Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Mar 08 2023 Valid 12Z Sat Mar 11 2023 - 12Z Wed Mar 15 2023 ...A series of frontal systems will continue to cause flooding threats in West Coast, particularly California, through much of next week... ...Overview... While one strong atmospheric river directed at California late this week has phased into the short range period, additional plumes of moisture focused by frontal systems are likely to come into the West Coast this weekend and into next week, 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... As the period begins Saturday, model guidance has decently clustered positions for an upper low over the northeastern U.S., but some variability in models and ensemble members/means for position of the related coastal surface low whose track will affect precipitation potential in the Northeast. Several runs of the ECMWF have shown a position of the low farther south than other guidance, which finally looks to be better clustered in the new 00Z run. Meanwhile the GFS suite has tended to be closer to the coast, which again the 00Z run seems more agreeable. Even minor wiggles in the low track could lead to important sensible weather differences. An average position of the models was right around where the ensemble mean lows were, so this compromise type position was preferred. Early next week, the eastern trough will track further offshore while another trough/low pushes into the north-central U.S./south-central Canada. There are some variations in timing and depth of these lows, but nothing too egregious for the medium range period. This upper low tracking into the central U.S. may dig a bit by Monday-Tuesday as it shifts into the Great Lakes region while ridging amplifies behind 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. Meanwhile, behind the 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 12/18Z 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. ...Weather/Hazards Highlights... In the West, the strongest bout of moisture is expected around Friday, but precipitation looks to continue to fall particularly in the Sierra Nevada and foothills through Saturday. Then 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. 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 have been added for these areas given the threat of high rain rates causing potential for flash flooding. 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. 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 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