Extended Forecast Discussion NWS Weather Prediction Center College Park MD 349 PM EST Wed Mar 08 2023 Valid 12Z Sat Mar 11 2023 - 12Z Wed Mar 15 2023 ...Continuing flooding threat for the West Coast, particularly California, through much of next week... ...Overview... A parade of highly anomalous moisture plumes will be focused along the West Coast by frontal systems this weekend and into next week, likely causing continued flooding concerns. Farther east, a fairly progressive pattern in the upper levels with low pressure/frontal systems at the surface could lead to a couple of precipitation events in the central and eastern U.S., with snow in the northern tier and rain and thunderstorms in the South. ...Guidance/Predictability Assessment... The latest runs of the guidance continue to show an active, wavy pattern across much of the CONUS through the extended period. The leading upper low over the Northeast continues to have variability with its proximity to the coast, with the persistent trends of the ECWMF offset a bit to the south and the GFS/GEFS favoring a position nearer to the coastline. A compromise of the two placements fall within the placement of the ensemble means, but any wobbles with the track of this low could affect precipitation potential in the Northeast lead to important sensible weather differences. An average of these two models places the low near the ensemble means , so this compromise was preferred. This low will continue to track offshore early next week. Following the departure of the East Coast trough another trough/low pushes into the north-central U.S from the Canadian Prairies. There is some degree of model spread with this feature however most are showing a shot of cooler temperatures and winter precipitation for the northern states into the Great Lakes region. While the upper trough/low shifts into the Great Lakes region a ridge is expected to amplify in the Interior West. The odds are becoming better that this will promote consolidation of another East Coast surface low, but again with considerable spread in the track and timing. On the backside of this ridge there are considerable variations in troughing across the eastern Pacific and potentially coming into the West by midweek. These details will impact the West Coast weather hazards and will continue to be refined with time. The WPC forecast was based on a blend of deterministic 06/00Z guidance early in the period with a gradual trend toward more GEFS and EC ensemble guidance as the period progressed to help curtail the individual model differences. Some aspects of the the forecast, particularly the QPF, included WPC continuity from the previous forecast cycle. ...Weather/Hazards Highlights... The strongest surge of moisture is expected around Friday, however precipitation looks to continue to fall particularly in the Sierra Nevada and foothills through Saturday. Additional rounds of moisture come into the West Coast (including the Pacific Northwest) early next week. Despite some continued model variations in the timing and placement of heaviest rain, around Monday-Tuesday looks favorable for another notable atmospheric river. Per the California Nevada River Forecast Center, the middle elevations may be particularly sensitive to flooding as warm air and rain falling atop the margins of the snowpack causes snowmelt. In higher elevations the snow is expected to be able to absorb the rain more. But the added weight of rain falling on snow could cause issues as well. Slight Risks of excessive rainfall are in place mainly for the Sierra/foothills of California this weekend. Rounds of precipitation could also impact the Interior West as well. Another Slight Risk was raised for portions of the California Coast and far Southwest Oregon on Day 5. Farther east, there may be some light snow reaching onshore on Saturday with a low in the western Atlantic, but there remains considerable uncertainty for this lead time with continued model differences. Southern New England looks most likely to see snow with this low but probabilities are not terribly high and have been varying. Meanwhile better chances for snow are forecast for the north-central U.S. on Saturday with some light to moderate amounts. Farther south, rain and thunderstorms are expected in the vicinity of a low pressure system affecting parts of the Mississippi Valley Saturday and pushing into the Southeast on Sunday. With instability and a moist airmass in place, along with likely wet antecedent conditions, Marginal Risks of excessive rainfall areas were maintained for these areas given the threat of high rain rates causing potential for flash flooding, although with minor adjustment to reflect the latest QPF trends. As low pressure systems may consolidate into a coastal low on Monday into Tuesday, snow may spread into parts of the Appalachians/Mid-Atlantic/Northeast, but with considerable uncertainty for that too dependent on the low track. A couple of rounds of below normal temperatures could affect the Northwest into the northern High Plains through early next week, with below average temperatures by around 5-15F pushing into the East as well. Above normal highs will likely be confined to the south-central U.S., with temperatures potentially over 90F in South Texas this weekend before a cold front moves through and takes temperatures back to near average for the Gulf Coast states by Monday-Tuesday. Campbell/Tate 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 Hazards: - Heavy precipitation across portions of California, the Central Great Basin, the Pacific Northwest, and the Southwest, Sat-Sun, Mar 11-Mar 12 and Mon-Tue, Mar 13-Mar 14. - Heavy precipitation across portions of the Central Rockies, the Central Great Basin, and the Southwest, Wed, Mar 15. - Heavy precipitation across portions of the Central Appalachians, Sun, Mar 12. - Heavy rain across portions of the Southeast, the Mid-Atlantic, the Southern Appalachians, and the Ohio/Tennessee Valleys, Sun, Mar 12. - Heavy snow across portions of the Pacific Northwest, Sun-Mon, Mar 12-Mar 13. - Heavy snow across portions of the Upper Mississippi Valley and the Northern Plains, Sat, Mar 11. - Flooding possible across portions of the Lower Mississippi Valley and the Southern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Northern Plains, Sat, Mar 11. - Much below normal temperatures across portions of the Northern Plains, Sat-Mon, Mar 11-Mar 13. - Much below normal temperatures across portions of mainland Alaska, Tue-Wed, Mar 14-Mar 15. - Much below normal temperatures across portions of the Alaska Panhandle, Sun-Tue, Mar 12-Mar 14. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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