Extended Forecast Discussion NWS Weather Prediction Center College Park MD 209 AM EST Sat Mar 04 2023 Valid 12Z Tue Mar 07 2023 - 12Z Sat Mar 11 2023 ...Despite some remaining uncertainty, chances are increasing for a multi-day heavy rain event in the Southern Plains/Lower Mississippi Valley along with north-central U.S. snow next week... ...Overview... The medium range period begins Tuesday with upper troughing over the western U.S., leading to cooler than average temperatures and some modest precipitation there. Though model guidance has been waffling quite a bit with the pattern midweek and beyond, in recent guidance there has been some building consensus towards energy digging to reinforce the trough and then the trough pressing east toward the central U.S. by late week, though ample timing differences remain. This generally more amplified pattern would lead to considerable rain and thunderstorm chances across the south-central U.S. in the vicinity of a wavy surface front around Tuesday-Thursday and potentially tracking eastward for late week. Additionally, cold air farther north under the influence of a cold Canadian surface high could support snow chances across central and northern parts of the Plains and into the Mississippi Valley. But model disagreements temper confidence in any details and the forecasts are certainly subject to change in future cycles. It does appear likely that temperatures will cool across most of the lower 48 as the week progresses, with the greatest cold anomalies across the Plains. ...Guidance/Predictability Assessment... Model guidance has been quite variable over the past few days even for the large scale pattern by midweek and beyond due to large differences in the way that western Canada energy evolves ahead of a strong eastern Pacific/Alaska ridge. Fortunately there seemed to be somewhat better agreement in the 12/18Z model cycle compared to previous ones. As mentioned above, the growing consensus seems to be for digging energy to maintain the western trough into Wednesday-Thursday, but timing of that trough ejecting eastward shows ample spread. Within the guidance envelope in looking at the deterministic guidance and ensemble members from the GEFS, ECens, and CMC, GFS deterministic runs were among the slowest with the trough's track eastward while the 12Z CMC was among the fastest. Fortunately the ensemble means were somewhat more in line with each other for a middle ground solution of the trough's timing, and the 12Z ECMWF was not far away from them either. Thus the WPC forecast favored a ECMWF/GEFS mean/EC mean blend for the latter part of the period, after a multi-model blend earlier on when there was better agreement. This also worked for ridging ahead and behind the trough and seemed to work reasonably well for surface low pressure and frontal positions. This WPC forecast indicated higher QPF across the south-central U.S. compared to the previous one due to models showing a more amplified upper pattern allowing for more moisture to stream in. Winter weather probabilities also rose in the central U.S. on the backside of a low pressure system forecast to consolidate. However, will need another cycle or two of model guidance to gain much confidence in the forecast. The 00Z CMC has slowed down with the trough closer to consensus, but differs with its pattern in the Northwest. The 00Z GFS remains on the slow side with the main trough, while the 00Z ECMWF shows a similar track to the 12Z run until speeding up by day 7/Saturday. Overall predictability is still rather low, with timing of the trough in particular remaining a limiting factor in the confidence. ...Weather/Hazard Highlights... The upper trough over the western U.S. should continue to support periods of rain along the northern/central West Coast and snow in the higher elevations for some modest amounts. Confidence decreases in terms of coverage and intensity of precipitation across the West by the latter half of the week, but overall precipitation amounts look to increase by Friday-Saturday with a possible frontal system approaching. Farther east, a wavy frontal boundary looks to be in place across the south-central U.S. for Tuesday-Thursday while subtropical moisture streams in from the Pacific in the midlevels and low level moisture comes in from the Gulf of Mexico. Rain and thunderstorms chances are forecast to increase Tuesday and persist through much of the week. Rain rates are likely to be high given the moist and unstable airmass. There is a Marginal Risk outlined in the Day 4/Tuesday experimental Excessive Rainfall Outlook and a Slight Risk for Day 5/Wednesday (with model rain amounts higher day 5 and it being the second day of the event) across parts of the Southern Plains into the Lower Mississippi Valley for the threat of flash flooding. With the caveat of timing uncertainty, generally it appears that the threat for heavy rainfall may move eastward into the Southeast/Tennessee Valley by Friday. Meanwhile farther north, snow seems likely to develop from the central Rockies expanding eastward into the central/northern Plains and Mississippi Valley where Canadian high pressure spills colder air into the region. Probabilities for wintry weather are broad for now given the uncertainty for timing and placement of notable snow, but are increasing overall. Below normal temperatures over much of the West should persist through much of the period while also spreading into the northern/central Plains, and eventually into the southern Plains, next week. There will be decent coverage of highs 10-20F or more below normal within this area, with 20-30F below average anomalies over the central Rockies and High Plains toward next Wednesday-Saturday. Meanwhile expect more above normal temperatures to extend across the south-central to eastern U.S. Tuesday in the warm sector ahead of Plains/Great Lakes/Northeast low pressure and trailing cold front. Areas in the East in particular will trend toward around average by midweek and beyond behind the cold front, but the Gulf Coast could hang on to above normal temperatures through Thursday or Friday. 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