Extended Forecast Discussion NWS Weather Prediction Center College Park MD 214 AM 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... To begin the week and the medium range period Sunday, a compact closed low looks to be atop the north-central U.S./south-central Canada, tracking into the Great Lakes region and the Northeast by Monday-Tuesday while its trough digs a bit. Recent model cycles have shown reasonably good agreement with this feature in the upper levels, while at the surface there have been some wiggles in track and timing of the surface lows across the Southeast and Midwest that are likely to combine into a coastal low. These variations are within the normal spread of the medium range period but could cause sensible weather impacts including the amount of snow in the northeastern CONUS. ECMWF runs have generally been slightly closer to the coast with the low track than other guidance. The WPC forecast tried to take a middle ground track for the low with a compromise position from the various models and the ensemble means. A mean ridge looks to build behind this low/trough in the central U.S., though some shortwave energy may affect it at times. The details of the flow over the eastern Pacific and into the West is the most uncertain portion of the forecast. While overall rounds of troughing are likely, guidance varies considerably with the timing of waves as energies may combine or not. These features have quite low predictability given they emerge from the far northern latitudes and drop south. At least in the 12/18Z model cycle, the ECMWF and most EC ensemble members indicated a couple of separate features tracking south and then east, while the GFS and CMC suites tended toward combining the features for one main trough feature. These differences will impact the West Coast weather hazards and will continue to be refined with time. The WPC forecast blend attempted to get to a middle ground with some components of the deterministic models but also favoring the NAEFS (for both GFS and CMC components) with some EC ensemble means as well. The 00Z model cycle looks like models are starting to converge somewhat for now, but with continued uncertainty. ...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. 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