Extended Forecast Discussion NWS Weather Prediction Center College Park MD 228 PM EST Mon Mar 06 2023 Valid 12Z Thu Mar 09 2023 - 12Z Mon Mar 13 2023 ...Excessive rainfall for the southern Plains into the Mid-South mid to late week as well as snow over the north-central to northeastern U.S. late this week into this weekend... ...Wetter pattern expected to develop along the West Coast with moderating temperatures... ...Overview and Guidance/Predictability Assessment... A system moves from the Rockies through the Great Lakes and Northeast between late this week and early next week leading to renewed heavy rain concerns near the Deep South and snow concerns closer to its mid-level path across portions of the Northern Tier of the Lower 48. Upstream, an upper level low should progress eastward into the Ohio Valley/Great Lakes with time which at this point appears to be less of a heavy rain/snow threat. A wetter regime along the West Coast is expected with moderating temperatures as onshore flow from the Pacific focuses in and near northern California both due to the first deep layer low and then due to renewed northeast Pacific troughing from late Saturday onward. Outside of the 00z UKMET, which is faster with the initial system moving from the Great Lakes through the Northeast, the guidance shows reasonable agreement. A compromise of the 06z GFS, 00z ECMWF, 00z Canadian, with some inclusion of the 00z NAEFS mean and 00z ECMWF ensemble mean was used for the 500 hPa heights, pressures, winds, and QPF. The remainder of the grids were more 13z NBM based, as it typical. ...Days 4-5 Experimental Excessive Rainfall Outlook... ...Southern Plains... Areas of heavy rainfall and thunderstorms are expected to persist from the short range period through Thursday from the Southern Plains into the Mid-South as subtropical Pacific moisture aloft combines with low level Gulf moisture streaming into and over a frontal system. Precipitable water values will be 1.25-1.75". The flow will be unidirectional out of the west-southwest with height, which argues for cell training. The degree of 850 hPa inflow/effective bulk shear (25-40 kts) along with CAPE of at least 1500 J/kg nearby and upstream argues for organized short, training bands and possible mesocyclones, both of which raise the heavy rainfall risk. An upgrade to a Moderate Risk was made to the experimental Excessive Rainfall Outlook for these areas for day 4/Thursday, which accounts for current soil saturation from recent rains and rainfall that precedes the day 4 period/before March 9/1200 UTC. ...West Coast... The western U.S. is forecast to see active weather for much of the period, with the longer duration of impacts in and near northern CA. On Friday, an atmospheric river of at least moderate amplitude (roughly 800 kg/ms IVT) will aim anomalously high moisture at California on the heels of 50-60 kt 850 hPa inflow off the Pacific and precipitable water values nearing 1.25". Snow levels look to rise late week, which could lead to rain falling atop melting snow at the margins of the snowpack (at modest elevations) which may cause flooding concerns. A Marginal Risk of excessive rainfall is in place for portions of the West on day 4, while a Slight Risk of excessive rainfall is in place for day 5/Friday given this potential. With time, these threat levels could be raised should the guidance strengthen the atmospheric river. Additional moisture plumes coming into the West could provide more chances for precipitation there this weekend into early next week, with current indications aiming a moisture plume at the time around the northern half of California. ...Other Weather Highlights... Up north, snow will likely be ongoing across northern parts of the Plains as the period begins Thursday and expanding quickly into the Upper Midwest where Canadian high pressure -- near record high for March for central Canada -- spills colder air into the region. Probabilities for 0.25" liquid equivalent from sleet and snow remain broad given the uncertainty for timing and placement, but continue to increase in both magnitude and coverage. Wintry weather is expected to extend into the Great Lakes and Appalachians around Friday and then into the northern Mid-Atlantic and New England Saturday, depending on specifics of the expected central/eastern U.S. into western Atlantic low pressure system. Below normal temperatures over much of the West (around 10F below normal) should persist through Thursday before the core of the colder than average air shifts into Montana and the Dakotas from Friday onward. The upcoming pattern change should lead to minimum temperatures rising to near/slightly above normal values for the West by Saturday-Sunday. Meanwhile, above average temperatures at least in terms of highs will likely be confined to the Gulf Coast states Thursday-Sunday. In modified form, expect the chilly air over the central U.S. to spread into the East Friday through Monday, more obvious in the afternoon highs. Roth 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