Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sun Mar 26 2023 Valid 12Z Wed Mar 29 2023 - 12Z Sun Apr 02 2023 ...Overview... A low pressure system just offshore of California as the period begins Wednesday morning could lead to some coastal rain and mountain snow continuing across California into Wednesday and Thursday. The upper trough and associated surface low is forecast to push eastward through the latter part of the week and emerge into the Plains. Thus precipitation will spread across the Intermountain West Thursday and then into the central U.S. Thursday-Friday. Rain and thunderstorms that could cause severe weather and flash flooding are likely through much of the Plains and Mississippi Valley, while wintry weather could affect the north-central U.S. on the backside. As this system pushes east, another upper trough is forecast to drop into the Northwest by next weekend. ...Model Guidance/Predictability Assessment... Recent model guidance is reasonably agreeable on the synoptic scale through much of the medium range period. Trends have been for a slightly weaker but still notable surface low near the California coast for Wednesday, with good consensus on its position. Meanwhile on Wednesday model trends have continued for a well offshore and suppressed Atlantic low. As the West Coast upper low tracks southeast into the West and east into the central U.S. Thursday-Friday while ridging amplifies ahead of it stemming from a Gulf of Mexico upper high, models show differences well within typical spread for this timeframe. GFS runs and even the 06Z GEFS mean indicated an embedded upper low within the trough by late Friday into Saturday as it lifts into the east-central U.S., unlike the ECMWF/CMC that just show a potent shortwave. Again, these are relatively minor differences but they are associated with variations in the surface low placement and the amounts/axis of precipitation that could be snow across the north-central U.S. to the northwest of the surface low around Friday. Shortwaves behind this main feature are more variable and could affect periods of upper ridging behind it. By the weekend another larger scale trough with possibly an embedded upper low is forecast to come southeast into the Northwest. While there are some differences in the timing and depth of this feature depending on how the energy digs, true outliers were not really noted other than perhaps the new 12Z CMC that was somewhat flat across the Pacific Northwest as it splits energy differently. The WPC forecast was thus based initially on a blend of the 00Z deterministic guidance and then gradually increased the proportions of ECMWF ensemble mean and less so the GEFS mean as the period progressed to reduce the impacts of individual models varying. ...Weather/Hazards Highlights... As the period begins Wednesday, the surface and upper low just offshore of California will be directing an (albeit somewhat weak) atmospheric river into California. This plume of moisture is likely to affect southern California on Wednesday before pushing south into Mexico. Meanwhile the considerable upper divergence and somewhat steep lapse rates caused by the upper low so nearby could cause scattered thunderstorms across the central California coast through Wednesday into Thursday. Precipitation amounts shown by the models are fairly weak, but some ingredients for heavy rainfall rates are in place so a Marginal Risk of excessive rainfall was added for day 4/Wednesday. Meanwhile in the higher terrain of the Sierra Nevada, additional snow is likely through midweek, and snow should spread into the Intermountain West and Rockies as well through Thursday. Then by late week into the weekend, another east Pacific/western Canada trough should direct moisture more into the Northwest this time, though still could affect northern California, with lower elevation rain and mountain snow possible in those areas. As the first trough and low system from the West eject into the Plains for the latter half of the week, precipitation will spread into the central and then eastern U.S. before clearing out by next Sunday. Strong to severe thunderstorms are possible in the southern half of the Plains Thursday and into large portions of the Mississippi Valley on Friday per the Storm Prediction Center's extended outlook. The day 5/Thursday experimental Excessive Rainfall Outlook also indicates a couple of Marginal Risk areas in place. One is in the Southern Plains where moisture and instability will be abundant and could cause high rain rates ahead of a cold front. The instability axis will also push farther north into the vicinity of a warm front centered in and around Iowa, so flash flooding is possible there as well. Moderate to heavy rain is forecast to shift into the eastern U.S. late week, though there has been some model variety in amounts. Meanwhile on the backside of the low, a round of snow is possible in the north-central U.S., though the positioning and amounts of notable snow totals will have to be refined with time. Windy conditions also could be a threat across the southwestern to south-central U.S. and progressing eastward toward the Ohio Valley with time behind the cold front. 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 teens to 30s and lows near or even below 0F. After a near average Wednesday in terms of temperatures elsewhere, ridging extending from a Gulf of Mexico upper high should cause relative warmth to 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 15-25F warmer than normal. A few record warm low temperatures could be tied or set, but record highs may be limited to Florida. 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