Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 AM EST Sat Nov 16 2024 Valid 12Z Tue Nov 19 2024 - 12Z Sat Nov 23 2024 ***Heavy rain for both the Pacific Northwest and the Central Gulf Coast through the middle of the week*** ...Overview... A rather active weather pattern is expected to be in place across the Continental U.S. next week. A large cut-off upper low is likely to develop over the Great Lakes and Ohio Valley region by the second half of the week, with colder weather moving in across this region, and a strong surface low is looking more likely to develop near the East Coast. In terms of the Tropics, although Tropical Storm Sara will likely remain well to the south across Central America and the far southern Gulf, a plume of enhanced moisture is likely to advect northward across the Gulf and bring the potential for heavy rain ahead of an approaching cold front for portions of the central Gulf Coast and the Florida Peninsula. Meanwhile, rain and mountain snow returns to the Pacific Northwest by midweek as multiple Pacific storm systems approach the region. ...Guidance/Predictability Assessment... Overall, the latest 00Z model guidance has come into better agreement on the overall synoptic scale set-up across the continental U.S. through the middle of the week. A potent southern stream shortwave will lead to the development of a deep and cut-off upper low over the Ohio Valley/Great Lakes region, and the guidance has also come into better agreement over the past couple of runs, in contrast to the big run-to-run differences seen yesterday. Out west, there are some mesoscale differences with a series of Pacific storm systems that will be impactful to the Pacific Northwest. The ensemble means were gradually increased to about 50% by next Saturday amid growing model differences, mainly across the Western U.S. as the flow pattern from the eastern Pacific becomes more uncertain. ...Weather/Hazards Highlights... Heavy rain is expected to make weather headlines across the central Gulf Coast region during the Day 4 period Tuesday. A surge of deep tropical moisture, some of which originates from Tropical Storm Sara well to the south, will fuel multiple rounds of heavy showers and thunderstorms ahead of a cold front. Given the potential for some 2-4 inch rainfall totals in a few spots, the existing Slight Risk area has been expanded some for the Day 4 ERO to include the Florida Panhandle, southern Alabama, and far southwestern Georgia. For the Pacific Northwest, multiple days of heavy precipitation associated with atmospheric rivers ahead of slow moving Pacific fronts will likely lead to several inches of QPF extending from northwestern California to the Olympic Peninsula, and this includes both the Day 4 and 5 periods where a Marginal Risk is in effect. The pattern does not change much going into the end of the week with additional heavy rainfall likely, and heavy snow for the Cascades. A dynamic storm system is forecast to develop over the Ohio Valley and then across the Great Lakes and Northeast U.S. as a deep upper low forms, and a potentially strong surface low developing near the Mid-Atlantic and then towards New England. The cold air aloft from the upper low, in combination with increasing moisture, will likely result in the first significant snowfall of the season for the central Appalachians, and perhaps a lake effect snowfall event by next Saturday as colder air moves over the warmer lake waters, especially across elevated downwind terrain. Depending on how strong the surface low gets, windy conditions and heavy rain near the coast could also be a problem. Temperatures are expected to be quite chilly across the Rockies and adjacent portions of the Intermountain West and then Plains in the wake of cold frontal passages, with a strong surface high building in. Some overnight lows could be near or even below zero for portions of the central and northern Rockies Tuesday and Wednesday mornings. Meanwhile, warm and humid pre-frontal conditions are likely for the Gulf Coast and to the Southeast U.S. with deep southerly flow from the Tropics through midweek. Pre-frontal warmth farther inland across the east-central U.S. and East/Northeast will be followed by a return to much cooler fall-like conditions to close out the work week. Hamrick 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