Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Fri Oct 4 2024 Valid 12Z Mon Oct 07 2024 - 12Z Fri Oct 11 2024 ...Potentially hazardous heat will persist across the Southwest through Monday... ...Chances for heavy rainfall across the Florida Peninsula will increase into next week... ...Overview... The upper pattern over the CONUS next week should settle into a slower-evolving pattern dominated by mainly large scale features. An upper ridge slowing moving from the Western to Central U.S. will support much above normal temperatures 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 into late next week and bring some precipitation to the northern parts of the region. Farther south, expect the main rainfall focus over the Lower 48 to be over parts of Florida, due to the combination of abundant tropical moisture in addition to Gulf of Mexico low pressure and a front draped across the peninsula. ...Guidance/Predictability Assessment... Guidance continues to show some typical differences in the details for the evolution of the upper trough settling over the East and the anchoring low. There is some question on timing and placement of the upper low and the associated surface low mid- to late week, with the 00Z/06Z GFS runs north of the CMC/EC, but a general model compromise seemed to provide a good middle ground solution. Timing of the next trough into the West and weaker shortwaves ahead of it become uncertain toward late next week. The 00Z ECMWF was a bit of a western/slower outlier with the trough axis compared to the other operational models and many AI/ML models. The WPC forecast started with a blend of the deterministic 00/06Z models and increased the proportion of the ensemble means into the latter part of the period as overall spread increased. The main feature of interest during the period though is still evolution of a possible tropical system in the Gulf of Mexico. Lows may spin up along the frontal boundaries in the eastern Gulf/northern Florida/western Atlantic while another feature that is more concerning for forming a possible tropical system (possibly partially consisting of remnant energy from what was Eleven-E in the East Pacific) may evolve over the western Gulf. Recent 12Z ECMWF/GFS/UKMET runs have converged in showing a developing tropical or subtropical system moving northeastward through the Gulf, crossing the central Florida Peninsula sometime on Wednesday, and moving into the western Atlantic for later week. This general agreement is relatively new and is thus uncertain, but tropical development is seeming more likely than it did with the 00/06Z model suites. In coordination with NHC, did discount the 12Z CMC with its low spinning in the Gulf for ten days, since this was an outlier and seems unlikely given the close proximity to a frontal boundary that should help steer it east at least eventually. Even within the EC/GFS/UK general cluster, there are differences with the track and central pressure of the low, which affects the QPF amounts. This cycle's WPC QPF amounts trended down across the Tampa Bay to Orlando corridor compared to the NBM because of the 00/06Z models were south with heavier amounts, but this may have been a mistake considering the 12Z models trended up in those areas. Uncertainty in tropical development across the Gulf remains high. Please consult the NHC outlooks for the latest information on features of interest over the Gulf. ...Weather/Hazards Highlights... The heaviest rainfall potential for next week will be over the Florida Peninsula, with low pressure (that could be tropical or subtropical) over the Gulf of Mexico shifting eastward with time but with considerable model variability on placement and timing. Regardless of tropical development, enhanced/above average tropical moisture will be in place and funnel into a couple of west-east oriented wavy fronts draped over the state. This should produce multiple days of moderate to heavy rainfall, and Marginal Risks are in place in the EROs for much of the Florida Peninsula for Days 4-5/Monday-Tuesday. Heavy rain potential may continue into Wednesday at least. Possible tropical system approach could further enhance precipitation, so stay tuned to NHC and WPC forecasts for updates. Elsewhere, the frontal system moving into the Northeast on Monday should generate moderate rainfall there, and some lake-enhanced rain is possible in westerly flow behind it through midweek. 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 non-zero chances of snow in the highest elevations. Widely scattered showers are possible at times across the Four Corners states into parts of the south-central U.S., but anything beyond light rain is unlikely. 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 Monday, with some highs at least 10-15F above normal, more into the Plains by Tuesday-Friday 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 Monday. Height falls slowly pushing in over the West Coast will support a trend toward near normal temperatures by Wednesday or Thursday. Temperatures over most of the East should cool starting Monday 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. Tate/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 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