Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 PM EST Mon Nov 11 2024 Valid 12Z Thu Nov 14 2024 - 12Z Mon Nov 18 2024 19Z Update: Overall, the 12Z model guidance is in excellent agreement across the Continental U.S. for both Thursday and Friday, and therefore a multi-deterministic model blend works well as a starting point in the forecast process. The UKMET is heavier with QPF across portions of the Carolinas that does not have much support from the model consensus, but otherwise it matched up well elsewhere. Beginning late Saturday, the CMC becomes more progressive with the trough across the Desert Southwest and then ejects it across the Southern Plains, whereas the GFS/ECMWF favor a closed low that is slower to lift out. Even greater differences emerge over the Pacific Northwest by next Monday, and this is where an ensemble mean approach works best, and ensemble means accounted for about 40-50% of the forecast by that time. No changes were needed for the Day 4 and 5 EROs, which remain void of any excessive rainfall areas given the relatively uneventful QPF that is expected. The previous forecast discussion is appended below for reference. /Hamrick ----------------------- ...Overview... A shortwave into the East as the period begins Thursday will bring light to moderate much-needed precipitation to parts of the Ohio Valley/Mid-Atlantic/Northeast on Thursday. This energy will exit the coast by Friday, combining with an upper low near Maine that lingers for a few days. Upper ridging will progress from the Central to Eastern U.S. bringing a return to seasonably warm temperatures. Meanwhile, a trough entering the West on Thursday will deepen as it slowly shifts east next weekend and into early next week. Another shortwave into the Pacific Northwest will bring a chance for enhanced precipitation to that region. ...Guidance/Predictability Assessment... The guidance continues to show good agreement on the overall large scale pattern across the CONUS for the upcoming medium range period, but plenty of uncertainty in the details and evolution of individual systems. Models have trended better in terms of timing/amplitude of the shortwave into the East, though some question on energy as it gets wrapped into the upper low parked off the Northeast coast. The trough into the West also shows good agreement the first half of the period, but by Sunday and Monday details get highly uncertain. There is some suggestion that an upper low may break off over/near Southern California around Sunday and the timing/evolution of this progressing eastward into Monday has big implications for the late weekend precipitation pattern across the south-central Plains. The ECMWF is much quicker to weaken the low and move it east, but the new 00z run (available after forecast generation time) did slow down. The GFS remains the strongest and slowest. WPC prefers a middle ground solution closest to the CMC and ensemble means. The WPC forecast blend for tonight used an equal blend of the deterministic guidance (minus the UKMET which was slower with the Eastern shortwave) for the first half of the period. After this, increased the ensemble means to 70 percent of the blend by Day 7/Monday to account for rapidly lower confidence in the flow specifics especially out West. ...Weather/Hazards Highlights... A shortwave into the Pacific Northwest on Thursday will continue to bring some additional rainfall and higher elevations snows, but the heaviest and most impactful precipitation should be in the short range period. Precipitation across the Northwest should trend lighter Thursday-Friday with moderate mountain snows continuing across the northern Rockies and into the Sierra Nevada range. Across the East, modest rainfall should accompany a cold front as it passes through the East on Thursday, but given much of this region remains in a drought, rainfall should be more beneficial than hazardous. As such, the Days 4 and 5 (Thursday-Friday) Excessive Rainfall Outlooks are blank across the CONUS. After Friday, moisture on the north side of a surface low ejecting into the Northern Plains may bring some light rain or snow from eastern Montana to the Upper Midwest. As the western trough deepens, it may begin to tap moisture to increase precipitation chances across the south-central Plains. However, extent and amounts of this rainfall remains unknown given the uncertainties in the evolution of this trough over the Southwest. A northern stream shortwave into the Pacific Northwest should bring another round of modest rain and mountain snows to western Washington/Oregon and the Cascades next weekend. Above normal temperatures will progress eastward with time from the northern Plains/Midwest Thursday-Friday, enveloping the East by Sunday into Monday. Daytime highs could be 10-15 degrees above normal in some spots. Out West, temperatures will trend much cooler with time as upper troughing amplifies over the region and persists into early next week. 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