Extended Forecast Discussion NWS Weather Prediction Center College Park MD 413 PM EDT Thu Apr 06 2023 Valid 12Z Sun Apr 09 2023 - 12Z Thu Apr 13 2023 ...Overview... Latest guidance is consistent with the general idea of an amplified eastern Pacific upper trough reaching the West Coast by the middle of next week, with the main question mark being embedded details that have lower predictability several days out in time. This feature and leading wavy surface front will promote multiple days of rain and high elevation snow over some areas from the West Coast into the northern Rockies. Meanwhile the models and ensembles have shown some pronounced spread/trending for the pattern farther east. Currently the guidance is gravitating toward the idea of a low amplitude pattern on Sunday evolving toward a more blocky regime next week as Plains energy drops into an evolving Gulf Coast upper low and ridging over portions of the West bridges across the northern half of the Plains and into the Midwest/Lower Great Lakes/Upper Ohio Valley. The upper low and associated surface reflection may produce areas of heavy rainfall along the eastern half of the Gulf Coast into the Southeast next week. ...Guidance/Predictability Assessment... Thus far the guidance has displayed better than average agreement and continuity for the overall handling of the upper trough moving into the West by mid-late week. An intermediate position looks good for a possible embedded upper low into early Wednesday, with the 00Z ECMWF a bit south of other solutions at that time. Not surprisingly, by day 7 Thursday more spread develops with respect to precise trough timing and upper low details--whether the Pacific Northwest low persists and tracks elsewhere, a separate low forms (and when, if the latter is the case), or the trough opens up per the ensemble means and 00Z CMC. The 00Z ECMWF continues to be farthest south with its upper low Thursday while the 00Z/06Z GFS were a bit slower than other solutions. The new 12Z GFS has adjusted to the consensus trough axis while waiting beyond early Thursday to close off a new southern upper low. The 12Z ECMWF offers a slight nudge toward preferences relative to the 00Z version, at least into early Thursday. Over the central/eastern part of the country, the operational models and GEFS/ECMWF ensembles have been rapidly converging toward the idea of early week Plains shortwave energy dropping into an upper low that settles along the Gulf Coast by midweek while ridging builds over areas to the north. Some ECMWF/CMC runs and a minority cluster of ensemble members advertised this possibility earlier than the GFS and most GEFS members, while the 00Z CMC mean was still weaker and more elongated than other guidance (as is the new 12Z CMC). The 00Z/06Z GFS runs were somewhat west/northwest of other models/means but the new 12Z GFS has adjusted somewhat closer to the GEFS/ECMWF means and 00Z ECMWF. The new 12Z ECMWF is close to the previous run. The updated forecast started with a 00Z/06Z operational model blend for the first half of the period and then transitioned to about one-third total 06Z GEFS/00Z ECMWF mean input. For the incoming western trough, this yielded the agreeable position for its axis while toning down the more extreme aspects of the 00Z ECMWF/00Z-06Z GFS runs. The blend provided a reasonable intermediate position for the Gulf Coast upper low. ...Weather/Hazards Highlights... Abundant moisture and a focusing wavy cold front on the leading edge of the approaching Pacific upper trough will support a period of moderate to heavy precipitation over favored terrain and coastal areas of the Pacific Northwest during Sunday-Monday. The experimental Day 4 Excessive Rainfall Outlook maintains a Marginal Risk area from the central Oregon coast to the Olympic Peninsula. As the upper trough and leading front slowly progress eastward next week, expect Pacific Northwest precipitation to trend gradually lighter while the northern Rockies and vicinity should see multiple days of snow and low elevation rain. Eventual coverage and amounts across California and the Great Basin are more uncertain, depending on precise details of the upper trough/possible embedded low. This issue will also influence precipitation specifics over the northern Rockies by Wednesday-Thursday. Central U.S. shortwave energy may produce some areas of rainfall over the region Sunday-Monday. As this energy forms an upper low along the Gulf Coast with a possible northern Gulf surface system, there will be an increasing potential for moderate to heavy rainfall over some areas from the eastern half of the Gulf Coast through the Southeast during Tuesday-Thursday. There has been a lot of guidance spread/trending for this evolution thus far, so overall confidence will remain below average until solutions exhibit better agreement and stability. Ahead of this evolution, easterly flow behind a cold front may already support periods of rain over the Florida Peninsula starting earlier in the period. The Southeast will be on the chilly side to start the period Sunday-Monday, with some locations seeing highs 10-20F below normal on Sunday. The region may continue to see near to slightly below normal temperatures through the rest of the period depending on specifics of clouds and rainfall with the expected Gulf Coast upper low. Meanwhile fairly warm temperatures over the West at the start of the week, with some readings 10-15F or so above normal, will trend cooler with time as the upper trough moves in. Highs may be 5-15F below normal by next Thursday. The greatest warm anomalies during the period will extend from the northern-central High Plains into the Great Lakes and New England, with plus 10-25F anomalies for highs/morning lows. The northern High Plains should trend cooler by midweek. 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 Hazards: - Heavy rain across portions of California, the Pacific Northwest, Sun, Apr 9. - Heavy rain across portions of the Southeast and the Lower Mississippi Valley, Tue-Wed, Apr 11-Apr 12. - Heavy snow across portions of the Pacific Northwest, Mon-Tue, Apr 10-Apr 11. - Flooding possible across portions of the Middle Mississippi Valley, the Southern Plains, and the Northern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Flooding likely across portions of the Southeast. 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