Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 PM EDT Mon Mar 25 2024 Valid 12Z Thu Mar 28 2024 - 12Z Mon Apr 01 2024 ...Heavy rain across the Eastern Seaboard into Thursday... ...Rounds of heavy rain and snow possible across much of the West... ...Overview... A cold front or two pushing ahead of a broad upper trough in the central-eastern U.S. and eventual Atlantic surface low development will help focus rainfall over portions of the Eastern Seaboard into Thursday and possibly lingering in Maine on Friday. Meanwhile, one or more Pacific systems will bring moderate to heavy rain and mountain snow to the West Coast states (shifting gradually south with time) and into the Rockies later this week into early next week. Increasing moisture may combine with shortwaves to produce some precipitation from the north-central to east-central U.S. Friday-Sunday, though with ample uncertainty in the details of coverage/totals and type. ...Guidance/Predictability Assessment... Overall, the latest deterministic and ensemble model guidance offered good agreement and run to run consistency for the synoptic features across the CONUS during the medium range period. Troughing will deepen along the East Coast as a quick moving storm system lifts from the SOutheast to New England Thursday-Friday. Trends in the guidance have been for the low pressure to track slightly closer to the coastline, especially as it reaches New England. The 06Z GFS (and now 12Z GFS) continue to lead the westward trend, spreading precipitation further west/inland, which will have implications for heavy rainfall but also winter weather for the interior/higher elevations. Meanwhile, a closed upper level low will slowly move southward along the West Coast through the weekend, tapping into an atmospheric river by late in the medium range period over California. Models are in decent agreement with this system, with the typical model bias/spread expected by Day 6-7. 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. However, in the newer 00Z guidance, the CMC seems quite aggressive with a shortwave in the northern Plains Friday tracking east over the weekend, which other guidance does not show. Given the general model agreement on the large scale, the WPC forecast was based on a multi-model blend of deterministic guidance favoring the GFS and ECMWF. Gradually incorporated more GEFS and EC ensemble mean guidance as the period progressed, but was able to maintain deterministic models as over half the blend through Day 7. ...Weather/Hazards Highlights... Low pressure organizing Thursday along the coastal regions of South and North Carolina will deepen as it lifts northeast along the Mid-Atlantic and Northeast coasts. Favorable forcing for ascent provided by a coupled jet structure and a plume of moisture lifting northward will allow for widespread moderate to locally heavy precipitation to develop. The latest trends in the model guidance have shifted the axis of heaviest precipitation slightly westward, bringing more coastal areas into the excessive rainfall risk from portions of central/northeastern North Carolina to southern New England. As the low deepens further in the Gulf of Maine on Friday, a surge of cold air on the backside may allow precipitation to changeover to wet, heavy snow across portions of Vermont, New Hampshire, and Maine though there are considerable uncertainties in the evolution of the low pressure and associated cold air. Brisk winds may also occur in New England on the backside of the low late week. For the West Coast, an active and unsettled weather period is expected. The first system arrives late Wednesday into Thursday, especially for the Pacific Northwest region where heavy mountain snow is possible. A brief break for the end of the week but quick on its heels is another weather system pushing toward coastal northern California by late Friday into Saturday. This system will bring a period moderate to locally heavy precipitation to California through early next week at least as an atmospheric river is increasingly likely to set up. Based on the latest forecast, moderate levels of IVT values are forecast, especially for central/southern California by Days 6-7. While heavy rainfall looks to peak in Southern California on Day 6/Saturday, there seems to be enough moisture coming in by Day 5/Friday to start to delineate a Marginal Risk from coastal California east toward the foothills of the Sierra. 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 may increase in coverage and amounts by early next week. Confidence remains low for where precipitation will occur as well as the precipitation types. Signals are increasing for a quick moving low pressure along the Northern Plains and far northern areas of the Upper Midwest which may bring a period of accumulating snow. 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 10-15 degrees above average will become widespread, with highs in the 70s reaching Kansas and Missouri while highs climb into the 90s if not 100F in far southern Texas. The above normal temperatures are likely to expand east into the Ohio/Tennessee Valleys, Mid-Atlantic, and Southeast over the weekend. 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 little above normal next weekend. Tate/Taylor 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