Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Mar 03 2024 Valid 12Z Wed Mar 06 2024 - 12Z Sun Mar 10 2024 ...Wet pattern for parts of the Eastern U.S., especially across the South and the East Coast... ...Overview... Models and ensembles advertise a couple wet systems affecting the eastern half of the country during the period, with one confined near the East Coast mid-late week and than a much broader one progressing across the central-eastern U.S. late week through the weekend as upper trough energy initially crossing the West continues eastward. This western upper trough and associated surface reflection will initially produce areas of light to moderate precipitation from California into the Rockies mid-late week. A trailing upper ridge will promote a drier trend over most of the West but then a trailing upper trough and one or more surface fronts should bring precipitation back to the Pacific Northwest/northern California by late week through next weekend. ...Guidance/Predictability Assessment... Overall, the guidance shows decent agreement on the overall large scale pattern during the medium range period, but still a lot of uncertainty in the details, which has some considerably implications for weather and hazard impacts across some parts of the country. Immediately, there are some placement/timing differences with the first of a couple low pressure systems through the East Wed-Fri. The 06z GFS was a little more offshore/faster with the low as it exits the Mid-Atlantic coast late Thursday/early Friday compared to the other deterministic solutions, but there was some support for that little bit faster motion from the 00z ECMWF-initialized machine learning models. Meanwhile, the next system should be ejecting into the Plains by Friday and there is more uncertainty on its track across the Midwest/Tennessee Valley and an eventual triple point low exiting the Mid-Atlantic Sunday-Monday. This seems related to how northern stream and southern stream energy combine/interact. The 00z CMC run had the most northerly track for this system, but the new 12z run came in today farther south, which has better consensus with the GFS/ECMWF and ensemble means. The next shortwave looks to enter the West around next Sunday into Monday, and has fairly good agreement at this point in the guidance. The WPC forecast for today used a blend of the deterministic guidance for days 3-4. For the later periods, incorporated more of the ensemble means to temper some of the detail uncertainties, but still maintaining 50 percent of the deterministic guidance amidst good overall agreement. This also maintained good agreement with the previous WPC forecast. ...Weather/Hazards Highlights... The eastern half of the country will see an active weather pattern during the period. A developing wave affecting areas near the East Coast mid-late week should produce one area of enhanced precipitation, but with details continuing to show lower than average confidence due to high sensitivity to exact surface low track. The new Day 4/Wednesday-Wednesday night Excessive Rainfall Outlook proposes some adjustment to the prior Day 5 Marginal Risk area, maintaining a significant portion of the original area that overlapped with relatively lower FFGs over parts of the central Appalachians/northern Mid-Atlantic (but with mixed signals for QPF totals) and expanding it to include southern New England where there is a little better of a signal for heavier rainfall potential. Some significant precipitation in the form of rain and/or snow may continue northeastward over New England into day 5/Thursday-Thursday night but confidence in totals or precipitation type distribution (with some snow possible even close to the New England coast if the surface track is far enough offshore) is too low to depict any risk area for the Day 5 ERO. As upper troughing emerges from the West, a fairly vigorous surface system should develop over the central U.S. late this week and continue into the East by next weekend. The Day 5/Thursday-Thursday night ERO depicts a Marginal Risk area from parts of the southern half of the Plains into the south-central Mississippi Valley as convection develops in association with low pressure and attached fronts. By Friday the past couple days of guidance have been signaling potential for heavier rainfall over portions of the Southeast and this will continue to be monitored. Otherwise, yet to be determined system details will dictate other areas in the eastern half of the country that may see significant precipitation from this system. Some locations from the central High Plains into the Upper Midwest, Great Lakes, and New England could see some snow but currently with 24-hour probabilities of 0.25 inch liquid in the form of snow staying under 30 percent thus far. Expect a wavy frontal system to bring light to locally moderate precipitation to California on Wednesday with some rain and higher elevation snow extending eastward into the Rockies mid-late week. Most of the West will see a drier trend as this moisture progresses eastward, though the Pacific Northwest and northern California should see a rebound in rainfall/mountain snow late this week into the weekend with the approach/arrival of an upper trough and a couple frontal systems. The upper trough crossing the West mid-late week will promote below normal temperatures across most of the region, with some areas of high temperatures as cool as 10-15F below average also extending into parts of the High Plains for a time. Negative anomalies for morning lows should be confined more to the Northwest/Great Basin. By next weekend clouds and precipitation may keep highs a little below normal over the Pacific Northwest next weekend but other portions of the West should trend toward normal as upper ridging builds in, while temperatures should rise to above normal levels over the northern Plains. The eastern half of the country should stay on the warm side of normal mid-late week with the most anomalous readings of plus 20-30F for morning lows on Wednesday from parts of the Ohio/Tennessee Valley into the Northeast. These values may lead to some daily records for warm lows if they hold for the calendar day. Eastward progression of the late week/weekend storm system will bring a cooling trend from west to east, with highs over the eastern third of the country declining to near or slightly below normal by next Sunday. Santorelli/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