Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Mon Oct 7 2024 Valid 12Z Thu Oct 10 2024 - 12Z Mon Oct 14 2024 ...Hurricane Milton should clear Florida by Thursday... ...Overview... By the start of the period on Thursday, Hurricane Milton should be beginning its extratropical transition and exiting the Florida peninsula. Much of the heavy rainfall should be offshore but some lingering backside showers are still possible across eastern parts of Florida. Elsewhere, an initial Northeast/Mid-Atlantic upper trough will lift out while a central U.S. ridge will continue to bring much above normal temperatures but with Pacific energy dampening the northern periphery on the way to crossing the Northeast by the weekend. Expect a strong low settling into the Gulf of Alaska to support an amplifying ridge-trough pattern to the east by next Sunday-Monday, with developing low pressure possibly generating some areas of rainfall over the Great Lakes/Northeast. A compact upper low/trough may bring some rain to the central West Coast around Friday-Saturday. Height falls and moisture ahead of the Gulf of Alaska system may begin to push into the Pacific Northwest by early next week. ...Guidance/Predictability Assessment... The updated forecast started with a 06Z GFS/00Z ECMWF blend for the early to middle part of the period, followed by a transition to 30-40 percent total ensemble means (06Z GEFS/00Z ECens). This solution yielded the closest starting point possible for the official National Hurricane Center track of Milton and best reflected the relative majority of guidance for other significant features, while accounting for typically increasing detail uncertainty later in the period. Model guidance still shows meaningful spread for the track/timing of Milton, though new 12Z runs are helping to narrow the spread especially for timing. The CMC/CMCens had been a slow extreme relative to other dynamical and machine learning (ML) guidance but the 12Z run has trended closer (but not yet to) the majority/average. Meanwhile, the fast UKMET has slowed down in the 12Z run (with the ECMWF slowing down some as well). After the departure of the initial Northeast upper trough, the progressive southern Canada/northern U.S. shortwave reaching southeastern Canada and New England by around Saturday displays fairly good clustering overall. Recent UKMET runs have been on the fast side though, with ML runs in support of the slower majority. Then as larger scale upper troughing amplifies into the eastern half of the country by Sunday-Monday, there is good agreement in principle but with typical detail differences which reflect at the surface with greater spread from the Midwest/Great Lakes through the Northeast. By next Monday the ML guidance shows surface low pressure somewhere between the eastern Great Lakes and southeastern Canada/New England, more in the realm of recent GEFS/ECens and GFS runs, with the 12Z/06 ECMWF closer to this idea versus the farther south 00Z ECMWF or east 12Z/07 ECMWF. Thus the late part of the forecast blend split ECMWF input between the 00Z ECMWF and 12Z/06 ECMWF. Near the West Coast, the 00Z UKMET/CMC strayed faster than other guidance with the compact upper trough/low arriving around late week or weekend. ML models ranged between the ECMWF/GFS cluster and intermediate timing, at least favoring a slower solution than the UKMET/CMC. The 12Z UKMET has trended back to the slower cluster and the new CMC is slower but still running ahead of other solutions. ML models vary regarding how much shortwave energy could reach the Northwest by next Monday, ranging between the dynamical model/ensemble consensus of fairly modest height falls at that time versus somewhat more energy arriving. ...Weather/Hazards Highlights... Continue to monitor National Hurricane Center advisories for the latest information on Hurricane Milton. Current forecasts indicate the system should be tracking eastward near the east coast of Florida as of early Thursday. The majority of the associated heavy rainfall should be in process of moving offshore but there is still some potential for lingering heavy rainfall to be impacting northeast parts of Florida and possibly as far north as the Georgia coast into Thursday so a Marginal Risk persists for that region on the Day 4 Excessive Rainfall Outlook. Otherwise, a large part of the lower 48 will see fairly dry weather for a majority of the period. A compact eastern Pacific upper trough/low may bring a brief period of mostly light rainfall to the central West Coast around Friday-Saturday, with a little light precipitation possibly reaching farther eastward into the West thereafter. Then a front approaching the Pacific Northwest may bring some moisture into the region by the start of next week. The amplifying upper trough and accompanying surface evolution should bring some areas of rainfall to parts of the Great Lakes/Northeast by late weekend into the early part of next work week, though with fairly low confidence in specifics at this time. A western-central U.S. upper ridge will continue to promote an extended period of well above normal temperatures with highest anomalies likely for the northern Plains on Thursday, and some locations should reach 20-30F above normal as highs in the 80s may extend to the Canadian border. Northern stream troughing and a cold front could push the greatest warm anomalies southeast into the central Plains and Mid/Upper Mississippi Valley Friday-Saturday. Meanwhile height falls slowly pushing in over the West Coast will support a trend toward near normal temperatures by Wednesday or Thursday. However, temperatures around 10 degrees above normal in the Southwest could set additional daily records though the temperatures have moderated compared to recent days and the short range. The northern Rockies/High Plains may see temperatures reach up to 10-20F above normal by Sunday-Monday. Meanwhile, cooler than average temperatures are forecast across most of the East late this week as upper troughing sets up near the East Coast, with some potential frost/freeze concerns across portions of the Appalachians and interior Northeast early Thursday. Expect temperatures to moderate next weekend as the upper low scoots out of the region, but cool again early next week underneath another trough. Rausch/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