Extended Forecast Discussion NWS Weather Prediction Center College Park MD 355 AM EDT Tue Apr 11 2023 Valid 12Z Fri Apr 14 2023 - 12Z Tue Apr 18 2023 ...Overview... As the medium range period begins Friday, a weakening low pressure system will be lifting across the Southeast and producing some rain and thunderstorms across the Southeast/Carolinas, with a few showers possible in the Mid-Atlantic on Saturday as well. Meanwhile an upper trough is forecast to push across the western U.S. late week and through the central U.S. this weekend and into early next week, allowing for some snow in the northern/central Rockies and perhaps into the north-central U.S. while rain chances increase farther south in the Plains and Mississippi Valley and pushing into the East early next week. After a brief bout of ridging in the West, precipitation should return to the Pacific Northwest this weekend and farther inland early next week as upper troughing and a cold front enter the region. ...Guidance/Predictability Assessment... As the shortwave trough and its low pressure system weaken in the southeastern U.S. late week, attention will turn to the initially phased/open upper trough tracking across the West and central U.S. through early next week. Most guidance from the 12/18Z and newer 00Z model cycles indicates an upper low forming within the trough by late Saturday-Sunday over the Midwest persisting while slowly moving northeast over the Great Lakes into southern Canada through Monday-Tuesday. This will also support a reasonably deep low at the surface with a similar track, sweeping a cold front across the central and then eastern CONUS. Model guidance has been waffling somewhat with the timing of the low/front tracking east, with the 12/18Z guidance a little faster than the previous couple of cycles, so the WPC forecast was adjusted similarly faster. The 18Z GFS rotated the upper low a bit more by around Monday, lessening the extent of troughing over the CONUS, with the 12Z GFS looking more reasonable compared to the ECMWF and the ensemble means. However the more dramatic outlier with this feature remains the CMC (including the new 00Z run), which has been persistently dropping energy farther south into the trough and closing off an upper low farther south, leading to a southwest/slower surface low position than the bulk of the models/ensembles. Models generally agree that behind this developing upper low, upper ridging will come in ahead of another trough/low that looks to approach the Northwest early next week after spinning in the northeastern Pacific late this week. Placement of energy within the Pacific upper low remains questionable, impacting the timing of the southeastern part of the low and the front at the surface pushing into the Northwest. These differences are within the typical spread of the late medium range timeframe though, and an average/middle ground position seemed like the best bet at this point. Given the above considerations, the WPC forecast was based on a blend of the deterministic guidance favoring the 12/18Z GFS and the 12Z ECMWF early in the period, phasing out any UKMET and CMC contributions in favor of the agreeable GEFS and EC ensemble means. ...Weather/Hazards Highlights... By Friday the low pressure system in the Southeast should be lifting northeastward with some rain possible in the Carolinas, but with uncertainty in how far west (into more sensitive areas like the Columbia SC metro and back into the southern Appalachians) the convection will spread. No risk area for excessive rainfall is depicted at this time given the uncertainty, since if heavy rain remains over coastal regions, it may not cause flooding issues over sandy soils that are not sensitive to flooding. Rainfall should trend lighter overall Saturday as the upper system opens up, though some showers could reach the Mid-Atlantic. Snow will be possible in higher elevations of the central Rockies by Friday behind surface low pressure in the central High Plains and with the support of a trough aloft. Precipitation is likely across the central U.S. late week as well. Some enhanced convection is possible in the central U.S. on Friday with a likely focus over the central Plains/Middle Mississippi Valley. There may be a nonzero flash flooding threat with this thunderstorm activity, but no excessive rainfall areas are depicted in the outlook for now given ample uncertainty in the placement of heavy rain and whether or not amounts would be heavy enough. Additionally, there remains some chance for springtime snow to extend into parts of the northern Plains and the Upper Midwest/Upper Great Lakes for Friday into the weekend and possibly lingering into early next week. Meanwhile expect rainfall to expand in coverage to the south and east late week into the weekend, with moist flow ahead of the cold front possibly helping to increase rainfall chances across the western/central Gulf Coast region once again. The best agreement in models and ensembles for the placement of heavy rain is across southeastern Louisiana on day 5/Saturday, where there is a Marginal Risk of excessive rainfall in place. Some guidance shows heavy rain farther west across southwestern Louisiana and coastal Texas as well, but there remains considerable spread with this for now, precluding an excessive rainfall risk stretching farther west. But this will continue to be monitored in future forecasts if models agree better on location of heavy amounts and especially if they are over areas sensitive to flash flooding. After a dry period for the West into Friday, precipitation chances will increase for the Pacific Northwest on Saturday as a weak atmospheric river channels into the region on the southern side of the northeastern Pacific upper low. Precipitation in the form of lower elevation rain and higher elevation snow is likely to expand east into the northern Rockies and perhaps south into northern California early next week. The most anomalous temperatures as the period begins Friday should extend through the Upper Midwest/Great Lakes to the Northeast with readings exceeding normal by at least 15-25F. These temperatures should gradually moderate closer to normal through early next week after an initial backdoor front and then the main cold front comes through. Meanwhile the trough pushing through the western and central U.S. and its attendant cold front will help produce cooler than normal temperatures especially in terms of highs across the West and portions of the central U.S. by the weekend. The West and then the central U.S. are expected to flip back above average early next week as upper ridging comes in behind the first trough. Tate 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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