Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 PM EDT Tue Oct 22 2024 Valid 12Z Fri Oct 25 2024 - 12Z Tue Oct 29 2024 ...General Overview... A quasi-zonal upper level flow pattern should be in place across most of the country on Friday, with broad cyclonic flow across the Great Lakes and a departing trough off the Northeast Coast. The pattern is expected to trend more amplified by early next week as a large scale trough builds in across the Western U.S., accompanied by a strong cold front and precipitation for the northwestern quadrant of the nation. Meanwhile, an upper ridge and widespread above average temperatures are likely from the Plains to the East Coast, with the surface high slowly moving offshore by Tuesday and allowing for southerly flow ahead of the Western U.S. trough. ...Guidance/Predictability Assessment... Updated 00/06Z model guidance remains in good agreement on the synoptic pattern late this week and into this weekend, depicting quasi-zonal flow with perhaps a couple embedded northern and southern stream shortwaves passing over the central and eastern U.S. However, despite an accompanying frontal system at the surface passing form the Midwest to the East Coast, this looks to be a relatively dry system with mostly scattered light to moderate showers anticipated. Broad scale ridging follows upstream over the central U.S. likely leading to periods of warmer temperatures for the weekend. Similar to the prior forecast, the main area of uncertainty lies in the latter half of the forecast early next week with respect to an upstream upper-trough dropping south from the northeastern Pacific over the western U.S. The overall trend for the deterministic models has been for the ECMWF/CMC to be on the faster side of the guidance with a northern stream trough digging southward over the West Monday and reaching the the Plains by Tuesday, with the ECMWF the most aggressive in deepening a surface low over the Midwest as the height falls expand over the region. On the other hand, the GFS has been slower, with the trough remaining positively tilted over the west, and even suggesting a split stream solution with energy hanging back over the southwestern U.S. This trend continued with the updated 12Z GFS. The ensemble means fall in the middle of the guidance, with smaller differences but the ECens furthest east and the GEFS furthest west. The suite of AI models from the EC are very similar to the CMC/ECMWF solution through Monday, but begin to show a slower progression into Tuesday, adding further uncertainty during this time frame and suggesting the GFS solution may not be quite the outlier at this point. For now, these differences don't appear to significantly impact the expectation that there will be precipitation spreading inland over the Interior West late weekend into early next week, but more on the potential for cooler temperatures and precipitation chances across the northern/central Plains by Tuesday with the more aggressive ECMWF solution. The updated WPC forecast blend remains similar to the prior forecast given good run-to-run consistency. A composite ECMWF/CMC/GFS/UKMET blend is used through the weekend given the noted similarities, with a shift towards the ECMWF/CMC and the ECens/CMC ensemble means for the latter half of the period. The contribution from the means is increased to 50% by the final day of the period as to not trend quite as far towards the ECMWF solution which is more aggressive in deepening the surface low over the Midwest. ...Weather/Hazards Highlights... It overall remains the case that most of the country should be void of heavy rainfall, especially early in the period, as moisture will be limited despite a few frontal boundaries and troughs passing through. Warm air advection across the Ohio Valley and Great Lakes on Friday around the backside of the surface high and ahead of the cold front should support scattered light to moderate showers, reaching into the interior Northeast by early Saturday. There may also be a few showers across portions of the Southeast states Friday/Saturday, but these should be more scattered and many areas should remain dry. The exception will be West. The Pacific Northwest/northern California will see onshore flow, and terrain enhancement will tend to increase rainfall prospects across this region, mainly next weekend as the moisture plume ahead of the Pacific front moves inland. A few inches of rainfall will be possible for the coastal ranges and the western slopes of the Cascades. However, the rainfall totals should be limited enough during the Day 4 and 5 period (Friday and Saturday) to warrant keeping the excessive rainfall outlooks for those days blank for now. There is still some discrepancy in the guidance for the arrival of this rainfall event and possible totals. Precipitation should spread inland with the system through the Great Basin and into the northern/central Rockies Monday-Tuesday. Some heavy snow will also be possible for the higher elevations of the Pacific Northwest as well as into the northern Rockies. Precipitation chances may increase over the northern/central Plains by Tuesday as moisture returns ahead of a deepening surface cyclone over the Midwest. In terms of temperatures, it will remain warm and mostly sunny across the southern tier of the U.S. through most of the forecast period. An upper trough crossing the Northeast behind a cold front should result in a brief return to colder temperatures on Sunday before a moderating trend commences next week. Some areas of the central/northern Appalachians could see a Frost or Freeze Sunday morning. Widespread highs 10-20+ degrees above average are looking likely across the Plains into the Great Lakes and Ohio Valley early next week as the ridge axis builds and a warm front lifts well to the north into southern Canada. Meanwhile, considerably cooler conditions are expected across much of the Western U.S. as a pattern change evolves and an upper trough develops over the region. Some gusty Chinook winds will be possible in the lee of the Montana and Wyoming Rockies Saturday into Sunday with downsloping winds, as well as across portions of the northern/central Plains as lee troughing sets up ahead of an approaching upper-level trough. Additional areas of gusty winds are possible Monday-Tuesday with the eastward progression of the upper-trough as cyclogenesis increases flow over the central Plains and Middle Mississippi Valley, as well as with a cold front passage through the central/southern Rockies and into the Southwest. Putnam/Hamrick 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