Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri Oct 13 2023 Valid 12Z Mon Oct 16 2023 - 12Z Fri Oct 20 2023 ...Overview... Most guidance shares the general theme of an amplified eastern U.S. trough (with possible embedded upper low) departing while Pacific trough energy reaching the Northwest by Tuesday amplifies into the central U.S. mid-late week. This latter amplification should be in response to a strong east-central Pacific storm originally associated with current Typhoon Bolaven helping to enhance upper ridging expected to build over the western U.S. Western Atlantic low pressure associated with the initial eastern upper trough/low may affect the East Coast to some degree, while a front reaching the West Coast early in the week may bring a brief period of enhanced precipitation to that region. Eastward progression of this front should bring lighter totals into the Rockies and perhaps somewhat heavier activity over parts of the central U.S. later in the week. Temperatures over the West will tend to be above normal aside from a brief midweek moderation while much of the East will see below normal readings especially during early-mid week. ...Guidance Evaluation/Predictability Assessment... Over the course of next week there are two primary forecast issues. The first involves details of eastern U.S. troughing and associated offshore low pressure during Monday-Wednesday. Currently, low-predictability shortwave differences have an important influence on exactly how closed or deep the upper trough will be by Wednesday, with corresponding effects on on the surface low. Among 12Z/18Z guidance, the GFS was on the western side of the model/mean spread due to being deepest with its upper low. On the other hand some other solutions are noticeably more open aloft, leading to a weaker and more offshore surface low. Continuity had been closer to the coast so ultimately favored a 12Z/18Z model blend that yielded a GFS/ECMWF compromise that provided just a modest eastward adjustment while awaiting continuation of the offshore trend (which seems to be the case so far in the 00Z cycle). The other challenge is determining the character of shortwave energy reaching the central U.S. mid-late week. In general ECMWF runs have been somewhat more stable in depicting a moderately progressive and phased trough that is more similar to the means, while some GFS runs (including the new 00Z version) have been pulling off southwestern energy into a High Plains upper low. The 12Z/18Z GFS runs waited until late day 7 Friday to close off a low over the central Plains/Midwest. The new 00Z CMC shows the most extreme southwestward elongation by next Friday, though this is not outside the fairly wide ensemble envelope. The 12Z CMC was more similar to the ECMWF and ensemble means. Increasing uncertainty for details at this later time frame led to transitioning the forecast to a blend of the 18Z GFS/GEFS mean and 12Z ECMWF/ECens/CMC. ...Weather/Hazards Highlights... Expect shortwave energy and a leading cold front to bring a brief period of moderate to heavy precipitation to the central and northern West Coast around Monday, with highest totals likely over southward facing terrain over the Pacific Northwest. There may be sufficient instability to promote some enhanced rainfall intensity on a localized basis but the combination of observed soil moisture/recent rainfall/stream flows suggest any runoff issues should be isolated enough not to merit any risk area in the Day 4 (Monday) Excessive Rainfall Outlook. Lighter precipitation should spread across the northern and perhaps central Rockies as the shortwave energy and front continue eastward. Coverage and intensity of rainfall should increase over the central U.S. by Wednesday-Friday but details become increasingly unclear, with various potential upper level and surface evolutions leading to a wide range of possibilities for location and magnitude of heaviest rainfall. Another area of moisture could reach northern parts of the Pacific Northwest by late next week. Early-mid week upper troughing and possible embedded low over the East should promote offshore low pressure development that may maintain brisk winds for a time near the East Coast. However latest trends for a weaker and/or somewhat more offshore system should keep rainfall on the lighter side. The eastern Great Lakes into the Appalachians may see some light precipitation as well. The amplified trough over the East early in the week will support below normal temperatures over much of the eastern half of the country early in the week, with a broad area of highs 10-15F below normal. Similar anomalies for morning lows should be confined more to the central Plains on Monday and the Gulf Coast/Florida regions Tuesday-Wednesday. Eastern U.S. temperatures should return closer to normal thereafter. The western U.S. will see above normal temperatures for most of next week, with a brief midweek lull separating warmth ahead of a leading Pacific cold front and then a rebound in temperatures as upper ridging builds over the region. Some locations may see one or more days of plus 10-20F anomalies. Rausch 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are 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/#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