Extended Forecast Discussion NWS Weather Prediction Center College Park MD 223 PM EST Sat Nov 16 2024 Valid 12Z Tue Nov 19 2024 - 12Z Sat Nov 23 2024 ...Heavy rain likely for the central Gulf Coast Tuesday... ...Heavy precipitation with long duration atmospheric river(s) in the Pacific Northwest and northern California... ...Overview... A rather active weather pattern is expected to be in place across the lower 48 next week. A large cutoff upper low is likely to develop over the Great Lakes and Ohio Valley region by the second half of the week, which will bring in cooler temperatures along with windier conditions as a surface low tracks across that region toward the Northeast. Ahead of this consolidating upper low, enhanced tropical moisture will 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 on Tuesday. Meanwhile, northeastern Pacific surface lows are forecast to bring in heavy precipitation to the Pacific Northwest and northern California as one or more strong atmospheric rivers take aim at the coastal areas. ...Guidance/Predictability Assessment... Model agreement is pretty good with the larger scale features of the pattern through much of next week. Consolidating energies from the Four Corners region, south-central Canada, and perhaps retrograding from the Great Lakes will produce a deepening upper low moving across the Great Lakes/Ohio Valley around Thursday, around 2 standard deviations below normal for 500mb heights. The upper low should be slow to move eastward into late week, and models vary somewhat with the low's eventual placement somewhere over the northeastern U.S. or nearby southeastern Canada or western Atlantic. Associated surface lows have similar spread but overall a model/mean blend seemed to be a good middle ground for these features. Mean upper ridging is forecast to set up over the interior West behind the eastern upper low. By later week the northeastward extent of the ridge is in question depending on where the eastern upper low ends up. Then into the eastern Pacific, the first compact upper low and deep surface low around midweek are in reasonably good agreement among models, yielding higher confidence in the strong AR, though the newer 12Z GFS does have the AR taking aim farther north of the non-NCEP models by Wednesday. Secondary lows show more spread in the models. The ECMWF has an upper low and surface low rather close to the coast by Friday, which WPC leaned against as it was against consensus of other models and AI/ML models. The newer 12Z GFS and ECMWF fortunately seem to be in better agreement with this system. The WPC forecast was based on a blend of the deterministic models early in the period, with a gradual increase of the ensemble means to half by Day 6 and 60 percent Day 7 amid increasing model spread. This showed reasonable continuity from the previous forecast. ...Weather/Hazards Highlights... As the medium range period begins Tuesday, deep tropical moisture (PWs likely over the 95th if not 99th percentile) in part from what is currently Tropical Storm Sara well to the south will surge into the southeastern U.S. ahead of a cold front. Instability should help create heavy rain rates of multiple inches per hour across the central Gulf Coast and there is some potential for backbuilding storms. A Slight Risk remains in place for portions of the central Gulf Coast into the Southeast. Expect the southern side to be a high-end Slight Risk, though storm motion potentially being on the faster side could limit widespread or significant flooding concerns that would be more to Moderate Risk levels. For the Pacific Northwest, multiple days of heavy precipitation associated with atmospheric rivers ahead of slow moving Pacific fronts should lead to several inches of QPF accumulating from northwestern California to the Olympic Peninsula. The peak of the AR is likely to come onshore around Tuesday night, and looks to stall on Wednesday as a mesoscale frontal wave propagating along the front causes it to meander. Added a Slight Risk to the Day 5/Wednesday ERO for this forecast cycle, as many hours of rain that may have 1" per hour rain rates could fall in the same areas. Burn scars will be particularly sensitive to heavy rain that occurs. Despite some continued model spread in exactly where the moisture plume will stall, drew in the Slight Risk from northwestern California to southwestern Oregon where the best model consensus is. Onshore flow is forecast to continue into later week for additional coastal heavy rain and higher elevation (Cascades) heavy snow. 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, along with a reasonably deep surface low. The cold air aloft from the upper low, in combination with increasing moisture, will first cause some accumulating snow in the north-central U.S. on Tuesday. Then the first significant snowfall of the season is likely for the central Appalachians, and perhaps some lake- enhanced and higher elevation snow in the Northeast Friday- Saturday. Windy conditions are expected in the northern Plains Tuesday-Wednesday behind the surface low, with some breezy conditions into the east-central U.S. and possibly the Northeast. Heavy rain may impact the Northeast coastal areas as well, with amounts partially dependent on the depth and position of the surface low as it may redevelop offshore. 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. Western areas are forecast to warm to above normal into later week as upper ridging develops. 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 workweek. Tate/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