Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Wed Mar 06 2024 Valid 12Z Sat Mar 09 2024 - 12Z Wed Mar 13 2024 ...Wet pattern over the Eastern U.S. this weekend with parts of the Interior Northeast possibly seeing some snow extend into early next week... ...Multi-day unsettled period along the northern-central West Coast starting late this week... ...Overview... Guidance continues to show strong surface low pressure development over the eastern U.S. this weekend in response to the likely closing of an upper low within the trough progressing into/through the East. This system will spread a broad area of moisture across the eastern states through the weekend with some areas seeing heavy rainfall and parts of New England likely to receive some snow. A broadening drier trend will extend from the central into eastern U.S. behind this storm. Meanwhile a series of shortwaves and surface fronts will push into the West, supporting multiple days of rain and higher elevation snow over the Pacific Northwest and northern California along with increasing snowfall over the northern Rockies by early next week. A mean ridge aloft setting up over the central U.S. will support an expanding area of well above normal temperatures from portions of the Plains into the Great Lakes during the first half of next week. ...Guidance/Predictability Assessment... Models have continued cluster around a single solution regarding the overall progression of the synoptic pattern across the CONUS and adjacent seas during the medium range period. Some uncertainty still exists with respect to the intensity of the negatively tilting upper trough. The North American models (06z GFS and 00z UKMET) trended a little weaker than the European models (00z EC/UKMET). The differences being that the North American guidance clustered around an open wave over the East on day 4 while the European models signaled a closed low. Fortunately all of the models, including the ensemble means eventually clustered around a deep closed upper low over the Western Atlantic on days 5, 6, and 7. A general model blend consisting of the 00z EC/UKMET/CMC and 06z GFS was used on day 3. On day 4, the 06z GEFS was introduced to the blend to account for some of that intensity uncertainty in the East. On day 5, the 00z ECE/ECE and 06z GFS/GEFS were favored over the UKMET and CMC. The blend weighting turns quickly toward the ensemble means on day 6 and continues into day 7 to account for typical deterministic uncertainty. ...Weather/Hazards Highlights... Guidance continues to show an active weather pattern for this weekend over the East, with an elongated axis of surface low pressure over the east-central U.S. as of early Saturday likely deepening and consolidating into a strong system near the Maine coast by early Monday. This storm should then track off to the east. During the Saturday-Saturday night period covered by the Day 4 Excessive Rainfall Outlook, guidance continues to emphasize two general areas of enhanced rainfall as the storm system develops. One is over the Southeast, reflecting eastward continuation of heavier convection expected Friday (now in the short range time frame). The ongoing Marginal Risk area continues to reflect favorable signals for at least one axis of heavy rainfall but so far guidance has been reluctant to improve its clustering for timing/location details. Therefore continue to hold off with an embedded Slight Risk area but with the expectation that improved guidance agreement should eventually support this refinement. The other area of enhanced rainfall potential extends across parts of the Ohio Valley/Lower Great Lakes/northern Mid-Atlantic, but with less instability suggested. A Marginal Risk area over this region accounts for a combination of model QPF signals, and over the Mid-Atlantic wet ground conditions from shorter-term rainfall, with new guidance not really providing a compelling need to change this risk area for now. Going into Day 5 (Sunday-Sunday night), areas along and a little inland from the New England coast may see a period of strong onshore flow that would support a period of heavy rainfall. Thus the Day 5 ERO has introduced a Marginal Risk area for this activity. Expect greater potential for snow farther inland over New England, with some locations possibly cold enough for some snow already as the system reaches the Great Lakes/Mid-Atlantic, followed by the pattern becoming increasingly favorable for snow as low pressure emphasis shifts to along/offshore the coast by Monday. The Winter Weather Outlook probabilities for at least 0.25 inch liquid in the form of snow still exceed 50 percent across portions of interior New England for the Sunday-Sunday night period. Expect some lake effect/terrain-enhanced snow to the lee of the eastern Great Lakes, while parts of northern New England may see some snow extend through Monday. This system will likely produce a period of brisk winds, especially over the Northeast. A rapid succession of frontal systems will bring multiple days of rain and mountain snow to the Pacific Northwest and northern California from the weekend through the first half of next week with snow steadily increasing over the northern Rockies as well. In the Saturday-Saturday night period covered by the Day 4 ERO, the lower elevations of the Olympics are the main region of interest for potential rainfall enhancement with lighter totals extending farther south. Guidance is still offering mixed messages for moisture/instability/QPF (GFS generally higher) so will maintain no risk area for now. By Day 5 (Sunday-Sunday night) there is a somewhat better, though not exceptional, moisture signal toward the California/Oregon border while rainfall from the prior day may provide a little more sensitivity from ongoing activity to the north. Thus the plan is to introduce a Marginal Risk area along the Pacific Northwest coast into northern California. Currently expect some locally moderate to heavy activity to persist into Tuesday, likely trending lighter by Wednesday as a building East Pacific upper ridge nears the coast. At the same time moisture may start to extend a little farther south inland from the coast. Supported by mean ridging aloft, the most pronounced temperature anomalies during the period will be on the warm side of normal and developing over the central U.S. around Sunday-Monday, then expanding to include the Great Lakes and vicinity by midweek. Some locations from the northern Plains into the central Great Lakes may see one or more days with daytime highs at least 20-25F above normal, with somewhat less coverage of similar anomalies for morning lows. To a less extreme degree, most of the country east of the Rockies should see above average temperatures by next Wednesday with the Southeast being closest to normal. Otherwise, the system affecting the East during the weekend will leave a cool day in its wake over the southern Plains on Saturday and the southern tier on Sunday with some near to slightly below normal readings over the East on Sunday-Monday. The west/snowy pattern over northern-central West Coast and eventually into the Rockies will maintain moderately below average highs and near/above average lows over the West. 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 WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw