Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM 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... Successive guidance runs have been gradually improving their clustering for the system developing over the eastern U.S. Most dynamical and machine learning models have generally been leaning toward maintaining two defined surface low centers from Saturday into Sunday, albeit with some differences in emphasis, before solutions converge toward the dominant surface low being near the coast of Maine as of early Monday. Latest UKMET runs have been a bit on the slow side for the upper low forecast to form over the Great Lakes and then track eastward. Recent ECMWF runs were also a bit on the slower side but the new 00Z run has shifted a little east (which also has better support from the machine learning models). An operational blend incorporating more 18Z GFS/12Z ECMWF along with some 12Z CMC and least weight from the 12Z UKMET reflected the best consensus and reasonable continuity for this forecast update. Along the West Coast, there is good agreement for the first shortwave of the series arriving on Saturday and then moderate clustering for the next one arriving around Sunday. The new 00Z UKMET strays to the slow side while remaining guidance shows typical spread for amplitude and timing. Confidence steadily decreases with time as these features continue eastward through the larger scale mean ridge covering the central U.S. By Tuesday-Wednesday recent guidance has been consistently in conflict regarding how much troughing will move from the East Pacific into western U.S., with GFS/GEFS runs generally leaning toward flatter flow and the ECMWF/CMC and their means (plus the ICON) advertising more amplified troughing. The majority of latest machine learning models would favor tilting more toward the ECMWF cluster, though the ML models along with teleconnections relative to the East Pacific upper ridge's positive height anomaly center offer potential for the western U.S. upper trough that develops by Wednesday to be a little east of the ECMWF. Note that while the new 00Z GFS still shows flatter flow over the West as of early Wednesday, it does trend more strongly toward the ECMWF idea a day later. The 00Z GEFS mean also shows a trend toward the ECMWF mean. These considerations through the 12Z/18Z guidance runs led to tilting the forecast about 70 percent toward the ECMWF/CMC/ECens scenario by late in the period. ...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. 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