Extended Forecast Discussion NWS Weather Prediction Center College Park MD 312 AM EDT Sun Mar 24 2024 Valid 12Z Wed Mar 27 2024 - 12Z Sun Mar 31 2024 ...Heavy rain threat across the Southeast may continue into midweek... ...Rounds of heavy rain and snow possible across much of the West through the period... ...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 for portions of the Eastern Seaboard for the latter half of the workweek. This activity could be particularly heavy for portions of Florida and Georgia into the Carolinas. Meanwhile, one or more Pacific systems should bring increasing totals of rain and mountain snow to the West Coast states and then into the Rockies mid-late week into next weekend. Increasing moisture may combine with shortwaves in the north- central to east-central U.S. Friday-Sunday, though with ample uncertainty in the details of precipitation placement and type. ...Guidance/Predictability Assessment... Most guidance agrees in principle for the large scale upper pattern evolution, with central-eastern U.S. upper troughing taking on a more negative tilt by Friday-Saturday as it reaches the East Coast/western Atlantic while deepening mean troughing becomes established near the West Coast. The most prominent forecast issues involve the East Coast/western Atlantic evolution late this week, along with individual features within the West Coast trough plus ejecting energy and its influence on central U.S. surface waviness. For the late week East Coast evolution, guidance continues to vary with the details of energy aloft. However, the past couple of model cycles have at least shown some convergence away from solutions showing separate closed low development aloft. In terms of the surface low, the GFS runs seem to align the best with the ensemble means while the ECMWF (both 12Z and new 00Z runs) appear quite slow/south. Clustering is still good for the strong storm forecast to be offshore the Pacific Northwest by Thursday. There is increasing spread both aloft and at the surface by Friday, as energy rounding the base of the upper low already in place causes potential for a surface low farther south to take over for the northern one. Ensemble means indicate one surface low in between but this could be due to averaging out ensemble members showing both lows. Most deterministic guidance briefly show two lows. Regardless, an upper trough with an embedded low is most likely to reach the vicinity of California by the weekend, along with a low at the surface. The ECMWF operational run and many of its ensemble members are slower than the CMC/GFS suites. ECMWF-based AI models are faster than the operational run, so favored a blend not as slow as the EC itself. Shortwaves farther east are variable but in general do not seem as focused in the north-central U.S. compared to some previous model runs. The WPC forecast used a multi-model deterministic blend early in the period, and with reasonable clustering for the large scale features was able to maintain a majority deterministic blend through Day 6. By Day 7 the GEFS and EC ensemble means reached just over half the blend. ...Weather/Hazards Highlights... A cold front or two and developing low pressure at the surface combining with a strong jet and its right entrance region to the east of an upper trough will support potentially heavy rain across portions of the Southeast/Carolinas on Wednesday. Though recent model guidance has backed off some from some of the extremely high rain totals from a day ago, ingredients are in place for some locally heavy rain that could cause flooding issues. A Marginal Risk remains in place in the Wednesday Excessive Rainfall Outlook, and an embedded Slight Risk cannot be ruled out in future updates. Some potentially heavy rain could linger into Thursday, currently most likely over eastern North Carolina, but with continued uncertainty. Will start with a Marginal Risk there with what seems to be a compromise in coverage of the guidance (some show heavy rain well west and south of the Marginal, while some really only hit the Outer Banks). There is also a broad range of possible outcomes for how much rainfall extends into the Mid- Atlantic/Northeast mid-late week, depending on the evolution and track of western Atlantic low pressure. In this forecast cycle, precipitation has trended down there in favor of a focus offshore. The surface low may produce a period of brisk winds over New England. Deeper moisture starts to stream into the West Coast by Wednesday as upper troughing and low pressure approach. Rain rates could be high enough to be concerning for flooding across portions of California into western Oregon, so the Day 5 ERO shows a Marginal Risk there for Wednesday. Heavy snow is likely into the Cascades/Sierra Nevada and toward the northern Rockies beginning Wednesday. Coastal/lower elevation rain totals look to lessen by Thursday but snow is forecast to expand into the Intermountain West Thursday and beyond. Then model trends have continued to indicate that California may see increasing heavy precipitation by Friday into the weekend based on where an eastern Pacific upper low/trough sets up and directs a possible atmospheric river into the state. This southern shift would also bring precipitation into the central Great Basin and Southwest for the weekend. 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 and Ohio Valley. Confidence is low for where precipitation will occur as well as the precipitation types. Colder than average temperatures by 15-20 degrees are likely to remain into midweek for the north-central Plains, with some gradual moderation but remaining on the cooler side into the weekend. Cool temperatures but with lesser anomalies are farther south in the central U.S. Wednesday, but then a notable warming trend is expected there as upper ridging builds. The south-central Plains into Mississippi Valley can expect temperatures around 10-15 degrees above average by Friday-Saturday and closer to 20 degrees above average by Sunday. This brings 70s and 80s into Kansas and Missouri. Meanwhile in the West, near to above normal lows but near to below normal highs are forecast, consistent with the clouds and precipitation there. 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