Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM 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, with effects from the Mid-Atlantic through New England very sensitive to strength/track of the low. 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 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... Most guidance is similar in terms of the big picture, with a western U.S. upper ridge broadening and settling over the central U.S. as central-eastern U.S. upper troughing takes 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 continue to 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. Guidance continues to have a difficult time resolving the finer but extremely important details of East Coast/western Atlantic evolution during the latter half of this week. Surface details will be very sensitive to evolution aloft, with sharpening/negative tilt of Southeast energy and uncertain interaction with northern stream flow suggesting below average predictability. Through the 00Z/06Z cycles, the full array of other models/means and 00Z ECMWF- initialized machine learning models (MLs) would say that the 00Z ECMWF is too slow with its Atlantic surface system. Indeed, the new 12Z ECMWF has trended faster. Meanwhile the 12Z UKMET/CMC closed low solutions ultimately track the deep surface low closer to the coast than most other model/ensemble guidance, with the MLs not supporting such a western track either. Some north-south spread develops for the strong storm forecast to be offshore the Pacific Northwest by Thursday with no clear clustering between the southern GFS and northern ECMWF (but the new 12Z run adjusting a bit south). Guidance remains split by Friday- Saturday regarding whether a separate well-defined system rounding the base of the upper low already in place will track toward California (ECMWF/CMC), while the northern system weakens, or a more ambiguous energy transfer takes place (with a weaker southern reflection initially) to yield the southern system eventually (GFS and ensemble means). The 00Z UKMET was between these two ideas while the 12Z run jumped to the ECMWF cluster. MLs were mixed with respect to how this pattern would evolve. Conflicting signals favored an intermediate approach closest to continuity that was somewhat closer to the ECMWF cluster in principle. As a higher- confidence upper trough/low reaches the vicinity of California next weekend, the guidance majority including the MLs plus recent trends favor timing slower than the 00Z and especially 06Z GFS (and the GEFS to some degree). The 12Z GFS has adjusted slower. Shortwaves farther east are variable but in general have been trending weaker, leading to a more suppressed surface pattern. GFS runs have been somewhat at odds with this theme. Forecast considerations led to an equal weight of 06Z GFS, 12Z ECMWF/UKMET/CMC, and remaining 20 percent input of the 06Z GEFS/00Z ECens means early in the period to balance considerations for the East Coast/Atlantic system and East Pacific evolution. Then the forecast shifted 70 percent toward the ECMWF/CMC and their means given timing preferences for the upper trough/low nearing California. ...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 and perhaps as far north as southern Virginia on Wednesday. Recent model guidance has continued to oscillate for timing and extent of significant rainfall, with yesterday's faster/lighter trends now countered by a slower/heavier trend in the 12Z runs thus far. Regardless of specifics, deceleration of moisture anomalies as a surface wave reaches near the Southeast coast by early Thursday along with some instability may promote some locally heavy rain that could cause flooding issues. Trends favored some north/northwest expansion of the existing Marginal Risk area in the Day 4 Excessive Rainfall Outlook. Erratic guidance behavior thus far and less than desired clustering for placement of highest QPF precludes adding an embedded Slight Risk area, but stabilization of current guidance themes would make such an upgrade more likely. By Thursday, guidance increasingly diverges regarding coverage of potentially heavy rainfall from the Mid-Atlantic northeastward. Overall the latest trends suggest greater coverage of significant rainfall. The expanded Day 5 Marginal Risk area from the eastern Carolinas and Mid-Atlantic coast into far southern New England made use of mid- higher end ensemble QPF probabilities as a starting point given the operational model spread. The precipitation uncertainty continues into Friday over the Northeast, with some potential for significant totals of rain--or even heavy snow on the western side of the moisture shield. The surface low may produce a period of brisk to strong winds over New England as well. 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 (with current soil moisture fairly high), so the Day 4 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 into Ohio Valley. Confidence remains 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 extend 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-Sunday, bringing highs in the 70s 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. One exception may be over the Pacific Northwest where enough upper ridging may build in to push highs a little above normal next weekend. Rausch/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