Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Thu Mar 09 2023 Valid 12Z Sun Mar 12 2023 - 12Z Thu Mar 16 2023 ...Continuing flooding threat for the West Coast, particularly California, through much of next week... ...Overview... A wet pattern will continue for the West Coast into next week with a series of frontal systems focusing moisture plumes/atmospheric rivers and likely causing continued flooding concerns. The timeframe around Monday-Tuesday looks to see the highest chances in the medium range period for precipitation causing flooding. 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 overall synoptic pattern was captured well by the current suite of guidance. A general model blend was utilized for days 3 and 4 to resolve troughing in the Upper Midwest and shortwave energy riding along mean zonal flow into the West coast beneath an upper-level low in the East Pacific. The 06z GEFS and 00z ECE were introduced on day 5 to account for run-to-run inconsistencies in the GFS and deterministic EC with respect to the East Pac trough approaching the West Coast. The 00z Canadian was removed on days 6 and 7 and the 00z CMCE was introduced to help mitigate continued deterministic uncertainty with the western U.S. trough. The non Canadian guidance appear to begin converging on a solution with regards to the southern stream trough over the Southwest/Southern Plains by day 7. ...Weather/Hazards Highlights... An active pattern across the West Coast will persist from the short range period into the medium range period. While model guidance indicates a relative break over the weekend in the heaviest totals, any additional rainfall may cause issues over sensitive areas over California in particular, so a Marginal Risk of excessive rainfall is in place for day 4/Sunday. But by around Monday-Tuesday another atmospheric river looks to come into California, despite some continued model variations in the timing and placement of heaviest rain. Higher moisture anomalies and precipitation forecast on day 5/Monday has led to a Slight Risk issuance in the Excessive Rainfall Outlook at that point. Impacts of this precipitation event could be dependent on what issues and hazards evolve in the short range period, but could include more flooding as well as issues caused by the added weight of rain falling on snow. Rounds of precipitation could impact the Interior West as well. Meanwhile, rain and thunderstorms are expected in the vicinity of a low pressure system pushing across the Southeast on Sunday. With instability and a moist airmass in place, along with likely wet antecedent conditions, a Marginal Risk of excessive rainfall was maintained for parts of the Southeast/Southern Appalachians Sunday given the threat of high rain rates causing potential for flash flooding, with typical minor adjustments to the area to reflect the latest QPF trends. Farther north, light snow is possible across the Great Lakes region Sunday. Then as low pressure systems likely consolidate into a coastal low on Monday into Tuesday, snow may spread into parts of the Appalachians/northern Mid-Atlantic/Northeast. Confidence is fair that portions of those areas should see some snow (though with some minimal chance the low tracks far enough offshore that this would not be the case), but amounts and type of the precipitation will be highly dependent on the exact low track, so the details will continue to be refined with time. There may be some moisture return to the Mississippi Valley by around midweek, but this remains quite uncertain. Below normal temperatures may be most anomalous in the northern Rockies/High Plains early next week, with lows around 10-20F and highs around 15-25F below average. Cooler air is likely to spread into the Mississippi/Ohio Valleys and into the East as well as the week progresses, though in moderated form. Above normal highs will likely be confined to the Gulf Coast region on Sunday, with temperatures well over 90F possible in South Texas, before a cold front moves through and takes temperatures back to near or slightly below average for the Gulf Coast states by Monday-Tuesday. Temperatures may rebound to above normal in the southern/central High Plains by Tuesday-Wednesday though. Kebede/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