Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 PM EDT Sat Mar 25 2023 Valid 12Z Tue Mar 28 2023 - 12Z Sat Apr 01 2023 ...Overview... The medium range period begins Tuesday with potent surface and upper low systems just off the West Coast that will lead to heavy mountain snow in California as well as coastal rain that could cause some flooding issues in sensitive areas. These systems should then track eastward through the latter half of the week, bringing some modest precipitation to the interior West around Wednesday-Thursday and then pushing into the central and eastern U.S. late week, with rain on the southern and eastern side and perhaps some wintry weather on the northwestern side of the system. Meanwhile in the East, the forecast remains somewhat uncertain for Tuesday-Wednesday with some model variety in shortwaves there and surface low positions in the western Atlantic, but in general the forecast for impactful precipitation has been decreasing. ...Model Guidance/Predictability Assessment... There remain some differences from model to model and run to run across the eastern U.S. and into the western Atlantic even as the period begins Tuesday. These differences are smaller scale but affect a round or two of QPF in the northeastern quadrant of the U.S. as well as surface low positions in the Atlantic. In general the trend has been for a more offshore surface low, and more recently also for a more suppressed (farther south) and weaker low shifting into the Atlantic compared to previous cycles. Interestingly the 00Z/06Z deterministic models were on the whole farther south than the bulk of the 00Z/06Z ensemble mean lows on day 4/12Z Wednesday, with some spread in ensemble members. This iteration of the WPC forecast thus decreased QPF from the previous forecast and this trend will likely continue even more in the next forecast, seeing the newer and drier 12Z guidance. Farther west, models are quite agreeable with the existence and positioning of the upper and surface lows just offshore and coming into northern California Tuesday. Some minor to moderate differences arise as the upper troughing tracks eastward. The GFS/GEFS suite has remained on the faster side of tracking the trough east, but is no longer quite as dramatically doing so. This seems to have improved slightly in the newer 12Z GFS/GEFS mean with a slight slowing trend, but the 12Z ECMWF looked slower than its old run too. CMC deterministic and ensemble runs seem to be a fair middle ground with the trough timing and thus the ridge placement east of it as well. Considering the above, the WPC forecast was based on a blend of deterministic models early but with time incorporated the EC ensemble and CMC ensemble means to find a middle ground solution and temper individual model differences including the faster GFS and slower ECMWF. ...Weather/Hazards Highlights... In the West, the storm system will direct a plume of moisture into California especially Tuesday and continuing southward on Wednesday. Though this would only be considered a weak atmospheric river, especially compared to recent events, some heavy rain totals may be possible along the coast and in higher elevations. A Slight Risk remains embedded along the central coast of California where conditions are particularly sensitive to flooding from wet antecedent conditions, with a broader Marginal Risk in the lower elevations. This is a colder system than previous recent ones for lower snow levels. The higher terrain of the Sierra Nevada and northern California mountains are likely to see 1-2 feet of snow accumulation. Meanwhile, lingering convection along a frontal boundary in the Southeast on Tuesday could cause high rain rates for some possibility of flash flooding there, especially in areas that see heavy rain during the days 2-3 time period early in the week. A Marginal Risk is in place across the Deep South. Meanwhile in the Northeast, models have been waffling on how much precipitation there could be, and recent trends have been for lower rain and higher elevation/northern snow amounts for Tuesday-Wednesday. Then by Thursday and Friday, the surface low ejecting from the West and the amplified upper pattern should lead to increasing showers and thunderstorms across the Plains and Mississippi Valley. The Storm Prediction Center has already delineated an area of potentially severe convection for parts of the southern half of the Plains on Thursday. Some snow is possible on the backside of the low in the north-central U.S. into later next week, but with considerable uncertainty in placement and amounts of wintry weather for now. Overall troughing in the West will lead to cooler than normal temperatures there for much of next week, with highs around 10-20 degrees below normal. The coolest anomalies are likely to be across the northern Plains though, where temperatures will range from 10 to 30 degrees below average depending on the placement and day, equating to highs in the 20s to 30s. Meanwhile the Southeast could see a warm Tuesday morning before a cold front comes through and brings temperatures nearer to average midweek. But as ridging extends from a Gulf of Mexico upper high, relative warmth should rebuild across the southern/central Plains Thursday and spread into the Mississippi Valley Friday and the East by Saturday, with highs generally 10-15F above average and lows around 15-20F warmer than normal. 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