Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Mon Mar 15 2021 Valid 12Z Thu Mar 18 2021 - 12Z Mon Mar 22 2021 19Z Update: Similar to yesterday, the 12Z models are in very good agreement across the continental U.S. through most of the medium range period, and therefore a general multi-deterministic model blend was used as a starting point in the forecast process. For the building western trough this weekend and into next Monday, there are some modest timing differences with the shortwave tracking across the Desert Southwest, with the 12Z GFS slightly ahead of the consensus. Greater differences are apparent on Monday with the next shortwave trough approaching the Pacific Northwest with the 12Z CMC/ECMWF more amplified and faster than the GFS. Otherwise the rest of the forecast remains on track and the previous discussion is appended below for reference. /Hamrick ...Overview... Upper ridging initially over the northern Rockies and southern Canada will translate eastward through the period, allowing a closed low to linger over the Southeast. Troughing in the northeastern Pacific will move ashore through the West this weekend, setting up another round of snow for parts of the central Rockies. ...Model Guidance/Predictability Assessment... Through the 12Z/18Z guidance, a deterministic/consensus blend sufficed for the Thu-Fri period as an area of low pressure moves through the Ohio River/Tennessee valleys and off the Carolina coast. By next weekend, guidance continues to indicate lingering troughing over the Southeast, supporting surface wave development along the front just offshore. Upper pattern (Rex block) would allow for such a feature but its proximity to the coast and strength remain unclear. For now, kept most of its associated precipitation just offshore but strong northeast flow aided by high pressure to the north could pose some marine hazards (see products from OPC and TAFB). Over the West, upper trough is poised to move inland a bit quicker than forecast 24 hrs ago (ECMWF EPS trended quicker towards the faster GEFS mean) but a blended solution between the GFS/GEFS and ECMWF/ECMWF ensemble mean was a good starting point as the 12Z Canadian was on the quicker side of the spread (possible outcome but not preferred at this time). Split between northern and southern portions of the trough could allow for a close low in the Great Basin/Four Corners region by early next week. ...Weather/Threats Highlights... Rainfall will slip eastward to the Mid-Atlantic ahead of the warm front on Thursday, perhaps with a bit of snow on the northern fringe of the precipitation shield. Much of the region has seen little to no precipitation the past 7-14 days. Rainfall may be confined to coastal portions of the Southeast into Florida next weekend as long as the offshore system stays far enough away. High pressure over the Northeast will maintain cool temperatures this weekend. Approaching Pacific systems will spread low elevation rain and higher elevation snow to the West Coast (NorCal and northern Sierra into coastal OR/WA) Thu-Sat until the upper trough axis moves inland. Another system may bring in light rain/snow late Sun into early Mon. Farther east, light to modest snow is likely for the terrain in the Great Basin to the central Rockies as the southern portion of the upper trough slows the system progression. Question will be how much moisture can be tapped from the Gulf early next week as the system pulls into the Plains. Mild temperatures over Florida (record highs in the upper 80s to near 90 are possible) will give way to 60s/70s behind the cold front over the Southeast/Florida Friday into the weekend. Temperatures will moderate in the northern tier by Sunday into Monday to about 5-10 degrees above normal. Over the West, cooler than normal temperatures behind the front are forecast for much of the region this weekend into Monday. East of the Rockies, well above normal temperatures are expected for the High Plains into the Upper Midwest and Great Lakes (perhaps 15-20 degrees above normal). Fracasso 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml