Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 PM EST Tue Feb 14 2023 Valid 12Z Fri Feb 17 2023 - 12Z Tue Feb 21 2023 ...Overview... After a progressive positively tilted upper trough (with leading cold front) crosses the East on Friday, guidance agrees that a strong eastern Pacific upper ridge should support gradual development of a mean trough over the Interior West/Rockies. Meanwhile an upper low will drift southward offshore California, with the digging trough over the West possibly beginning to interact with the low as it reaches the vicinity of northern Baja California by around next Tuesday. Expect shortwaves within broadly cyclonic flow across southern Canada and northern tier U.S. to produce one or more waves along a mean frontal zone that sets up near the Canadian border during the weekend, followed by a southward push of this front early next week as the longer-term mean pattern becomes established. Aside from rapidly departing precipitation along the East Coast on Friday, the main areas of precipitation during the period will be rain and mountain snow spreading from the Northwest into the northern-central Rockies from the weekend into next week and possibly over parts of the east-central U.S. early next week. ...Guidance/Predictability Assessment... The overall synoptic pattern over the CONUS was captured reasonably well by a combination of today's guidance. The 00z UKMET becomes a bit of an outlier with respect to the evolution of the southern stream/Southwest upper low after day 3. There's a notable amount of timing uncertainty between 00z ECE and CMCE members surrounding the SW system beginning on day 4. As far as the blends go, we went with a general model blend of the 00z EC/GFS/UK/CMC and 06z GFS through day 4 with less UK/00z GFS and CMC weighting on day 4. The 00z ECE and 06z GEFS were introduced on day 5 and progressively increased in weighting through day 7 to account for uncertainty around the southern stream system. The 00z CMCE was used on days 6 and 7 but with lesser weighting due to spread between its members. ...Weather/Hazard Highlights... The cold front reaching the East Coast on Friday will bring an increasingly progressive area of mostly rain across the region, with precipitation type over New England depending on the still uncertain latitude of the low track. Currently expect the rain/snow line to be fairly far north. There may be a brief period of lake effect/terrain-enhanced snow behind the front. Latest trends for the upper low dropping southward offshore California late this week through the weekend have been a little farther west, and just enough to decrease the potential for much precipitation along the coast, with most moisture staying offshore. Precipitation totals will still be sensitive to small changes in upper low track, so continue to monitor forecasts. Trends continue to be slower for the eventual arrival of the upper low into the Southwest, but eventually it should bring at least some moisture into parts of the Southwest/southern Rockies early next week, with precipitation possibly starting to develop over parts of the southern Plains/east-central U.S. sometime during the first half of next week. Meanwhile, northern stream flow rounding the upstream Pacific ridge should bring an increase of rain and mountain snow to areas from the Northwest into the northern-central Rockies, especially by Sunday-Monday. Favored terrain may see meaningful precipitation totals. The East Coast will see above normal temperatures (especially for morning lows) extend into Friday as a cold front approaches and then passes through during the day. Cool high pressure behind the front will support a broad area of chilly highs from the east-central U.S. through the southern Plains into parts of the West, with readings up to 10-15F or so below normal on Friday. Highs of 5-10F below normal may persist over the South into Friday. The West will briefly moderate closer to normal through the weekend. As the longer term mean pattern sets up, expect the central-eastern U.S. to trend warmer (highs up to 10-20F above normal and lows slightly more anomalous) by Sunday-Tuesday while the northern-central Plains into the Rockies/Great Basin trend colder (highs up to 10-15F below normal) by Tuesday. Kebede/Rausch 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 Hazards: - Heavy snow across portions of the Northern and Central Plains, the Northern and Central Great Basin, the Northern and Central Rockies, and the Pacific Northwest, Sun-Tue, Feb 19-Feb 21. - Heavy snow across portions of the Central Rockies, the Central Plains, the Northern and Central Great Basin, Mon-Tue, Feb 20-Feb 21. - Flooding possible across portions of the Southeast and the Mid-Atlantic. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Much below normal temperatures across portions of the Central and Southern Plains, the Central and Southern Rockies, the Northern and Central Great Basin, sin, and the Southwest, Fri, Feb 17. - High winds across portions of mainland Alaska, Fri, Feb 17. 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