Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Mar 23 2024 Valid 12Z Tue Mar 26 2024 - 12Z Sat Mar 30 2024 ...Heavy rain threat across the Southeast Tuesday-Wednesday... ...Heavy rain and snow possible across much of the West by midweek and beyond... ...Overview... As snow starts to exit the Upper Midwest on Tuesday, the main focus for potentially hazardous weather will be much farther south, as heavy rain and thunderstorms are likely ahead of a cold front tracking across the southeastern U.S. Tuesday-Thursday. Then upper troughing that reaches the East late this week should promote western Atlantic surface development but with continued uncertainty for the coverage/magnitude of precipitation and wind, especially for the Northeast. 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. Some of this moisture may reach the northern Plains by Friday. ...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. By the end of the period, differences between phasing or having a separate closed low lead to wide differences in Atlantic surface low track and precipitation/wind effects for the Northeast. 00Z ECMWF-initialized machine learning (ML) models did not offer much help as they varied as well, highlighting the uncertainty of this aspect of the forecast. The 06Z GFS/GEFS and 00Z ECMWF/ECens/CMC seemed to provide the best clustering for the time being. Clustering is still good for the strong storm forecast to be offshore the Pacific Northwest by Thursday. The 00Z UKMET appeared somewhat weak/fast but the new 12Z run compares better to consensus. However more spread and variability arise by Friday- Saturday. Most of the recent guidance through yesterday had advertised an upstream Pacific system displacing the Thursday feature, but 00Z through 12Z model runs are suggesting that upstream energy may shear around the southwest side of the initial upper trough, tracking surface low pressure more toward California. Either way an upper trough with an embedded low is most likely to reach the vicinity of California by day 7 Saturday. The new 12Z ECMWF has furthered its detail differences to yield a slower trough/low by this time. Farther east, latest changes have led to a weaker depiction of shortwave energy ejecting through the western/central U.S. late in the week--yielding a weaker surface system. ML models continue to show differences for West Coast details and most support fairly weak surface waviness over the central U.S. by the end of the period. The updated forecast started with main emphasis on the 06Z GFS/00Z ECMWF and a little 00Z CMC for the first half of the period, with the UKMET still not comparing well to consensus for some features. Then the forecast incorporated 40 percent total 06Z GEFS/00Z ECens means (with even weight of GFS/ECMWF/CMC runs) by days 6-7 Friday- Saturday. This provided an intermediate way of beginning the potential trends along the West Coast and started to nudge a little weaker with central U.S. surface waviness. ...Weather/Hazards Highlights... Some snow may linger across northern parts of the Upper Midwest on Tuesday as the surface low tracks across the Upper Great Lakes. Meanwhile farther south, ample moisture and a strong jet aloft will support heavy rain. This is likely to be ongoing in parts of the Southeast as the period begins early Tuesday, with model guidance indicating a squall line crossing Mississippi and Alabama. This could produce rain rates that support Slight Risk levels in the Excessive Rainfall Outlook for Tuesday, though areal coverage of heavy QPF seems to be less so compared to Monday (now in the short range). Some enhanced rain potential extending into Tennessee and Kentucky and over to the southern Appalachians is covered by a Marginal Risk. Then on Wednesday, the axis of heavy rain continues to shift eastward as the cold front(s) press toward the Eastern Seaboard. Models still vary for the placement and amounts of East Coast rainfall at that time but the latest model runs are narrowing the spread a bit (12Z GFS lighter than earlier runs, 00Z ECMWF heavier than the 12Z/22 run). The Marginal Risk area in the Wednesday Excessive Rainfall Outlook still reflects the situation well, while the combination of moisture and instability would keep the door open to the possibility of an eventual embedded Slight Risk area. Most models are suggesting a slightly quicker end to rainfall, so the afternoon update nudged the far western edge of the Days 4-5 risk areas a fraction to the east. Some potentially heavy rain could linger into Thursday, currently most likely over the eastern Carolinas, but with continued uncertainty. 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. This system may produce a period of brisk winds over New England as well. Farther west, some snow is likely in the Intermountain West on Tuesday. Deeper moisture starts to stream into the West Coast late Tuesday and especially 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. Latest model guidance maintains support for this area. Heavy snow is likely in higher elevations midweek and beyond, stretching into the Intermountain West as well. Starting in this past night's cycle, guidance has started to shift East Pacific surface details by late in the week. If the emerging trends hold, duration of significant precipitation over southwestern Oregon/northern California would be a little shorter as better emphasis shifts into the southern two- thirds or so of California. Farther east, there is a general signal for precipitation to expand into the north-central U.S., possibly in the form of snow again, by Friday. However precise totals are fairly uncertain at this time. Much colder than average temperatures (more comparable to January) are likely for the northern and central High Plains in the wake of the strong Plains/Midwest storm. Highs will only be in the 20s and 30s for these areas on Tuesday. The central U.S. will see a warming trend going into the middle and late part of next week, though Montana and the Dakotas will still remain chilly by late March standards. The West should see generally cooler than average highs and near average lows next week. The East may see mixed above and near normal temperatures until late week when the relative warmth from the central U.S. (highs reaching up to 10-15F above normal over the central Plains) expands east. 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