Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri Mar 17 2023 Valid 12Z Mon Mar 20 2023 - 12Z Fri Mar 24 2023 ...Another strong storm with heavy rain/snow and wind threats to impact California... ...Overview and Guidance/Predictability Assessment... Latest guidance agrees fairly well upon the idea of the large scale upper pattern featuring a mean trough along or a little inland from the West Coast. The most prominent system within this pattern should reach the West early-mid week with an atmospheric river containing some moisture of tropical origin, and then track across the Plains/east-central U.S. later in the week with an expanding shield of precipitation over that part of the country. Another system dropping into the mean trough may reach the West Coast by next Friday. At least in a general sense, guidance agrees fairly well for the dominant system of interest affecting the West and then areas farther eastward. There are some moderate timing differences as the vigorous upper trough moves into the West by midweek, with the new 00Z CMC/UKMET leaning a bit on the faster side of the spread versus other models/means. Farther east later in the week, the evolution of surface low pressure appears to be quite sensitive to the specifics of the dynamics aloft. Recent GFS runs have tended to be deepest with low pressure reaching the Great Lakes while the GEFS mean has been the strongest among the ensemble means. The new 00Z CMC has introduced the idea of a more persistent closed upper low, which would keep the surface low west of the Upper Great Lakes through most of next Friday. A general model/mean blend by this time seems to provide a good representation of the current most likely track for this system with intermediate depth. The next upper trough forecast to reach the West Coast late in the week has good guidance clustering in concept but recent GFS runs have been somewhat sharper and more amplified than most other solutions. Meanwhile the upper flow details across southern Canada and northern tier U.S. from the Plains through New England appear to be among the more contentious aspects of the forecast, with effects on a wavy front that may affect areas from the northern Plains eastward. The 12Z ECMWF/CMC were fairly amplified with upper troughing that reaches New England by Thursday while GFS runs have generally been much weaker/faster. The new 00Z UKMET/CMC are even slower than the 12Z ECMWF with a fairly amplified trough, with the UKMET representing a significant change from its 12Z run. Ensemble member spread keeps the means fairly modest in the depiction of this trough, so an intermediate solution looks reasonable. Finally, confidence is fairly low for small-scale shortwave specifics across the southeastern quadrant of the lower 48 early-mid week. The 00Z GFS represents a somewhat faster/weaker trend for that model's wave off the southeastern coast on Wednesday, with most other guidance still showing higher surface pressures offshore at that time. The array of 12Z/18Z guidance led to the updated forecast starting with an operational model blend for about the first half of the period. Then the increasing detail uncertainties favored a fairly rapid transition to 40-50 percent total 18Z GEFS/12Z ECMWF means along with some continued input of the 18Z GFS/12Z ECMWF and a little 12Z CMC. ...Weather/Hazards Highlights... A fairly weak leading system will support areas of rain and higher elevation snow that should be in progress across a decent portion of the West at the start of the period on Monday. Then a stronger upper trough digging toward the West Coast, along with associated low pressure/frontal system, will direct a plume of enhanced moisture into parts of California and southwestern U.S. around Tuesday. Expect a period of moderate to heavy lower elevation/coastal rain and heavy high elevation snow especially in the Sierra Nevada. Rainfall totals should be less extreme than this past week's events while snow levels remain lower, somewhat reducing the flooding/runoff issues. However the forecast rainfall totals along with wet ground conditions do appear to support a Marginal Risk in the day 5 (12Z Tuesday-12Z Wednesday) Excessive Rainfall Outlook) for areas from the lower elevations of the Sierra Nevada to the southern California coastal ranges, as well as favored terrain over central Arizona. Eastward progression of this system should then bring significant snowfall into parts of the Great Basin and central Rockies Tuesday-Wednesday. This system should then emerge over the Plains and Great Lakes by Thursday-Friday, with some snow possible over far northern areas and rain/thunderstorms over the eastern Plains/east-central U.S. Currently expect the heaviest rainfall to be extend from the ArkLaTex region into the Lower Ohio Valley. Another Pacific system may bring rain and mountain snow to the Pacific Northwest and parts of California toward the end of the week. The early part of the week will see well below normal temperatures across the southern tier with some localized readings of 20F or so below normal. Near or below freezing lows over some areas may be harmful to any new spring vegetation. There will be a steady warming trend over the eastern half of the country to above normal temperatures by the latter half of the week, ahead of the system tracking out of the West. The mean trough aloft over the West will maintain a broad area of below normal highs across that region, with minus 10-20F anomalies most common Tuesday onward. The far northern Plains will also see below normal temperatures for most of next week, especially on Monday-Tuesday. 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, 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