Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1209 PM EDT Sun Oct 07 2018 Valid 12Z Wed Oct 10 2018 - 12Z Sun Oct 14 2018 ...Tropical Depression Fourteen Likely to Affect the Gulf Coast and Portions of the East... ...Moisture from Sergio Should Reach into Arizona/Southern Rockies... ...Pattern Overview and Guidance/Predictability Assessment... To commence the forecast on Wednesday morning, what is currently Tropical Depression Fourteen will likely have intensified to near hurricane strength on its approach toward the western/central Florida panhandle. This system is forecast to weaken upon landfall with a general northeastward track along the coastal Carolinas before exiting into the Atlantic. Across the Pacific oceanic basin, Hurricane Sergio will be in the process of accelerating toward the central Baja California coast with a landfall sometime early Friday morning. As usual, the cyclone should shear apart across interior Mexico as it encounters mountainous terrain. Aside from the tropical systems, the longwave pattern will remain quite amplified in nature with leading height falls tracking toward the Mississippi River on 10/1200Z. The compact shortwave migrating toward the Upper Great Lakes will support a lengthy period of cyclogenesis as the parent wave lifts into southern Ontario. The baroclinic zone attendant to the cyclone is forecast to continue marching eastward with possible tropical interactions along the Eastern Seaboard. Shifting focus back to the western/central North America, broad ridging will extend across the eastern Pacific into the immediate West Coast for Days 5-7, October 12-14. This will support another shortwave racing from the vicinity of Alberta/Saskatchewan toward the Great Lakes although a great deal of uncertainty is noted. Well to the south, an upper low may close off along the California/Arizona border with gradual motion downstream to the Southern Rockies by next Sunday. Model spread remains high with the pair of tropical systems although notable trends are worth acknowledging. Beginning with Tropical Depression Fourteen, models which were closer to the Central Gulf Coast have moved eastward with most solutions in the vicinity of the Florida panhandle. It is mainly a timing issue now with the 06Z FV3-GFS well out ahead of the pack while the 00Z ECMWF holds the solution back by about a day from the official forecast. The 00Z GFS/UKMET best represent this track through Day 6/Friday although the latter is much deeper given less interaction with land. Back to Hurricane Sergio, models have trended south in the past day with the 06Z/00Z GFS ahead of the slower 00Z ECMWF. Like the other cyclone, the National Hurricane Center forecast leans more in the GFS direction. Across the remainder of the country, timing issues still loom with the frontal zone sweeping toward the eastern U.S. late in the work week. The 06Z/00Z GFS continue to sit ahead of the 00Z ECMWF while the 00Z UKMET is muddled in the middle. Larger differences exist upstream with the next round of height falls tracking across the international border with Canada down to the north-central U.S. While ensemble spaghetti plots have shown a slow decrease in spread, there are still notable phase differences among the operational models. And finally across the Desert Southwest, there has been a westward trend in the guidance with the migratory closed low. Many ensemble members are even further west suggesting further changes may occur. The preference was heavily weighted toward tropical influences with the 00Z GFS/UKMET being the closest fits to both systems. Thus, over 2/3 of the forecast blend was dedicated to such solutions through Day 5/Friday with smaller contributions from the 00Z ECMWF and 00Z GEFS/ECMWF ensemble means. Thereafter, given the growing spread into next weekend, leaned more toward an ensemble mean-based approach. ...Weather Highlights/Threats... The heavy rainfall which had stagnated over the central U.S. will finally be moving toward the eastern third of the country as the frontal zone ejects eastward. Heavy rainfall will be likely up and down the boundary as tropical moisture from the cyclone intercepts this front. In particular, the 06Z GFS shows some fairly hefty totals along the Appalachian chain given tropical upslope influences. Down along the Eastern Gulf Coast, heavy rainfall accompanied by gusty winds and rough surf are possible in response to the advance of the tropical cyclone. If the more progressive solutions verify, then the precipitation impacts should be shortened in duration. However, the past several runs of the ECMWF and its ensembles continue to be slower. Back across the southwestern U.S. and Four Corners region, heavy rainfall may move into the region as Sergio shears across Mexico. While the circulation center will significantly weaken, its tropical moisture will likely stream into the mentioned regions across the U.S. Up to the central U.S., some threat for precipitation is possible but too much spread exists to hone in on any one sector. Well below average temperatures should continue to overspread large sections of the western/central states. The most significant anomalies will likely center over the northern/central Rockies into the adjacent Great Plains with numbers around 20 to 30 degrees below climatology. This would support highs that will not get out of the 30s through Thursday across the north-central U.S. While chilly conditions will push eastward with the frontal zone, the air mass will modify in time with anomalies back into the 10 to 15 degree range. In advance of the cold front, it will be quite mild with highs around 10 to 15 degrees above climatology over the Great Lakes, Ohio Valley, and northeastern U.S. Although a cool down is anticipated beyond Thursday given abundant precipitation as well as a frontal passage. Most impressively, overnight lows temperatures may sit in the upper 60s to lower 70s within the vast moist/warm sector on Wednesday/Thursday. Current anomalies are in the 20 to 25 degree range with dozens of daily records likely broken given the lack of radiational cooling effects each night. 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