Extended Forecast Discussion NWS Weather Prediction Center College Park MD 350 PM EST Tue Mar 07 2023 Valid 12Z Fri Mar 10 2023 - 12Z Tue Mar 14 2023 ...A wet pattern with warmer temperatures causing flooding threats is in store for the West Coast late this week... ...Overview... Strong atmospheric rivers directed toward the West Coast to cause potential concerns as additional rounds of precipitation coincident with multiple frontal boundaries are possible into early next week. 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 have decently clustered positions for the upper troughs/lows, one just offshore of Washington state and one over the Upper Midwest. The former will help push the first AR into California while the latter provides support for a surface low with snow north of and behind the low in the Great Lakes. There continues to be variability with the surface low in regards to strength and how close it is positioned near the coast as it redevelops by Saturday. The ECWMF once again is depicting a position further south then most of the guidance. The GFS/GEFS mean continues to have a more western track, translating to being more aggressive with the snow potential and amounts for the Northeast than other solutions. The EC ensemble mean low position continues to provide a good proxy for a compromise low position for the weekend. At the beginning of next week, the eastern trough will track further offshore with the western trough 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. The 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. This could help develop another eastern U.S./coastal low but these details are still to be determined. The WPC forecast followed a approach as the previous forecast by utilizing a blend of the deterministic 06/00Z GFS, 00Z ECMWF and CMC along with some EC and GEFS ensemble means to help curtail the individual model differences. ...Weather/Hazards Highlights... In the West, flooding concerns are likely to begin in the short range period late Thursday and persist into Saturday, with Friday looking like the wettest day. This is coincident with a strong atmospheric river pushing anomalously high moisture into especially California (specifically precipitable water anomalies of +4 standard deviations not only shown by individual models, but even per the GEFS and EC ensemble means). As snow levels are forecast to rise late week, rain falling atop melting snow at the margins of the snow pack (at modest elevations) may cause flooding concerns. Given the increase in QPF forecast and the confidence of the strength of the Atmospheric River prior to the start of the extended and the first day of the extended, an upgrade to a Moderate Risk was made for parts of the Sierra Nevada range and adjacent locations. A Slight risk was maintained for the day 5 period as any additional moisture over this area will keep an elevated threat for excessive rainfall and flooding concerns. Then there is the possibility for additional rounds of precipitation in the West into next week, especially for the Pacific Northwest into California, as more Pacific fronts approach. At this point these systems do not look to be as strong as the event late this week, but this will continue to be monitored. Precipitation is also expected into the Interior West including the Rockies through late week into the weekend. Farther east, snow is possible late week across the Great Lakes region and into the central/northern Appalachians north of a low pressure system. Snow may also extend into parts of the Mid-Atlantic and Northeast into Saturday but there remains considerable uncertainty in the amounts of precipitation and the precipitation type depending on the track of a coastal low that could develop. Farther south, showers and potentially thunderstorms could push quickly across the East on Friday. Then another couple of fronts could push through the central and eastern U.S. this weekend for another round of rain in parts of the Middle/Lower Mississippi Valley toward the Southeast, which may cause some flooding issues depending on if rain falls over areas that see ample rainfall in the short range period. Meanwhile light to moderate snow is possible in the north-central U.S this weekend and perhaps moving into the Northeast by Monday, but with uncertainty once again. Temperatures are likely to be cooler than average for the northern High Plains with the coolest anomalies likely on Friday, but with perhaps another couple of rounds of below normal temperatures into early next week. Highs a few degrees below average are forecast farther east toward the Ohio Valley as well, with lows near normal. The southern tier can expect periods of above average temperatures through the weekend with highs in the 70s/80s and perhaps exceeding 90F in southern Texas, before a cold front moves through and takes temperatures back to near average for the Gulf Coast states by Monday-Tuesday. Campbell/Tate Hazards: - Heavy precipitation across portions of California, into western Nevada and southwestern Washington, Fri-Sat, Mar 10-Mar 11 and Mon-Tue, Mar 13-Mar 14. - Heavy precipitation across portions of the Pacific Northwest, Fri, Mar 10 and Mon, Mar 13. - Heavy precipitation across portions of upstate/western New York, and along the central Appalachians, Fri-Sat, Mar 10-Mar 11. - Heavy precipitation across portions of the central to southern Appalachians, Sun, Mar 12. - Heavy snow across portions of the northern Rockies, Fri, Mar 10. - Heavy snow across portions of the upper Mississippi Valley and into the Northern Plains, Sat, Mar 11. - Heavy rain across portions of the Southeast, Sun-Mon, Mar 12-Mar 13. - Flooding possible across portions of the mid- and lower Mississippi Valley, as well as the southern Plains. - Flooding occurring or imminent across portions of the lower Mississippi Valley. - Flooding likely across portions of the lower Mississippi Valley and the southern Plains. - Much below normal temperatures across portions of the northern Plains, Fri, Mar 10 and Mon, Mar 13. 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