Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sun Jan 24 2021 Valid 12Z Wed Jan 27 2021 - 12Z Sun Jan 31 2021 ...Heavy precipitation likely for California mid-late week with another possible event focusing along the central West Coast next weekend... ...Rapidly deepening low pressure increasingly likely near the East Coast next Thursday... ...Weather Pattern Overview and Guidance/Predictability Assessment... Guidance continues to show an active pattern with mean troughing aloft along/offshore the West Coast and ejecting embedded features periodically crossing the lower 48 to produce areas of significant weather. A leading vigorous shortwave initially over the Plains will approach the East Coast by early Thu and support rapid strengthening of low pressure tracking into/over the western Atlantic. At the same time an amplifying/sharpening trough just off the West Coast will support a period of enhanced moisture flow into California during mid-late week. Another developing trough farther upstream is likely to be fairly deep but somewhat more shallow, which would direct moisture more toward the central West Coast next weekend. Progression of the late week West Coast trough will likely spread an area of precipitation across the eastern half of the country next weekend. During the first half of the period models continue to display some spread for eastern U.S./western Atlantic evolution at the surface and aloft but overall are trending closer together. In particular the latest GFS runs have backed off somewhat on the depth of a closed low that may develop within the upper shortwave just before reaching the East Coast along with the amount of precipitation the system produces, while the 12Z ECMWF/UKMET adjusted stronger aloft and northward/deeper with the surface low. The new 00Z CMC is on the slow and weak side but other 00Z solutions including the ECMWF compare fairly well with the emerging consensus. Through the 12Z/18Z cycles an operational model compromise appeared reasonable. Behind this system, a steadily increasing percentage of models wants to pull a compact upper low over southern Canada southeastward through the Northeast instead of lifting it northward as the majority had indicated earlier. Fortunately this feature should have a fairly modest effect on sensible weather aside from supporting colder temperatures. Guidance spread for the upper trough off the West Coast is now fairly narrow for the days 3-4 Wed-Thu time frame, while there has been some variability for the exact details of associated low pressure nearing the coast. Some timing spread develops mid-late period in response to upstream flow differences that tend to have moderate to low predictability several days out in time. Preferences leaned slightly to the slower GFS/GEFS/12Z CMC in principle due to earlier trends settling upon the slower timing. However somewhat faster timing at least partially toward the faster 12Z ECMWF is still possible given the speed of upstream flow. Interestingly the extra upstream shortwave in the 12Z ECMWF ultimately drops into the consensus position of the leading shortwave over the central U.S. by day 7 Sun. ...Weather/Threats Highlights... Expect the upper trough sharpening just off the West Coast along with the associated low pressure/frontal system to support areas of heavy rain and mountain snow over California in the mid-late week time frame. There will likely be a gradual southward progression from the central Coastal Ranges and Sierra Nevada into the southern Coastal and Transverse Ranges. Guidance continues to suggest the potential for some 5-10 inch liquid totals in this event. Less extreme precipitation totals will spread through other parts of the West and into the Rockies. Acceleration of the upper trough should produce a surface system and expanding precipitation area from the Plains eastward next weekend. A warming trend ahead of this system should limit most of any wintry weather to the Great Lakes/central Appalachians/Northeast. Also next weekend upstream Pacific flow should begin directing another round of moisture into the West Coast. Currently the best signal for highest totals with this next event exists over northern California and southwestern Oregon with somewhat lesser amounts farther north over the Pacific Northwest. Although guidance has not yet fully converged, agreement is improving for the evolution of initial Gulf Coast/southern U.S. low pressure that should deepen rapidly as it tracks into and over the western Atlantic. This storm should be just off the Mid-Atlantic coast by early Thursday and then continue farther offshore with time. Currently expect the best potential for some snow to extend from the Middle Mississippi Valley through the Mid-Atlantic with highest totals over the central Appalachians/Mid-Atlantic. Finer details that can take into the short range to be resolved will ultimately determine accumulations at any particular location. Progressive nature of this storm may be a limiting factor for snowfall totals. Meanwhile this storm may bring an area of moderate to heavy rainfall to the Southeast. Consensus still shows a period of strong winds along and offshore the East Coast in the gradient between the deepening storm and trailing high pressure building toward the eastern states. Ahead of the Gulf Coast wave that will deepen rapidly as it moves into the western Atlantic, lows over the Southeast will be quite warm to start the period early Wednesday--as much as 15-25F above normal. Cool high pressure building into the East behind this developing storm will bring readings down to moderately below normal levels for a couple days late this week. Meanwhile around midweek the Southwest and vicinity may see highs as much as 10-15F below normal with parts of the central Plains 10-20F below normal. The southwestern states will moderate slightly after Wednesday but likely remain below normal. On the other hand the upper ridge building into the central and eventually eastern U.S. will promote a warming trend to areas east of the Rockies. The northern Plains will see plus 10F or greater anomalies first, followed by an expanding area to the south and east. By the weekend an increasing proportion of the eastern two-thirds of the U.S. should see plus 10-25F anomalies for lows and some areas of plus 10-15F anomalies for highs. 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, 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