Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sat Oct 06 2018 Valid 12Z Tue Oct 09 2018 - 12Z Sat Oct 13 2018 ...Central U.S. Excessive Rainfall Threat Extends Into Mid-week... ...Separate Areas of Tropical Moisture May Reach into the Gulf Coast-Southeast and Southwest-Southern Rockies... ...Active Pattern Continues over the West Through at Least Mid-week... ...Pattern Overview and Guidance/Predictability Assessment... It will prove to be a very active period with multiple hazards to monitor next week. Amplified flow across the country will feature impressive height anomalies as shown by the latest model guidance. On Tuesday morning, a mid-level low centered over the Southern Rockies should be comprised of anomalies around 3 to 4 sigma below climatology while a sprawling 594-dm ridge around 2 sigma above average sits over the western Atlantic. This former feature is expected to lift up through the Central Plains late Tuesday while attaining a negative tilt. The induced area of low pressure is forecast to press north-northeastward while deeping on its approach into the Upper Great Lakes on Wednesday. Eventually the southern extension of these height falls will force the western Atlantic ridge farther downstream. Shifting focus back to the west, a renewed area of lower heights will sink down from British Columbia thus re-establishing a trough across the western U.S. by mid-week. Various scenarios are evident thereafter but the general idea is for this system to drag eastward toward the Rockies and Great Plains by early next weekend. Looking down in the Tropics, a pair of threat areas are worth noting. Across the Baja California peninsula into the Desert Southwest and Four Corners region, all eyes are on Hurricane Sergio. Although it will be in the process of decaying as it crosses the cooler eastern Pacific waters, its moisture is expected to move into the southwestern U.S. into the Southern Rockies by as early as Thursday. Farther east, an area of disturbed weather sitting over the western Caribbean has become better organized the past 24 hours. The National Hurricane Center currently supports an 80 percent chance for tropical cyclogenesis the next 48 hours. What evolves should lift into the middle of the Gulf of Mexico with many scenarios in play. Regarding the guidance, there has been an improved handling of the initial shortwave/attendant surface low lifting through the Great Plains and Great Lakes on Tuesday/Wednesday. The previously quicker GFS runs have come back to the rest of the pack with ensemble low scatter plots much lower spread than previous cycles. The 00Z ECMWF trended quite a bit deeper in its latest run which brings it closer to the consensus which is around 995-mb on Wednesday night. Downstream ridging should force the system up toward northern Quebec while high pressure builds over the north-central U.S. late in the week. Moving west, the next round of height falls dropping down in the Pacific Northwest has a few unknowns attached with it. Some of the latest ensembles in the spaghetti plot have shown more amplification carrying shortwave energy into the Desert Southwest. The 00Z ECMWF ensembles are most emphatic about this evolution with other model suites farther downstream. This leads to considerable timing differences as the longwave trough tracks eastward. The 00Z GFS sits well ahead of the 00Z ECMWF while the 00Z CMC positions itself somewhere in the middle. Back to the Tropics, the 06Z/00Z GFS are out ahead of the 00Z ECMWF with Hurricane Sergio but it appears the latest National Hurricane Center forecast leans more toward the former camp. Eventually the surface low will shear while encountering the higher terrain of Mexico late in the week. Moving to the Gulf of Mexico, there is increasing confidence of a defined area of low pressure primed to lift northward toward the Central/Eastern Gulf Coast sometime the middle of next week. Solution spread is high with position and timing which reduces confidence in the determinitic forecast. Notably quick are the 06Z GFS/00Z CMC while the 00Z ECMWF still attempts to stall the feature across the Gulf Coast, albeit in a more weakened state. Additionally, the GFS/UKMET runs sit east of the ECMWF/CMC which has been a theme the past day or two. Overall, uncertainty remains but the region bears watching given the tropical implications. The medium range package was driven by a combination of the overnight continuity along with some of the latest model guidance. Attempted to lean a bit more toward the 00Z ECMWF/ECMWF ensemble mean although incorporated the 00Z GFS/UKMET into the mix as well. The 00Z UKMET is rather aggressive with the cyclone strength moving toward the Florida panhandle so attempted to weight it less in the blend. Toward the end of the period, mostly went toward an ensemble approach led by the 00Z ECMWF ensemble mean with smaller contributions from the 00Z GEFS mean. ...Weather Highlights/Threats... Threats will be numerous during the period given the amount of amplification and tropical moisture affecting the U.S. Cold temperatures will prevail across the western U.S. extending into the Northern/Central Plains as well as the Middle/Upper Mississippi Valleys with 20 to 30 degree departures from climatology centered over the Central/Northern Rockies into the adjacent Great Plains. While daily records are not anticipated, it should prove to be rather chilly for several days as highs struggle to escape the 30s along the northern tier while overnight lows plunge into the 20s from the Central Great Basin through the interior western terrain and into the north-central states. Farther downstream within the vast warm/moist sector, well above average temperatures are likely, particularly with the overnight lows given limited radiational cooling effects. Anomalies around 20 to 25 degrees above climatology are noted over the Ohio Valley and Great Lakes extending into the northeastern U.S. The overnight warmth will likely lead to several daily records being broken over a rather broad region. With tropical moisture pumping northward ahead of the trough advancing into the Great Plains, heavy rainfall will continue to be an issue across the Middle/Upper Mississippi Valleys. This would fall on top of anything that occurs before the medium range which suggests hydrologic issues may be possible. Additionally, comma-head precipitation, much in the form of snow will be likely on the backside of the system. This would primarily impact the Dakotas with additional activity back into the Rockies as well. The cold front will advance eastward spreading a broad axis of moderate to locally heavy rainfall with it. Shifting focus to areas of tropical impacts, conditions should become wet across sections of Arizona/New Mexico into the Southern High Plains as the tropical moisture from Sergio surges inland. Rubin-Oster WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml