Extended Forecast Discussion NWS Weather Prediction Center College Park MD 227 AM EDT Fri Oct 05 2018 Valid 12Z Mon Oct 08 2018 - 12Z Fri Oct 12 2018 ...Central U.S. Excessive Rainfall Threat/Active Western U.S. Pattern... ...Separate Areas of Tropical Moisture Possibly Arriving into the Gulf Coast and Southwest/Southern Rockies... ...Pattern Overview and Guidance/Predictability Assessment... The forecast period will start on Mon with a deep neutrally-tilted western U.S. trough between two very strong ridges (max heights 594+ dm) over the eastern Pacific and Mid-Atlantic region/western Atlantic Ocean. The majority of guidance agrees in principle that with the passage of time the mean trough will take on more of a positive tilt as the eastern Pacific ridge slowly weakens and the East Coast ridge flattens. For specifics within the overall trough, one uncertainty will be with how energy over the Gulf of Alaska/northwestern North America at the start of the period digs into the mean trough and in the process ejects initial trough energy into the Plains and beyond. There are also question marks over how this ejecting trough energy will interact with energy that may be lingering over southern Canada. Models/ensemble means provide decent clustering regarding consolidation of low pressure over or near the upper MS Valley by around early day 5 Wed in association with the ejecting western energy but with a fair amount of spread on exact character of the low due to detail differences aloft. This system should provide a pronounced eastward push for the wavy moisture-focusing front over the Plains during the first half of the period. Upstream Pacific energy will intrude upon the ridge off the West Coast later in the period with solutions providing meaningful differences. There will be two features of interest in the tropics. Guidance indicates that the evolution of flow aloft will encourage Hurricane Sergio in the eastern Pacific to track northeastward and potentially bring another episode of significant rainfall to parts of the Southwest/southern Rockies in the latter part of the period. GFS runs are on the faster side of the spread but recent trends and occasionally slow biases of the ECMWF suggest that the latter model and its mean may be too slow to bring the system and its moisture northeastward. Meanwhile NHC's tropical weather outlook shows a medium/50 percent chance of tropical cyclone formation over the next 5 days for a disturbance that is over the extreme western Caribbean and may ultimately track northward through the Gulf. To varying degrees other operational models suggest a better defined system than the GFS. Guidance spread is very wide for this feature so confidence is below average at this time. The updated forecast reflects a combination of continuity and modest adjustments reflected in the majority of guidance. The first half of the forecast placed more emphasis on the operational solutions (18Z GFS and 12Z ECMWF and less 12Z CMC) while the latter part of the forecast leaned almost exclusively to the 18Z GEFS/12Z ECMWF means given the increasing detail uncertainty. Manual adjustments were made for Hurricane Sergio and yesterday's coordinated track for the possible tropical feature in the Gulf of Mexico. ...Weather Highlights/Threats... During the first half of the period the combination of low level Gulf inflow and very slow motion of a wavy front will likely result in an excessive rainfall threat over portions of the central U.S., with episodes of heavy rainfall/convection and potential training. During the latter half of the week the front should progress into the East with the faster motion promoting lower rainfall totals than expected over the Plains and vicinity. However the potential tropical feature tracking through the Gulf--which may bring enhanced moisture to parts of the central/eastern Gulf Coast region--could contribute additional moisture to the eastern front depending on the relative timing of the two features. Meanwhile deep-layer Atlantic flow may bring moisture into the Southeast Coast. The amplified western trough aloft will generate periods of rain and snow with cold air extending far enough south to bring snow potential into the central Rockies for at least a part of the period. Expect some precipitation enhancement from terrain/upslope flow over portions of the Great Basin/Rockies/High Plains. Highest 5-day totals should be over the northern-central Rockies and eastern Great Basin. Farther south there is increasing potential that moisture from Hurricane Sergio will spread northeastward and reach Arizona and the southern Rockies late in the week. This moisture will have to be monitored closely as it may affect some locations that recently received significant rainfall from Rosa. There will be a persistent core of much below normal temperatures from the northern Plains/Rockies into the Southwest with the northern half of this area likely to see high temperatures 20-30F below normal on multiple days. On the other hand most of the eastern half of the country will see much above normal temperatures. Highest anomalies should be for morning lows with plus 20F or greater anomalies possible over the Ohio Valley/lower Great Lakes/central-northern Appalachians. Some of these readings may exceed daily record warm low values. Rausch 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