Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Fri Oct 18 2024 Valid 12Z Mon Oct 21 2024 - 12Z Fri Oct 25 2024 ...Overview... A closed upper low tracking across the Great Plains will produce some scattered shower and thunderstorm activity across the region on Monday, before opening up and phasing with a northern stream trough that will amplify and sharpen over the East by midweek. This latter trough will push a cold front through the eastern half of the country with most associated rainfall likely to be over the Great Lakes/Northeast. A trailing upper ridge will prevail across the western and eventually central CONUS through next week. An elongated trough extending into the Pacific into the start of next week will contribute to a weakening atmospheric river event across the Pacific Northwest next Monday, followed by an upper low closing off over the eastern Pacific. Progression of this system should renew potential for rainfall and perhaps high elevation snow over the Pacific Northwest toward the end of next week. An anomalously warm airmass over the northern tier will shift into the Northeast through midweek. ...Guidance/Predictability Assessment... With guidance finally coming into good alignment for the upper system tracking through the Plains/Midwest early in the week, the primary focus of the forecast will be on the splitting of initial western Canada into eastern Pacific troughing and then tracking the evolution of separate resulting features--in particular a mean trough reaching into eastern North America and an upper low that closes off over the eastern Pacific. Between these, upper ridging that builds over the West should approach or reach the central U.S. by late week. Models/ensembles are still at least more similar in principle than they can be sometimes in such flow separation scenarios, though meaningful differences/trends remain. Northern stream upper trough energy progressing through central/eastern North America has exhibited better clustering in latest runs with a trend toward a sharper and more neutrally tilted feature by the time it reaches the Great Lakes and New England, leading to a more pronounced surface front. Meanwhile, differences still exist for the strength/location of western U.S. upper ridging and in turn for the timing of the eastern Pacific upper low/associated surface system. Upstream flow diving southeast from the Aleutians/Bering Sea may begin to have some influence on this system by the end of the week as well. Latest machine learning (ML) models have trended toward the upper ridge being more over the Interior West/West by midweek or so versus being farther westward 24-36 hours ago, while remaining guidance leans away from the stronger side for residual shortwave energy that could be over the Northwest for a time (per UKMET runs and the 00Z ECMWF). Relative to 6-7 day forecasts, spread for eastern Pacific system timing late next week looks fairly typical. However the 12Z ECMWF/ICON have trended to the slow side of the spread while the 12Z CMC is on the faster side and 06Z/12Z GFS runs have adjusted to an intermediate timing after being somewhat faster in the 00Z run. As for surface low track, the average of ML models would suggest a path toward Vancouver Island, versus 06Z/12Z GFS runs that are south or the 00Z ECMWF that is deep and farther northwest. Latest GEFS mean runs have been suspiciously weak and thus could not be used in the forecast. The updated forecast started with a blend of the 06Z GFS, 00Z and 12Z/17 ECMWF runs, and some 00Z CMC/UKMET for the first half of the period and then added only slight weight of the 00Z ECens/CMCens means late in the period (replacing the UKMET and a fraction of the GFS). This blend was still close to the means in principle over the East while maintaining reasonable definition with a favored track for the system expected to affect the Northwest by next Friday. ...Weather/Hazards Highlights... This weekend's Four Corners upper low will shift into the Central Plains by Monday, marking the end of an active period across the Four Corners/High Plains. Some lingering showers and thunderstorms will impact parts of the Central/Southern Plains and to a lesser degree Middle Mississippi Valley on Monday. The upper low will track far enough north that it will avoid tapping moisture from the Gulf of Mexico, which will keep rainfall totals in the light to moderate range. Any rain that does fall will be beneficial given the dry ground conditions across much of the central U.S. Then a northern stream cold front may produce some light rain across the northern tier, possibly becoming better organized over the Great Lakes and Northeast Wednesday-Thursday as supporting upper dynamics sharpen. Highest elevations in New England could see a little snow behind the front before any remaining moisture departs. The one other precipitation focus will be over the Northwest. Leading shortwave energy and associated surface front will produce some rain and high elevation snow from the Pacific Northwest into far northern Rockies on Monday. Then the approach/arrival of an eastern Pacific system may bring another plume of moisture into the Pacific Northwest/northern California by around next Friday. There is still significant spread for the timing, track, and strength of this system so it will take some time to resolve more precise details of when the precipitation arrives and its intensity. Expect most of this activity to be rain, with a little snow potential in the highest elevations of the northern Cascades. During early-mid week, the warm sector ahead of an advancing cold front will feature well above normal temperatures over the central/eastern U.S. The greatest anomalies should be in the plus 20-25F range over the Northern Plains/Upper Midwest on Monday but there will also be broad coverage of plus 10-20F anomalies farther south through the Plains and also extending as far as the Northeast. Temperatures could challenge daily records over parts of Texas on Wednesday. Near to slightly below normal readings will progress from the northern half of the Plains into the East mid- late week while temperatures rebound from the Interior West through the Plains, with High Plains locations likely seeing the warmest anomalies (up to plus 15-20F) by next Friday. The far southern tier should stay above normal through Friday. The West Coast states will tend to be near/above normal Tuesday-Thursday, sandwiched by slightly cooler highs early and late in the week. Rausch/Kebede 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