Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 AM EDT Sun Mar 17 2024 Valid 12Z Wed Mar 20 2024 - 12Z Sun Mar 24 2024 ...General Overview... The blocky upper level pattern with the upper high over the Pacific Northwest and the upper low over the Desert Southwest early in the week is expected to break down, while the trough over the Northeast U.S. should gradually lift out some and result in more of a zonal flow pattern across most of the nation. A southern stream shortwave trough originating from the closed upper low over the Southwest will likely sustain a weak surface low over the Gulf that will increase chances for showers and storms from southern Texas to Florida. An upper trough is expected to reach the Pacific Northwest next weekend and bring a result to more unsettled conditions. ...Guidance/Predictability Assessment... The 00Z model guidance suite is in reasonably good agreement on the synoptic scale for start the forecast period Wednesday, but timing differences are more apparent by Thursday with shortwaves tracking east across the Rockies and then the Northern Plains in a more zonal flow pattern. There is better overall agreement in the southern stream shortwave crossing the south-central U.S., but uncertainty as to where the corresponding surface low develops over the Gulf by late in the week. The forecast gets tricky going into next weekend for the East Coast regarding the likely evolution of an offshore surface low. At the time of fronts/pressures composition, both the 12Z/18Z runs of the GFS were outlier solutions with the very progressive and northward track of the surface low, even compared to the GEFS mean which is much more suppressed and closer to the ECMWF and ECENS guidance. The 00Z GFS maintains the highly progressive and amplified solution as well. The 00Z CMC also takes the low more towards the north, but not nearly to the same degree as the GFS. The machine learning guidance from the ECMWF also does not favor a GFS scenario, so a non-GFS blend was used for the second half of the forecast period with overall below average confidence. The WPC forecast incorporated a general deterministic model blend for Wednesday, with slightly more weighting applied to the ECMWF and CMC. After that, there was a gradual increase in the use of the ensemble means to about 50-60 percent of the blend by next Sunday. ...Weather/Hazards Highlights... There will likely be an increase in showers and some thunderstorms across portions of Texas and Oklahoma on Wednesday with warm air advection from the western Gulf and lift from a frontal boundary. It currently does not appear that the convection would be widespread nor heavy enough to warrant any Marginal Risks areas at this time, although trends will continue to be monitored. With the low beginning to develop over the western Gulf on Thursday, an axis of greater moisture flux near the coast may result in heavier rainfall totals on the order of 1 to 2 inches from southeast Texas to southern Louisiana, where a Marginal Risk area is planned. The greatest rainfall totals will likely remain offshore. Farther to the north from Montana to the Upper Midwest, multiple rounds of mainly light snow are forecast, and heavier snow for the higher terrain of western Montana and northern Idaho. Heavier rain is also expected to make a return to much of California by Friday and into Saturday as the next storm system moves into the region, and heavy snow for the central and northern Sierra. For the Eastern U.S., the eventual evolution and track of the surface low off the Southeast Coast will play a role in how much rain and wind coastal portions of the Carolinas will get, and for now it appears the worst weather conditions should remain offshore. There will likely be a stark temperature contrast from the Central Plains/Midwest to the Canadian border. Winter-like readings will continue across northern Montana, North Dakota, and into northern Minnesota with a strong arctic high situated over central Canada providing cold northerly flow. Above normal temperatures across the northwestern U.S. midweek are expected to cool down to near or slightly below average levels with the upper ridge breaking down and a upper trough developing near the West Coast late in the forecast period. Elsewhere, temperatures should generally be within 10 degrees of late March averages for most locations. Hamrick 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