Extended Forecast Discussion NWS Weather Prediction Center College Park MD 435 PM EDT Tue Oct 04 2022 Valid 12Z Fri Oct 07 2022 - 12Z Tue Oct 11 2022 ...Overview... A strong mean ridge, initially near the West Coast on Friday and likely retrograding offshore thereafter, will anchor a fairly amplified upper pattern during the period and support very warm temperatures over the West into at least Monday. Meanwhile a downstream upper trough along with trailing Plains into eastern U.S. high pressure will bring a period of well below normal temperatures through early Monday. Guidance has been waffling with details of a possible shortwave/surface system that may drop into the northern tier around Sunday and then with a larger scale trough expected to reach the Canada and the northwestern U.S. during the first half of next week. A weak upper low may open up/eject from its position over far northwestern Mexico after Sunday but could be replaced by other weak energy descending over the West, keeping a modest mean trough over the Southwest. Overall this pattern will support fairly low coverage/intensity of precipitation over the lower 48. The best defined rainfall focus will be over the Four Corners states and vicinity with the combination of the persistent upper trough/low near the western Mexico border and monsoonal moisture in place. Other areas of rainfall will be possible over the Great Lakes/Northeast early in the period and the Florida Peninsula. Uncertain details of the northern/central U.S. front Sunday onward as well as how much troughing reaches the Northwest will determine corresponding precipitation over those areas during the first half of next week. ...Guidance Evaluation/Predictability Assessment... Within the eastern mean trough, there is still some spread and variability for the leading shortwave energy/surface system crossing southeastern Canada and New England late this week. Relative timing has switched from 24-36 hours ago, with the 00Z ECMWF now a bit on the slower side with other solutions faster to varying degrees. The 00Z UKMET was close to the GFS/CMC but the new 12Z run has strayed faster than the others. An intermediate forecast appears best for this system. Trailing shortwave differences have less impact aside from perhaps how much of a surface trough could brush the Great Lakes/Northeast. The ECMWF has been most consistent with the shortwave and surface low/front reaching the northern tier by Sunday. The 00Z UKMET was similar to the 00Z ECMWF. Remaining models have had varying ideas, including splitting the original Gulf of Alaska energy (00Z GFS) or depicting a slower closed low/trough (00Z CMC/06Z GFS). The new 12Z UKMET trended a little slower and deeper/more closed but the GFS/CMC are now a better fit with the overall cluster. Preference sides with a blended/continuity approach. Spread an run-to-run variability become significant for the trough expected to being amplifying into Canada and the northwestern/northern U.S. early next week. ECMWF/ECMWF runs have tended to show more troughing into the Northwest, with multi-run continuity of the ECMWF overall showing a trend toward more troughing into the Northwest. The 12Z GFS and 00Z CMC are on the other extreme, with a farther east Pacific ridge/downstream trough configuration. The 06Z GFS was closer to the latest GEFS/CMCens runs that are just a little east of the ECMWF mean. Multi-day D+8 means show some differences in the position of the expected core of positive height anomalies over the northeastern Pacific/Gulf of Alaska but teleconnections relative to this anomaly center generally support some degree of trough elongation into the West near and soon after the end of the medium range period. This would at least recommend leaning away from the 00Z/12Z GFS and 00Z CMC. Guidance preferences ultimately led to using a 00Z/06Z model composite early in the period, followed by trending the forecast to even proportions of the 06Z GFS/00Z ECMWF, their ensemble means, and WPC continuity. For the QPF, the National Blend of Models once again seemed too confined with its coverage, especially across the Southwest and for early-period lake effect/terrain enhanced activity in the Northeast. The NBM also appeared somewhat underdone with rainfall over the central U.S. early next week with the emergence of some Southwest monsoonal moisture and possible frontal approach, though with low confidence in the magnitude. ...Weather/Hazards Highlights... The combination of persistent monsoonal moisture and an upper low over the northern Gulf of California will produce diurnal showers and thunderstorms over parts of the Southwest/southern Rockies through the weekend, with locally heavy rainfall possible in some spots. Intensity and coverage may vary from day to day. The upper low should open up after Sunday but may be replaced by additional weak energy. This evolution may help to spread some of the moisture eastward into the Plains where it could interact with a front, while leading to a somewhat lighter and more scattered trend for rainfall over the Southwest. The system anchoring the Plains front could also bring light rain into the Midwest/Great Lakes but with low confidence in specifics at this time. Upper troughing and a cold front reaching the Northwest/northern Rockies by next Tuesday could bring some rain and perhaps even high elevation snow into region, again with fairly low confidence for now. In the East, lake effect/terrain enhanced precipitation should taper off after Friday. Light to locally moderate rainfall will likely spread northward over the Florida Peninsula as easterly low level flow strengthens behind a front that ultimately replaces an initial front stalled near the Keys. Much of the West will see above normal temperatures at least through Monday, with highs 10-15F or so above normal most likely over the Northwest. Even with these anomalies, the resulting temperatures peaking in the 70s and 80s (90s for parts of California) are unlikely to create any more than isolated/moderate heat related issues. Expect a cooling trend by Tuesday. On the other hand, an area of well below normal temperatures will extend from the northern two-thirds of the Plains into the East late this week into early Monday. Some areas may see highs 15-20F below normal Friday-Saturday. Morning lows may be a little less extreme but should still drop into the 30s for many locations from the northern half of the Plains into the central Appalachians, and even 20s for North Dakota and northern Minnesota on Friday morning. Rausch/Santorelli Hazards: - High winds across portions of the Great Lakes, Fri, Oct 7. - Much below normal temperatures across portions of the Central Plains, the Northeast, the Central Appalachians, the Northern Plains, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Upper Mississippi Valley, the Southeast, the Great Lakes, and the Ohio Valley, Fri-Sun, Oct 7-Oct 9. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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