Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM 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... An active pattern is likely across the West late this week as a strong atmospheric river takes aim at California in particular, causing potential flooding concerns. Additional rounds of precipitation coincident with multiple frontal boundaries are possible into early next week as well. 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 Friday, model guidance has come into better agreement for positions of a couple of 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. This surface low does show some variability in strength and position especially as it redevelops near the coast by Saturday. ECMWF runs have been showing this low developing farther south than the bulk of guidance, while the 12/18Z GFS runs were well on the western side of the spread, with the new 00Z GFS looking in somewhat better alignment. The EC ensemble mean low position seemed to be a good proxy for a compromise low position Saturday-Sunday. By early next week the eastern upper low should be moving offshore while the western one 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. Overall the WPC forecast was based on a blend of the deterministic 18Z GFS and 12Z ECMWF and CMC along with some EC and GEFS ensemble means to temper the quirks of individual models through the forecast period. Predictability looks nearer to average after a spate of below average predictability over the last few days from the anomalous and blocky Pacific/Alaska ridge and western Canada lows. ...Weather/Hazards Highlights... In the West, flooding concerns are likely to begin in the short range period late Thursday and persist into Friday and 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 snowpack (at modest elevations) may cause flooding concerns. Slight Risks are in place in the experimental Excessive Rainfall Outlooks for days 4-5 given this potential. Though Friday is likely to see the most anomalous moisture and precipitation, precipitation may continue to be heavy Saturday in parts of the Sierra Nevada, so continued a Slight Risk in the foothills where it should stay rain. 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. 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