Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Fri Mar 15 2024 Valid 12Z Mon Mar 18 2024 - 12Z Fri Mar 22 2024 ...General Overview... It remains the case that an amplified upper level pattern is expected to be in place across the continental U.S. for the beginning of next week, featuring a deep upper trough across the Great Lakes/Ohio Valley region and the East, and a Rex block over the Western U.S. with a closed upper high over the Pacific Northwest and a loitering upper low over the Desert Southwest. The trough over the Midwest/East slowly lifts out some by midweek, and the Southwest upper low eventually evolves into an open wave and then ejects eastward across the Southern Plains/The South going into Thursday/Friday. A more zonal flow pattern aloft is likely by mid- later next week, although a developing low pressure system over the south-central U.S. will increase the prospects for showers and storms for the Southern Plains/Gulf Coast/The South, with potential for widespread enhanced rains/convection to then work downstream over the Southeast and to start working up the East Coast in about a week to be determined. ...Guidance/Predictability Assessment... Model guidance from the 00/06 UTC cycle seems reasonably well clustered into Monday/early Tuesday and a composite seems to offer a good forecast basis along with the National Blend of Models (NBM) and WPC continunity. Forecast spread increases more readily for the rest of next week, especially with the handling of features working over the East Pacific and into the Western U.S. along with the handling of the lead southern stream closed low slated to stall over the Southwest and subsequent downstream progression of the weather focusing feature. The WPC medium range product suite for this period was mainly derived from a composite of GEFS/ECMWF ensemble means and best matching guidance from the 06 UTC GFS/00 UTC ECMWF and the NBM. This solution maintains good WPC continuity and is in line with a composite of the latest 12 UTC cycle guidance. ...Weather/Hazards Highlights... After multiple days of heavy rainfall in the short range forecast period, there will likely be a welcomed abatement in both rainfall coverage and intensity going into early next week. The front crossing central Florida on Monday will probably produce some moderate to locally heavy downpours in places, but not enough to warrant any Marginal Risk areas at this time. The same holds true for the Rio Grande region of southwestern Texas with southerly flow ahead of the ejecting southwest U.S. upper low with mainly light to moderate rain expected. An even quieter pattern is likely on Tuesday for the Day 5 period with no risk areas necessary. To the north, a secondary cold front crossing the Great Lakes/Northeast should bring some generally light precipitation across the region. Sufficient cold air and low level moisture may combine to produce some accumulating snows for portions of the central Appalachians and downstream of Lake Erie and Lake Ontario. Looking ahead into mid-late next week, showers and thunderstorms will likely increase across The South as Gulf moisture advects northward ahead of a developing surface low, with the greatest rainfall near the coast and offshore into Thursday, especially across the central Gulf Coast. There is a growing guidance signal then suggesting potential for widespread enhanced rains/convection to then work downstream over the Southeast and to start working up the East Coast in about a week. Meanwhile, the expected upper flow transition into later next week may also lead into a more favorable pattern for energy to work increasingly into the Northwest and farther inland to the northern Intermountain West/Rockies. This would introduce renewed snow potential across the broad region as enhanced locally by terrain. Temperatures from the Midwest to the East Coast are forecast to be 5 to 15 degrees below average on Monday and Tuesday as a Canadian surface high will be in place. There will likely be some frost/freeze concerns for any areas where spring green-up has started as temperatures dip well into the 30s to near freezing, especially Tuesday morning. Temperatures across the Southwest will trend warmer with time as the upper low weakens, and above normal temperatures expanding over the Northwest early in the week will move into the northern/central Plains and Upper Midwest by next Wednesday, and colder weather arrives to Montana and North Dakota by the end of the week. Hamrick/Schichtel 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