Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 PM EST Fri Mar 08 2024 Valid 12Z Mon Mar 11 2024 - 12Z Fri Mar 15 2024 ...Strong storm exiting the Northeast on Monday will continue a threat for high winds... ...Unsettled weather along the northern-central West Coast will continue into early next week... ...Heavy rainfall potential returns to the South late next week... ...Overview... A strong low pressure system will be exiting the Northeast by the start of the period on Monday, but the threat for high winds will continue on the backside of this system at least through Monday. Meanwhile, a couple of shortwaves will move through the West Monday-Wednesday, bringing some unsettled conditions across much of the region before a strong upper ridge builds over the coast next Thursday-Friday. This should allow for amplification of the system over the West late week, with guidance all suggesting an eventual deep upper closed low over the Southwest/Four Corners region. This would support a possibly heavy rainfall threat by next Thursday and Friday across parts of Southern Plains/Lower Mississippi Valley. A mean upper ridge shifting from the Central to Eastern U.S. next week will bring an expanding area of above normal temperatures, with the greatest anomalies likely to be over the northern Plains/Upper Midwest early-mid week. ...Guidance/Predictability Assessment... Model guidance continues to show good agreement on the upper level pattern through the medium range period, with a typical amount of spread in forecast details. All operational models agree that there will be a deep upper low exiting the Northeast while a few shortwaves move into the West, then the pattern will amplify later in the week, forming a deep upper low over the Southwest and a strong ridge over the northern West Coast. The largest uncertainty in the forecast is concerning shortwave energy that moves into the West early in the period and then transits through the mean ridge over the eastern U.S. mid-week. Agreement has improved since yesterday, but there are still considerable differences in timing and location of this feature, which stem from subtle differences in the upstream amplification. The latest trend in guidance has been to pull the wave further north over the Great Lakes region while a trailing frontal system moves across the Central U.S. and Midwest. Even with high uncertainty, the effect on sensible weather should be minimal since this relatively weak wave should only produce light precipitation. Late in the medium range period, model guidance has been showing a potent shortwave moving through the deep trough over the Southwest and ejecting across the southern U.S. late Thursday into Friday. All available model guidance is showing an area of heavy precipitation in the South Thursday night into Friday and shifting into the Southeast late on Friday. Most guidance is showing max QPF values of 3-5 inches somewhere in the South, but there is a high amount of uncertainty concerning the exact location of the heaviest totals, which will be better resolved as we get closer to the event. If this forecast holds and models maintain high QPF values, this system could pose a threat for flash flooding. Since models are in good agreement, WPC used a general deterministic blend for the first half of the period, and gradually added ensemble means to smooth out minor forecast differences in the second half of the period. ...Weather/Hazards Highlights... As the strong surface low exits the Northeast, much of the precipitation will be well offshore by Monday, with the exception of some light to moderate snow showers continuing into the day across Maine and the higher terrain on the backside of the low. Gusty to high winds will also continue into Monday and maybe Tuesday across much of the Northeast. Behind this, weather across the Eastern and Central U.S. should trend quieter for the first half of the week, but the Western U.S. remains unsettled as a couple of shortwaves bring rounds of rain and higher elevation snow. The Day 4/Monday Excessive Rainfall Outlook highlights a marginal risk along the coast from northern California through Washington. Guidance continues to show some uncertainty for details of magnitude/distribution of moisture and higher rain totals, but wet ground conditions following heavy rain the prior day (Sunday) may lower the threshold for localized runoff issues in some areas that see overlapping heavy rain/high rates. As the final upper shortwave in the series amplifies and moves inland, the best focus for rain and mountain snow should shift into the central Rockies by Wednesday, with rainfall beginning to increase in coverage and intensity over the Plains and Mississippi Valley as well. Moisture streaming ahead of a closed low developing over the Southwest should increase the threat for another round of heavy rain and potentially flash flooding across the Lower Mississippi Valley and vicinity next Thursday shifting more into the Gulf Coast/Southeast by next Friday. Areas in the Mid-South may be more susceptible to flooding with this system considering the heavy rainfall forecast for this region in the short range period. Some threat for locally heavy rain/mountain snows will exist across the Southwest as well in association with this system. Mean ridging aloft over the central/east-central U.S. will support an expanding area of above normal temperatures to the east of the Rockies. The core of warmest anomalies should cover parts of the northern Plains and Upper Midwest Monday through Wednesday, with highs potentially reaching 20-30 degrees above normal. Above normal temperatures will gradually expand into the South and East mid to late next week week. Over the West, the wet/snowy pattern over northern-central West Coast and eventually into the Rockies will maintain moderately below average highs and near/above average lows through early next week. Upper troughing that amplifies inland by Wednesday-Thursday should spread a broader area of cool daytime highs across the West with temperatures beginning to rebound again along the immediate coast next Friday as strong ridging builds in aloft. Dolan/Santorelli 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