Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Mar 22 2023 Valid 12Z Sat Mar 25 2023 - 12Z Wed Mar 29 2023 ...Overview and Guidance/Predictability Assessment... Most guidance shows progressive upper level flow across most of the lower 48 from the weekend into the first part of next week, between lower latitude ridging and a Canadian upper low whose surrounding flow should extend into the far northern tier states. The main systems of note will be a leading strong storm that will bring snow to some northern areas and rain to the south, followed by a weaker system Sunday onward, but with considerable uncertainty over potential effects over the Northeast by midweek. Weak energy between these systems may also produce some precipitation on a more limited scale. The second system and a leading warm front lifting up from the Gulf Coast may promote some heavy rain over the South. Meanwhile an upper trough/low dropping southward over the eastern Pacific should ultimately reach near the West Coast by next Tuesday-Wednesday, most likely bringing increased rain/mountain snow to northern and central California. The pattern will support a very chilly period over the western U.S. and parts of the northern Plains. Based on the array of 12Z/18Z guidance available for the updated forecast, an operational model blend provided a reasonable starting point for depicting the system tracking northeastward from the Ohio Valley/Great Lakes. Recent ECMWF runs have tended to be a bit on the northwestern side of the guidance though consensus has been nudging very gradually in that direction. The new 00Z ECMWF has weakened the upper system a bit, leading to a slight south/southeast adjustment for the surface low as of early Saturday--close to the favored guidance average. Guidance continues to have a challenge in resolving the specifics of trailing energy. There is still a signal for some compact energy to slide across the north-central Plains during the weekend before shearing out, but such small scale features have low predictability a few days out in time. What remaining energy there is over the West continues to be inconsistent in the guidance from day to day, with the most common theme now being the existence of an upper low for a time before the overall trough opens up and progresses eastward to yield a modest surface wave tracking across the eastern half of the country. The new 00Z UKMET goes quite astray versus other guidance, altering Pacific/western U.S. flow to produce an amplified trough/embedded upper low reaching into the East. Then solutions are all over the place as to what may happen along/offshore the East Coast by midweek. There is somewhat of a majority (12Z ECMWF/CMC/GFS and 12Z ECens/CMCens) that suggests low pressure may deepen near the Canadian Maritimes, though regardless of specifics as of Wednesday there is better general agreement upon a gradient over the Northeast, between east-central U.S. high pressure and offshore low pressure. Models/means show typical scatter with the exact path of the upper low dropping down the eastern Pacific. There has been a somewhat slower trend with the southward progression over recent days, though the new 00Z ECMWF has reversed that longer term trend. A model/mean average looks good at this time, as there has been some east-west shifting among some model runs over the past day or so. In order to arrive at the desired depiction of significant features, the forecast started with a 12Z/18Z operational model composite early in the period and then transitioned to a mix of the operational guidance and 18Z GEFS/12Z ECMWF means. ...Weather/Hazards Highlights... The system tracking northeast from the Ohio Valley/Great Lakes on Saturday will bring rain and snow to the Great Lakes and Northeast with some light to locally moderate rain farther south along the trailing cold front. The best potential for meaningful snow will be from the central-upper Great Lakes through interior and northern portions of New England, with a developing triple point low off the New England coast by Sunday helping to keep cold air in place. This system's trailing front should lift back north across the Gulf Coast as a warm front Sunday into Monday, providing a focus for potentially heavy rainfall. Therefore the day 5 (12Z Sunday-12Z Monday) Excessive Rainfall Outlook introduces a Marginal Risk area over portions of the South. Current guidance signals suggest that an embedded Slight Risk area may be warranted at some point once a better defined axis for highest rainfall totals becomes evident. Elsewhere, a weak shearing system may produce some precipitation across the central Plains and vicinity during the weekend and a wave crossing the East early next week should spread some rain (perhaps some snow north) across the region. The shortwave energy and possible upper low moving through the West during the weekend will support periods of rain and mountain snow from the northern/central West Coast into the Rockies. Most amounts should be in the light to moderate range. Then the upper trough with embedded low forecast to amplify just off the West Coast should start to increase precipitation totals across northern/central California based on the most likely trough/low position. Model spread and variability is still enough to temper confidence in specifics though. Expect much of the West and portions of the northern Plains to see well below normal temperatures during the period. In general, the coldest readings should be from Saturday into Monday with the Great Basin, central Rockies, and eventually far northern Plains possibly seeing highs at least 20-25F below normal. Some of the cold air may spill into the central Plains on Sunday-Monday. Some locations may approach or set daily records for cold highs during the weekend. Temperatures over the Northwest should be less extreme but still be mostly below normal. Meanwhile from Saturday into early Tuesday the East will tend to see near to above normal temperatures and some variability with system progression. Warmest readings versus normal will tend to be over the Southeast, with some record warm lows possible if the early morning temperatures hold for the calendar day. After a system passes through around Tuesday, the East may begin to see some moderately below normal readings while winds could strengthen over the Northeast by Wednesday. Rausch 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