should produce some organized rain. Farther south, expect the heaviest rainfall over the lower 48 to be over parts of Florida, due to the combination of abundant moisture plus a combination of one or more Gulf of Mexico surface features and a front draped across the peninsula. ...Guidance/Predictability Assessment... Guidance continues to show typical detail differences for the evolution of the upper trough settling over the East and the anchoring upper low. General trends of the GFS/ECMWF and their means over the past couple days have been a little slower with the upper low, offering some support for ideas from recent CMC runs that have been on the western side of the spread. However by the end of the period the full array of dynamical and machine learning (ML) guidance would recommend a farther east position (over or east of northern Maine) versus the CMC. The 12Z UKMET held the core of the upper trough northwest of consensus by the end of its run but the 00Z version compares better to consensus. An average of the GFS/ECMWF and their means has good support from the average of ML models by late in the period. Some aspects of the forecast over and near the Gulf of Mexico still have low predictability due to a combination of smaller- scale features. One wave may track into the central/east-central Gulf from the northwestern Caribbean while another feature may evolve over the western Gulf, with the surface/QPF details also potentially affected by multiple impulses aloft. Lately there has been some improvement in the signal for these features to phase early next week as they approach and interact, with this potentially messy evolution involving fronts and moisture gradients possibly yielding a more conventional wavy front that sags just south of the Florida Peninsula by the end of the period. Continue to monitor outlooks from the National Hurricane Center for the latest information on features of interest over the Gulf. A blend of most guidance looks good for the height falls reaching the Pacific Northwest by the latter half of the period. The one exception was the 12Z CMC which brought a stronger shortwave into the region. The new 00Z CMC looks better in this regard. Based on 12Z/18Z guidance, the early part of the forecast started with an operational model composite. The forecast phased out the last part of the UKMET mid-period and then the 12Z CMC, while increasing 18Z GEFS/12Z ECens weight. This yielded a half GFS/ECMWF and half GEFS/ECens mix by the end of the period next Thursday. ...Weather/Hazards Highlights... The majority of the lower 48 should see dry weather for most or all of the Sunday-Thursday period, with most precipitation focus around the corners. By far the heaviest rainfall potential should be over the Florida Peninsula, with slow-moving low pressure over the Gulf of Mexico along with a wavy front draped over the state and a broad area of anomalous moisture extending over/east of the Gulf. There is still a fair amount of uncertainty for details though. During Day 4 Sunday, most dynamical models/ensemble probabilities keep the highest rain rates just offshore the western peninsula but machine learning models show faster progression of Gulf low pressure and leading enhanced rainfall. For now will maintain no risk area in the Day 4 Excessive Rainfall Outlook until a dynamical model trend toward the ML models becomes evident. On Monday there is better consensus that enhanced moisture and potentially heavy rainfall will extend farther eastward across at least the southern half of Florida, meriting the introduction of a Marginal Risk area for the Day 5 ERO. Southern Florida may continue to see periods of significant rainfall for another day or so, with the axis of highest totals sagging south with time along with the focusing wavy front. Elsewhere, the frontal system crossing the eastern Great Lakes and Northeast Sunday-Monday may generate rainfall of varying intensity. This region may then see showery conditions at times as shortwave impulses flow around an upper low expected to track over or near northern New England and the Canadian Maritimes. A mean frontal zone with one or more embedded waves should reach near the northern Pacific Northwest by early-mid week, producing some periods of light to locally moderate rain and nonzero chances of snow in the highest elevations. The strong and persistent western-central U.S. upper ridge will promote an extended period of well above normal temperatures over that part of the country. Gradual eastward drift of the ridge axis will shift the highest anomalies from the West on Sunday, with some highs at least 10-15F above normal, more into the Plains by Tuesday-Thursday when some northern locations could reach 20-25F above normal. Expect numerous daily records for warm minimum and maximum temperatures, especially over the Southwest through Sunday-Monday. Height falls slowly pushing in over the West Coast will support a trend toward near normal temperatures there by Wednesday or Thursday. Moderately above normal temperatures over most of the East as of Sunday should then decline to near or somewhat below normal as upper troughing sets up near the East Coast. This may lead to frost/freeze concerns across portions of the Upper Great Lakes region and interior Northeast, with the timing around the median to a bit late for the first frost/freeze of the season. 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