Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Thu Oct 10 2024 Valid 12Z Sun Oct 13 2024 - 12Z Thu Oct 17 2024 ...Overview... Most models and ensembles maintain the theme of an amplifying ridge-trough pattern during the first part of next week, followed by somewhat more progression as a trough moves into the West by next Thursday as downstream ridging crosses the central U.S. and eastern troughing relaxes. Much below normal temperatures and frost/freeze may be a concern over some areas from the northern Plains into the East, while the Rockies and portions of the Plains will be the main focus for above normal temperatures. Some daily record highs will be possible over the far southern tier early in the period. Expect the majority of precipitation to occur over three areas: the Great Lakes/Northeast in association with developing low pressure and chilly flow in its wake, southern Florida early in the week with a lingering front, and over parts of the West with the approach/arrival of a Pacific upper trough and leading front. ...Guidance/Predictability Assessment... In terms of the larger-scale sensible weather effects, the most contentious part of the forecast involves what happens with energy in the southern part of the eastern U.S. upper trough from around Tuesday onward. The majority of guidance, including recent runs of machine learning (ML) models, have generally favored maintaining an open trough aside from perhaps a transient upper low forming near the East Coast and then continuing into the Atlantic. This scenario would support surface high pressure and dry conditions over the East. However CMC runs have been persistent in closing an upper low (but trending eastward over the past couple days) and sporadic GFS runs have as well--most notably the new 12Z version that is like the 00Z CMC. Such a closed low would develop low pressure and rainfall over parts of the East. Among the ensemble systems, CMC members unsurprisingly show a greater closed low potential though some ECMWF members have that idea as well, while that solution is much more in the minority among GEFS members. The combination of ML models, overall dynamical guidance majority, and multiple days of CMC runs eventually trending more toward consensus when depicting stray solutions several days out, favor the open trough solution for a single deterministic forecast. There is enough of the minority signal to require monitoring for potential future changes though. As for more localized but still important detail differences, guidance still shows some spread for specifics of Northeast low pressure development by Monday. Latest ensemble means and most ML models support a primary low track in the middle/western part of the overall spread in contrast to the coastal track of the 12Z GFS/UKMET/CMC. Meanwhile guidance shows better than average agreement in principle for the upper trough reaching the West by next Thursday, though some low-predictability shortwave detail differences arise and will affect the coverage and amounts of associated precipitation. Based on 00Z/06Z guidance, a blend incorporating more 06Z GFS/00Z ECMWF relative to the 00Z UKMET/CMC represented preferences for significant features during the first half of the period. Then the blend transitioned to mix of the GFS/ECMWF and their corresponding ensemble means, with the latter reaching 50 percent total weight by the end of the period next Thursday. ...Weather/Hazards Highlights... The lower 48 should see fairly limited precipitation coverage overall. One area of organized focus will be over the Great Lakes and Northeast this weekend into the first half of next week, from a combination of the strengthening Sunday-Monday surface system as upper troughing begins to amplify and then lake effect/terrain- enhanced precipitation behind the system and underneath the upper trough. After system passage there may be some pockets of snow over high elevations and perhaps other locations during the colder overnight hours. Over the past day, guidance has trended lower with max QPF totals over the Northeast on Sunday in association with surface low development (with instability still lacking), further supporting no risk area in the Day 4/Sunday Excessive Rainfall Outlook. Farther south, a front lingering near south Florida will be accompanied by anomalous moisture and some instability Sunday- Monday, potentially leading to some areas of enhanced rainfall over the southern part of the peninsula. Guidance signals are mixed as to how much of the heavier rain may fall over land versus offshore, so for now will maintain no risk area in the Days 4-5 EROs. However will monitor future guidance updates for refinements in determining potential for localized flash flood threats, keeping in mind initially wet ground conditions. The upper trough and leading surface front moving into the West will gradually push precipitation into the northern/central West Coast through the northern Rockies. While there is good confidence in the overall event, unresolved specifics of the upper trough lead to greater uncertainty with respect to the southward/eastward extent of precipitation as well as magnitude. Expect relatively higher totals over favored coastal areas and the Cascades. Some higher elevation snow will be possible by midweek. An upper ridge aligned over the Rockies into early next week and then shifting eastward by midweek will promote well above normal temperatures in its vicinity. Plus 10-20F anomalies for highs will focus over the Interior West/Rockies into early next week and expand into the northern High Plains by Tuesday, followed by more emphasis over the northern/central Plains by next Thursday. Highs could locally exceed 20F above normal over the northern Plains around midweek. The southern Plains should remain unseasonably warm (at least plus 10-15F anomalies) into the start of next week before northern stream troughing and a cold front suppress above normal readings to the south. Daily records will be possible from the Southwest into the Lower Mississippi Valley Sunday-Monday. Meanwhile, the central/southern parts of the East will start out above normal but then trend colder with time toward highs 5-15F below normal next Monday-Wednesday underneath amplified upper troughing. Morning lows next week may be a tad less anomalous but still cold enough to raise some frost/freeze concerns from the north-central U.S. through the Midwest/Great Lakes and Northeast/Appalachians. Expect some moderation by next Thursday. Arrival of a Pacific upper trough will likely bring below normal temperatures to the West by next Thursday. 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