Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Wed Oct 9 2024 Valid 12Z Sat Oct 12 2024 - 12Z Wed Oct 16 2024 ...Desert Southwest record heat moderates/lingers into Saturday... ...Overview... Guidance continues to show development of an amplified ridge-trough pattern over the lower 48, downstream from a deep system just south of Alaska. A shortwave diving through the north-central U.S. this weekend will begin the process of amplifying eastern U.S. troughing and support a Northeast surface system, with multiple trailing impulses helping to reinforce the trough. Much below normal temperatures and frost/freeze may be a concern over some areas from the Plains into the East. The front leading the colder trend should suppress this weekend's central-southern U.S. heat that may challenge daily records at some locations. The upper ridge over the Rockies will support persistent warmth over the Interior West and Rockies, focusing more over the northern Rockies/High Plains toward the middle of next week. Expect the leading edge of a Pacific trough to push a front and some accompanying rainfall into the Northwest by next week. ...Guidance/Predictability Assessment... Comparisons of latest dynamical and machine learning (ML) guidance show good agreement for the large scale pattern evolution overall, with the most prominent uncertainties being the specifics of surface low development over the northeastern quadrant of the lower 48 around Sunday-Monday and how energy evolves within the eastern U.S. upper trough by next Tuesday-Wednesday. Solutions have been waffling for precise strength and track of the Sunday-Monday surface system, with sensitivity to details aloft. At the very least, ML solutions offer good support for more of a closed upper low and better defined surface system than what multiple CMC runs have been depicting. GEFS/ECens means have been tilting toward a more occluded system reaching New England than depicted in the 00Z ECMWF or past couple UKMET runs, but the ML model spread includes the range of dynamical model details. The dynamical model average has weakened the surface low somewhat versus the prior 12-hourly cycle, with the average of ML models fairly close to the dynamical guidance. Later in the period, model/ensemble spread develops regarding whether an upper low could form within the eastern U.S. trough. For now such an upper low is a minority solution, so the deterministic preference continues to be the open trough per a blend/ensemble mean approach. There was hardly any guidance support for the Midwest upper low in the 00Z CMC, while the 12Z CMC has at least trended eastward with it toward the 00Z GFS (which was still a bit on the western side among the subset of guidance that had a closed low). On the other hand, the new 12Z GFS becomes more progressive with the trough versus the majority. Latest consensus has the Pacific upper trough axis reaching just offshore the West Coast by early next Wednesday. ML models show similar or slightly faster timing, but multi-day trends have been a little slower--favoring the dynamical guidance for the time being. Guidance considerations led to starting the early-middle part of the forecast with a blend of the 06Z GFS, 00Z and 12Z/08 ECMWF, and a small weight of 00Z UKMET. Then the blend added in some of the 06Z GEFS/00Z ECens means by next Tuesday-Wednesday with the increasing detail uncertainty. ...Weather/Hazards Highlights... 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 into 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 and light to moderate rainfall into the region by the start of next week. The amplifying upper trough and accompanying surface system affecting the East late this weekend into early next week should produce some areas of locally enhanced rainfall across parts of the Great Lakes/Northeast. Little if any instability and fairly modest moisture anomalies even on the higher side of the spread suggest that short-term rain rates will likely not be high enough to pose a flash flood threat. Therefore the Days 4-5 Excessive Rainfall Outlooks continue to show no risk areas. An upper ridge aligned over/near the Rockies for most of the period will maintain well above normal temperatures over the Southwest/Interior West into Rockies, and extending into the northern High Plains toward midweek as the ridge shifts eastward a little. The central/southern Plains will also have a very warm start to the weekend before northern stream troughing and a cold front suppress above normal readings to the south/southeast. Greatest anomalies of plus 10-20F should be over the northern Rockies multiple days and eventually the northern High Plains, as well as the southern Plains during the weekend. Daily records will be possible from the Southwest through the southern Plains this weekend. Meanwhile, the East should see some near to slightly above normal temperatures this weekend followed by a colder trend 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. 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