Extended Forecast Discussion NWS Weather Prediction Center College Park MD 328 AM EDT Tue Mar 26 2024 Valid 12Z Fri Mar 29 2024 - 12Z Tue Apr 02 2024 ...Rounds of heavy rain and snow likely across California and into the Intermountain West... ...Overview... Snow and rain may continue into Friday across the Northeast as a low pressure system lingers. Meanwhile, an upper trough/low in the eastern Pacific late this week is forecast to slowly move into California and the Southwest by early next week. This feature and its surface low will direct moisture into California eastward and potentially produce heavy rain and snow for California into the Southwest/Great Basin and Rockies. Farther east, increasing moisture should combine with shortwaves rounding a south-central U.S. upper ridge to produce rounds of precipitation from the north- central to east-central U.S. late this week with increasing coverage early next week. There is still quite a bit of uncertainty in the details of precipitation coverage, amounts, and type, but snow and freezing rain are possible across the northern tier states. ...Guidance/Predictability Assessment... Overall, the latest deterministic and ensemble model guidance offered reasonable agreement for the synoptic pattern across the CONUS during the medium range period, though with typical uncertainty in the details. The first feature of note is a lifting negatively tilted trough over the northeastern U.S. late week and the western Atlantic surface low reflection. Some ECMWF runs have been on the slow side of the guidance envelope with the surface low track, including the 12Z. West-east wiggles in the low/frontal position are generally small but unfortunately have big implications for the precipitation forecast in New England. Through late week, there is good model agreement and high confidence for mid-upper ridging across the south-central U.S., but farther north models vary with ejecting shortwaves riding the ridge, impacting placement of precipitation. There were not necessarily outliers within the 12/18Z guidance envelope, but just variations that will have to be resolved at shorter forecast timescales. Meanwhile, a closed upper low will slowly move eastward toward and into the West Coast late week into the weekend, supporting an atmospheric river over California. By the early part of next week, models vary with the timing of the trough/low tracking eastward, with relation to it possibly phasing with deepening northern stream troughing in the north-central CONUS. Some guidance from the 12/18Z cycle started to show more phasing after previous runs kept the streams separate through Tuesday. The 18Z and now 00Z GFS as well as the ECMWF are on the phased side. AI/machine learning models generally have phasing as well. Overall this can be a tricky pattern though, so the phasing/separation and timing could certainly change in future updates. The 00Z guidance suite generally has the southern stream West low moving more quickly eastward, making phasing more likely. The WPC forecast was based on a multi-model blend of 12/18Z deterministic guidance through the early-mid medium range period. Given the increasing spread into Days 6-7, quickly increased the amount of GEFS and EC ensemble means in the blend to over half by the latter part of the period. ...Weather/Hazards Highlights... Surface low pressure in the western Atlantic tracking toward the Canadian Maritimes could cause some precipitation to linger across New England into Friday. The westward extent of the precipitation, especially any heavy amounts, remains uncertain as it is quite dependent on exact low track. There is some potential for heavy rain to occur over Downeast Maine though, and a surge of cold air on the backside may allow precipitation to changeover to wet, heavy snow across higher elevations farther west across New England. However, there is also some chance that little to no precipitation occurs in New England if the low is farther offshore, so continue to monitor future forecasts. Brisk winds may also occur in New England on the backside of the low late week. For the western U.S., an active and unsettled period is expected as a persistent jet on the southeast side of an upper low directs moisture into California eastward. At least a weak atmospheric river should start to take aim at California on Friday and produce some locally heavy precipitation. A Marginal Risk looks to be in good shape for Friday's Excessive Rainfall Outlook, from coastal California toward the foothills of the Sierra Nevada, while the higher elevations of the Sierra can expect heavy snow. By Saturday, anomalous moisture likely reaching above the 95th percentile will be directed at southern California. Notable upslope flow into the Transverse Ranges means heavy amounts could occur, so a Slight Risk is planned for urban areas to the upslope portions of the Transverse Range below the snowy higher elevations for Saturday. A Marginal Risk is in place farther east, stretching into southwest Nevada as moisture spills well inland. Ample moisture remains in place into Sunday-Monday across much of California into the Desert Southwest and Great Basin and into the Rockies. Moderate to heavy snow is likely in the higher elevations of the Intermountain West. Farther east, shortwaves generally riding along the northern side of a central U.S. ridge could combine with moisture to produce some light to moderate precipitation Friday into the weekend across the north-central U.S. and Great Lakes region into Ohio Valley. Precipitation looks to increase in coverage and amounts by early next week. Confidence remains lower than desired for where precipitation will occur as well as the precipitation types. However, chances for wintry precipitation are increasing across the northern Plains and Upper Midwest by the weekend, and stretching into the Great Lakes region and higher elevations of the Northeast by Monday. Some chilly temperatures may linger across the north-central U.S. through much of the period, but farther south from the central to southern Plains/Mississippi Valley, a notable warming trend is expected late week into the weekend as upper ridging builds. Temperatures of 10-20 degrees above average will become widespread, with highs in the 70s and 80s reaching Kansas and Missouri while highs climb into the 90s if not 100F in southern Texas. The above normal temperatures are likely to expand east into the Ohio/Tennessee Valleys, Mid-Atlantic, and Southeast by early next week. A cold front is forecast to finally drop temperatures in the south-central U.S. by Tuesday behind a cold front, focusing this warmth in the Southeast. Meanwhile in the West, near to above normal lows but near to below normal highs are forecast, consistent with the clouds and precipitation there. One exception may be over the Pacific Northwest where enough upper ridging may build in to push highs a above normal by the weekend and early next week. 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw