FXUS02 KWBC 151933 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 333 PM EDT Tue Oct 15 2024 Valid 12Z Fri Oct 18 2024 - 12Z Tue Oct 22 2024 ...Overview... Deep upper troughing with a developing closed low in the Interior West will lead to a cool and wet pattern there from late this week into the weekend, including higher elevation snow and frost/freeze concerns in some of the valleys. Heavy rain chances could increase in the southern/central Plains by the weekend. Upper ridging ahead of this trough/low will promote warmer than average temperatures in the northern/central Plains to Midwest. The ridge should gradually warm the East as well and promote dry conditions for the most part, except for South Florida. Additional rounds of precipitation may come into the Pacific Northwest late week into the weekend. ...Guidance/Predictability Assessment... Most guidance still agrees with the large scale pattern. This evolution consists of eventual ejection of the Southwest/Four Corners closed low into the Plains/Midwest as the leading edge of an elongated mean trough over the northeastern Pacific/western Canada pushes southeastward, while weak shortwave energy over the southern Appalachians/Southeast may separate a couple ridges that will comprise a larger scale overall mean ridge covering the East. The primary model discrepancies involve the eventual progression of the upper low and details of influencing upstream shortwave energy. Most machine learning (ML) models offer the best support for the intermediate timing of recent ECMWF runs. Latest GFS runs shift the upper low a little north of consensus by around Sunday or so and then stray faster, though incoming northern tier energy ultimately yields a mean trough not too far off the guidance average for position. By Day 7/Tuesday the GFS details aloft result in a deeper southern Canadian surface low compared to other dynamical or ML models. In contrast, per the ML models the 00Z ECMWF could be underdone with surface low pressure due to slow timing of upstream shortwave energy. CMC runs represent the slower extreme for the ejecting upper low, though the new 12Z version has adjusted closer to the favored intermediate timing. Also of note by the end of the period, dynamical/ML guidance develops significant divergence for how energy will be distributed within the western Canada into eastern Pacific mean trough aloft. This spread begins to influence sensible weather over the Northwest by next Tuesday but has the potential to produce more significant spread and run-to-run variability over more of the lower 48 thereafter. Guidance comparisons led to starting the updated forecast with a composite of 00Z/06Z operational models, replacing the GFS with the GEFS mean around mid-period and then splitting ECMWF input between the 00Z/15 and 12Z/14 runs. The 00Z ECens run was not available locally for blending purposes, so the last part of the forecast consisted of those ECMWF runs, the 06Z GEFS mean, and WPC continuity. ...Weather/Hazards Highlights... The upper low closing off across the Southwest late week into the weekend will push precipitation into the Four Corners states, Rockies, and High Plains. Expect enhanced precipitation totals in terrain with higher elevation snow, which will be the first snow of the season in some locations given the warm Fall in the West thus far. The Day 4/Friday Excessive Rainfall Outlook has introduced a Marginal Risk area centered over the central portion of northern New Mexico into far southwestern Colorado in light of guidance signals for potentially high localized rain rates over terrain that may be more sensitive after rainfall during the Day 3 period, with the ECMWF also indicating some pockets of instability. Some precipitation could continue across the central/southern Rockies into the weekend. By later Friday into the weekend, Gulf moisture may stream in just ahead of the upper low for increasing rain chances in the central/southern High Plains. While forecast details vary, models have been fairly persistent in showing some moderate to heavy rain potential focused in northeastern New Mexico and nearby, with some modest instability and slow movement of the upper low around Saturday in particular. Thus the Day 5/Saturday ERO continues to depict a Marginal Risk in that region and heavy rain would be most concerning atop sensitive burn scar areas. Latest guidance still suggests that the south- central Plains will continue to be a focus for enhanced precipitation on Sunday before rainfall finally moves eastward Monday, though with decreasing intensity. Meanwhile, the Pacific Northwest will likely see more precipitation during the period. Initial focus for enhanced totals will be over the Olympics and vicinity Friday into the weekend, on the southern periphery of a strong jet pushing into British Columbia. With time, expect upstream shortwave energy to expand the precipitation shield southward and eastward but with gradually lesser max amounts. Uncertainty about specifics of eastern Pacific/West Coast flow aloft decreases confidence in precipitation details over the Northwest by next Tuesday. Meanwhile, the eastern U.S. is forecast to be generally dry late week into early next week, with the exception of parts of the Florida Peninsula. There, return flow of moisture near a meandering front could increase rain and thunderstorm chances especially for South Florida by Friday and into the weekend. There could be a nonzero chance for flash flooding with this activity especially if heavy rain rates occur in the sensitive urban areas. However, as models continue to vary with how much rain may fall onshore versus offshore, the Days 4-5/Friday-Saturday EROs continue to depict no risk areas. Will continue to monitor this region for any trends in model clustering. An upper ridge aligned over the east-central U.S. Friday will promote above normal temperatures in its vicinity. The biggest anomalies of plus 10-20 degrees are forecast across the Midwest. Meanwhile another round of chilly morning lows may cause frost/freeze concerns for the Interior Northeast and Appalachians. Northeastern temperatures will rebound during the weekend when upper ridging moves overhead, bringing anomalies up to 5-10 degrees above normal. Then the ridge rebuilding over the central into eastern U.S. early next week will lead to warmer than average temperatures in the northern Plains to Upper Midwest once again, with expanding coverage of plus 5-15 degree anomalies over the East. On the other hand, the upper trough and frontal surges in the West will cause below normal temperatures (by up to 10-20 degrees for highs) to spread across the Great Basin/Intermountain West and Southwest on Friday with slow moderation into the weekend. These colder temperatures may also lead to additional frost/freeze concerns for any remaining susceptible growing areas/valley locations. Rausch/Tate 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 $$