Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM 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 expected large scale pattern evolution has above average confidence in light of the agreeable forecast of the eastern Pacific ridge and teleconnections supporting the resulting downstream pattern seen in the multi-day means of model/ensemble guidance. As is typically the case, there are some noticeable embedded uncertainties. The greatest spread in specifics appears to be across southern Canada and northern tier U.S. from Friday onward, with differences for fast-moving and low-predictability shortwaves leading to divergence for one or more surface lows and frontal position. A conservative blend approach seems best for this aspect of the forecast. For the upper low offshore California, recent continuity/trends have generally tilted toward most operational runs that show a track just far enough west to keep all but perhaps a little light precipitation offshore. Ensembles are getting closer to the model scenario but still spread a little more moisture along the coast than the model consensus. This gradual offshore trend leads to a slower eastward progression later in the period, with the 12Z GFS/ECMWF/CMC and new 00Z GFS clustering near northern Baja California by day 7 Tuesday--at least a day slower than some guidance was showing a day or so ago. Still slower adjustments in the 00Z CMC/ECMWF (due in part to a flatter western trough) say this trend may not be done yet. The 18Z GFS looked less favorable, straying a bit faster with the upper low and the trough over the West. An average of other guidance looked reasonable for the developing western trough. Finally, there is still rather pronounced north-south spread for the system forecast to be over or near New England by early Friday. GFS runs have been on the northern side but at least are now a bit south of 24 hours ago. Other models have pretty much held their relative positions over the past day with the CMC tending to be a little faster and in the middle of the latitude spread while the UKMET is a southern extreme. A model composite with greater emphasis on the GFS/ECMWF provides the best long-term continuity. ...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. 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 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