Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 PM EST Sun Mar 05 2023 Valid 12Z Wed Mar 08 2023 - 12Z Sun Mar 12 2023 ...Despite some remaining uncertainty, a multi-day heavy rain event in the south-central and southeastern U.S. as well as snow over the north-central to northeastern U.S. still look probable this week... ...Wetter pattern expected to develop along the West Coast with moderating temperatures... ...Overview... Guidance is still showing a lot of variability/trending and spread for important details of upper flow from the eastern Pacific eastward, but some primary common themes have been evident from the guidance. The first is that a vigorous West Coast upper trough should progress eastward and produce a system (with leading wavy front) that could produce areas of potentially heavy rain and thunderstorms across the south-central U.S. through Wednesday-Thursday with rainfall of varying intensity reaching into the East late in the week. Additionally, cold air farther north associated with a very strong Canadian surface high could support meaningful snow across central and northern parts of the Plains, Mississippi Valley, and eventually parts of the Northeast. This system should tend to have a generally eastward track across the East given the influence of ridging/above normal heights aloft over Canada. The second major theme of this forecast period will be the trend toward a wetter regime along the West Coast with moderating temperatures. The impacts of this wetter and somewhat warmer trend in relation to areas that have had significant snow up to this point will require additional time to resolve given the current guidance spread. As the West moderates, the coldest temperature anomalies will shift more into the Plains by late week into the weekend. ...Guidance/Predictability Assessment... Latest models and ensembles continue to have considerable difficulty in resolving the details of complex Pacific/western North America upper flow. This specifically involves the precise evolution of initial Pacific through Alaska ridging along with an initial northwest Canada upper low and the flow between these two features. On the positive side, by early day 4 Thursday there has been a gradually improving signal that the northwest Canada upper low will drop down to a position offshore Vancouver Island/Washington state, putting enough of a dent in the ridge just to the west to support a fairly progressive upper trough/low across the lower 48. The new 12Z GFS has made a significant adjustment in this progressive direction (consisting of recent ECMWF/CMC runs and their means) versus previous runs, while GEFS runs started latching onto this trend about 24 hours ago. From a multi-day perspective, there has been some occasional adjustment within this general faster trend, so current preference for the lower 48 system is to go with a compromise between the faster 00Z ECMWF/ECMWF mean and prior ECMWF run/current GEFS and CMCens means. This provides a faster trend from continuity while not yet going to the fastest side of the spread. By day 7 Sunday, the best model/ensemble clustering has low pressure just off the southern New England coast. Even with the GFS adjusting closer to the prior majority, there is still the potential for significant changes in the "most likely" scenario, as the 12Z CMC now digs the trailing trough containing the Thursday Pacific Northwest upper low so much that it ultimately becomes more dominant than the leading system which all the other guidance is emphasizing. As for that upper low near the Pacific Northwest by early Thursday, solutions still stray all over the place after that point. It may come inland over the Northwest per the GFS and CMC, persist for a time and then lift into British Columbia (latest ECMWF runs), or dissipate (most ensemble means). In the most extreme 12Z UKMET case it drops far enough southwest to merge with undercutting Pacific flow and create its own trough (12Z UKMET). The spread for this small-scale upper low and surrounding flow leads to unusually low confidence in the details of moisture expected to reach the West Coast by the late week through weekend time frame. This is reflected in the dramatic QPF differences seen in recent operational model runs. All that can be said at this point is that the overall signal from guidance is for flatter mean flow from the Pacific that would increase precipitation and raise temperatures to some degree, with additional time needed to resolve the important details. Based on the best guidance comparisons over the forecast domain, the updated forecast employed a 00Z/06Z operational model composite into early day 4 Thursday and then transitioned to a blend consisting of the past two ECMWF runs along with the 00Z CMC plus the 06Z GEFS and 00Z CMCens/ECens means. ...Weather/Hazard Highlights... The upper trough initially over the western U.S. should produce periods of mostly light to moderate rain and higher elevation snow over the region through midweek or so. Then as this system emerges from the West and a wavy frontal boundary remains in place across the south-central U.S. through Wednesday-Thursday, subtropical moisture aloft will be streaming in from the Pacific with low level moisture arriving from the Gulf of Mexico. This combination will produce rain and thunderstorms that should be ongoing as of the start of the period early Wednesday and persist into late week. Rain rates are likely to be high given the moist and unstable airmass. The experimental Days 4-5 (Wednesday-Thursday) Excessive Rainfall Outlooks both maintain Slight Risks of flash flooding across areas of the southern Plains to Lower Mississippi Valley, atop areas that have seen ample rainfall recently and should receive another round of heavy rain during the day 3 (short range) period Tuesday. The risk expands into the Mid-South by Thursday though there remains uncertainty in the details. Latest model trends are suggesting a little faster eastward progression, leading to some eastward expansion of the day 5 risk area. After that time frame, rainfall may end earlier given this faster trend. Meanwhile farther north, snow will likely develop from the central Rockies expanding eastward into the central/northern Plains and Mississippi Valley where Canadian high pressure spills colder air into the region. Probabilities for wintry weather are broad for now given the uncertainty for timing and placement of notable snow, with some run-to-run shifting with consensus trends, but are increasing overall. Wintry weather looks to extend into the Great Lakes and Appalachians and then into the Northeast as well, depending on specifics of the expected central/eastern U.S. into western Atlantic low pressure system. Overall expect the pattern to support increasing precipitation by the late week/weekend time frame, but with unusually low confidence in the specifics of amounts and location. Already by Thursday-Friday, issues with the pattern aloft lead to meaningful spread for where enhanced rainfall could focus along the northern-central West Coast with some higher elevation snow. In order to introduce the signal for the general threat along the coast, the Day 5 Excessive Rainfall Outlook to be issued shortly will depict a Marginal Risk area along coastal portions of Oregon and northern third of California. Currently it appears likely that best focus may be within a more narrow north-south zone but will await future guidance to refine the Marginal Risk area. Additional precipitation is likely into the weekend, again with considerable uncertainty in the amounts and location of rain/mountain snow. The overall pattern should promote higher snow levels than seen in some recent events. Below normal temperatures over much of the West (highs up to 10-20F below normal) should persist especially through Thursday or so. The upcoming pattern change should lead to min temperatures rising to near/slightly above normal values by Saturday-Sunday while daytime highs may still remain a little below normal. Meanwhile the northern/central Plains will become the more prominent focus for cold temperatures with most days featuring highs 10-20F below normal with the best potential for locally colder highs mid-late week (and including the central Rockies during that time). Meanwhile above average temperatures at least in terms of highs will likely be confined to the Gulf Coast states Wednesday-Friday, but even these areas should drop to near or below normal over the weekend after the cold front passes through. In modified form, expect the chilly air over the central U.S. to spread eastward from the Plains after Friday. Rausch/Tate 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