Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Sat Mar 02 2024 Valid 12Z Tue Mar 05 2024 - 12Z Sat Mar 09 2024 ...Wet pattern for parts of the Eastern U.S., especially across the South... ...Overview... Models and ensembles advertise a mean trough aloft crossing the West and then emerging into the central U.S., while a leading diffuse shortwave trough possibly helps to enhance East Coast low pressure. The combination of these features will favor a couple episodes of significant precipitation over the eastern half of the country with relatively more confidence in highest totals being over the South, while temperatures stay above normal. The West will be on the cool side with some areas of rain and higher elevation snow. ...Guidance/Predictability Assessment... Guidance continues to exhibit similarities for some aspects of the large scale pattern evolution but significant differences for some embedded details. One area of contention is with the diffuse upper trough which most guidance develops over the East in one way or another mid-late week, with effects on surface low pressure near the East Coast. ECMWF runs have tended to be on the slow side with this feature and the GFS somewhat fast (plus northwest for the surface low). CMC runs and the GEFS/ECens means depict a surface low track a little farther east, as does the new 00Z UKMET. 12Z ECMWF-initialized machine learning models favor the eastern side of the surface low envelope. Farther west the guidance has been having difficulty in resolving shortwave details across the Northwest/northern Plains into southern Canada. The 18Z GFS actually compares better in principle to other guidance than the new 00Z run which seems to become too slow and sharp aloft by Wednesday. After midweek there is some continuity in the concept of multiple pieces of energy comprising an overall upper trough crossing the West and reaching the Plains. However by next Saturday there is still spread for timing of this trough and an upstream eastern Pacific trough. The best clustering of guidance suggests that the GFS may stray a little fast with both. Guidance comparisons among 12Z/18Z solutions led to updating the forecast with an operational model composite early in the period (Tuesday-Wednesday), followed by incorporating some 18Z GEFS/12Z ECens means by Thursday and then 50-60 percent total weight of the means by Friday-Saturday. This transition helped to navigate the increasing detail spread within the more agreeable large scale pattern. ...Weather/Hazards Highlights... The large scale pattern evolution will favor a wet period over portions of the eastern half of the lower 48, with highest rainfall totals likely to be over the southern tier from one event around Tuesday and another late in the week. The first episode will feature a weakening front near the Gulf/Southeast coasts and a stronger front approaching from the northwest, with an axis of enhanced deep moisture passing through. Models disagree on the northward extent of instability and the full model/ensemble range varies considerably for where heaviest rainfall may occur. However latest GFS/ECMWF runs and their ensembles roughly favor maintaining the Marginal Risk area over the Southeast in the new Day 4/Tuesday Excessive Rainfall Outlook close to where depicted in the prior Day 5 outlook, so will maintain continuity until more confident changes can be made. By Day 5/Wednesday the guidance diverges considerably for evolution of diffuse eastern U.S. upper troughing and associated frontal waviness/rainfall distribution. Some solutions offer potential for significant rainfall between the central Appalachians/Mid-Atlantic into parts of the Northeast, while there is an alternate side of the envelope that is more suppressed with the moisture. For now the signal for details is not yet coherent enough to reach the point of depicting a risk area. Some of this activity may extend into Thursday. Behind this system, stronger dynamics emerging from the West should generate a central-eastern U.S. low pressure and frontal system late in the week with another potential episode of heavy rainfall from the Lower Mississippi Valley into the Southeast and less extreme precipitation extending farther north. Depending on system details, some precipitation over the central Plains could be in the form of snow. Northern New England may also see some snow with moisture from the leading East Coast system. Over the West, a frontal system may focus a narrow axis of enhanced precipitation over northern California on Tuesday. There is still spread in the guidance for exact details including how much moisture this system will incorporate. For now the Day 4 ERO does not depict any risk area for excessive rainfall while awaiting better clustering. At the same time, an upper trough to the north will bring an episode of organized snow to the northern Rockies. What is left of this moisture should sink southeastward over the West through midweek or so, with some snow lingering over the southern half of the Rockies into late week. Modest amounts of precipitation may reach the Pacific Northwest later in the week as an upper trough/leading front approach. Upper troughing moving through the West will promote below normal temperatures across most of the region, with some cool readings extending into the High Plains at times. Highs will tend to be 5-15F below normal while negative anomalies for morning lows should be confined more to the Northwest/Great Basin. Temperatures may begin to moderate toward the end of next week from west to east as an upper ridge builds in. Meanwhile the eastern half of the country should stay on the warm side of normal through the period. The most anomalous readings of up to plus 20-30F should be from Tuesday into early Wednesday, with best potential for some daily records likely to be over the Northeast. Rausch 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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