Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Mar 20 2023 Valid 12Z Thu Mar 23 2023 - 12Z Mon Mar 27 2023 ...Heavy rainfall threat over parts of the east-central U.S. after midweek... ...Overview and Guidance/Predictability Assessment... Models and ensembles agree on an active pattern with a series of moderately progressive systems affecting the lower 48, while a slower-evolving upper trough/surface system amplifies just off the West Coast by next Sunday-Monday. Upper ridging centered over the Gulf of Mexico as of Thursday should progress eastward for a while but could rebuild back to the west-southwest by the start of next week. The most prominent focus of this forecast period will be on a heavy rainfall event forecast over the east-central U.S. around Thursday-Friday as an upper trough emerges from the West and leads to abundant moisture interacting with a wavy front trailing from a northeastern U.S./southeastern Canada system, as well as the wave/front supported by the western dynamics. The Storm Prediction Center is also monitoring severe weather potential over some areas from the southern Plains eastward. The Interior Northeast could see meaningful snow from this system by Saturday. Meanwhile, areas from the northern/central West Coast into the Rockies should see periods of light to moderate rain/mountain snow late this week into the weekend. Then the upper trough expected to develop just off the West Coast late in the period may lead to higher totals over some coastal areas by the start of next week, depending on exact details of the upper trough. Guidance clustering has been gradually improving for the system crossing the northeastern U.S./southeastern Canada Thursday into early Friday. Latest GFS runs are most at odds with other solutions by straying to the fast side with leading shortwave energy that supports the wave tracking east of northern New England by Friday. Including the 00Z version, latest GEFS means offer much better support for the non-GFS cluster for the specifics of this system. Confidence is low with specifics of possible weak waviness along the trailing front over the East into Friday. The past 12 hours of model runs are finally attempting to converge regarding the details of the upper shortwave tracking out of the West and through the Plains, along with the associated low pressure evolution. The prior wide array of guidance is generally gravitating toward recent ECMWF mean runs that have been depicting the surface system reaching the vicinity of the Ohio Valley by 12Z Saturday and near the New England coast or Canadian Maritimes 24 hours later, corresponding to some sharpening and possible closing of shortwave energy. Operational ECMWF runs have been a bit on the northwestern side of the spread at the surface. Having the current/recent solutions finally look more similar is definitely progress, though exact track is still an issue and quite a bit of spread still exists for how strong the system will be by the time it affects New England. From the 12Z/18Z cycles that were used for the updated forecast, the main consideration was to lean away from the more suppressed 12Z UKMET by the end of its run early Saturday. Recent model runs have been waffling with respect to the character of upstream coming into the West late this week into the weekend, though latest consensus has returned to a fairly broad and diffuse presentation which has been most persistent in the ensemble means and in half or more operational models from prior cycles. As this energy continues east, the means agree quite well on the shortwave reaching near the Mississippi Valley by early day 7 Monday with low pressure over the Great Lakes. Operational model runs, especially the GFS, have been inconsistent with the strength and other details of this system. This favors a blended/ensemble mean approach for the time being. There are still plenty of detail uncertainties for the upper trough expected to amplify just off the West Coast by Sunday-Monday. Most guidance including the ensemble means continue to advertise a fairly clean departure of leading energy followed by amplification of separate energy dropping southeast from Alaska, while ECMWF runs still hold onto more of the leading energy and have some form of interaction with the Alaska energy. Thus far the ensemble means have been providing the best overall template, showing the upper low reaching just off the Pacific Northwest coast by next Monday. The new 00Z CMC has adjusted to this idea too after being more along the coast in the prior run. GFS runs are tending to pull the upper low a bit westward of consensus, leading to an overall trough somewhat west of what teleconnections would recommend based on the location of strongest positive height anomalies upstream over the North Pacific. While the ECMWF details are still a question mark, by the end of the period the 12Z run does end up getting closer to the preferred means. Based on above considerations, the updated forecast based on 12Z/18Z guidance started with an operational model blend but with some split of 18Z GFS/GEFS mean input. Then by late in the period the blend rapidly trended toward increasing 18Z GEFS/12Z ECMWF mean guidance with minority remnants of GFS/ECMWF ideas. ...Weather/Hazards Highlights... The leading system crossing the northeastern U.S. and southeastern Canada will spread some light to moderate precipitation across parts of the Northeast, including some snow over northern New England. Then the focus will turn to a potential heavy rain event over the east-central U.S. during Thursday-Friday, as an upper trough initially over the West progresses eastward while already-established moist flow from the Gulf of Mexico persists into late week and interacts with a wavy front left behind by the Northeast system. Even with wide array of solutions for specifics at the surface and aloft over recent days, there was a consistent signal for this event in principle, and now some of these details are starting to get better refined--but some additional adjustments are still likely. The new day 4 (12Z Thursday-12Z Friday) Excessive Rainfall Outlook will be quite similar to the previous day 5 issuance, reflecting only minor adjustments for latest guidance with respect for a Slight Risk area from eastern Oklahoma and vicinity northeast into western Ohio. The day 5 (12Z Friday-12Z Saturday) will follow along with the southeastward progression of heaviest rainfall, with a Slight Risk area from parts of Arkansas/northern Louisiana through much of Kentucky. Strengthening of low pressure later in the day 5 period should eventually lead to some acceleration of rainfall by early Saturday. Continue to monitor Storm Prediction Center outlooks for the latest information regarding severe weather threats from the southern Plains eastward. As the system continues northeastward, interior portions of the Northeast may see meaningful snowfall depending on the exact low track. The initial upper trough crossing the West followed by upstream energy will support periods of rain and mountain snow from the northern/central West Coast into the Rockies. Most amounts should be in the light to moderate range but localized enhancement will be possible. Once this trailing energy departs from the West, another system may bring some precipitation of varying intensity to portions of the central/eastern U.S. by Sunday-Monday. Also in that time frame, the upper trough most likely amplifying just off the West Coast may start to increase precipitation totals along the central coastal areas based on the consensus trough position. The large scale pattern will keep much of the West and parts of the northern Plains quite chilly through the period, with broad coverage of highs 10-20F below normal each day. The Great Basin and central Rockies could see highs as cold as 20-25F below normal during the weekend. Some record cold highs may be possible. Temperatures over the Northwest should be less extreme but still be mostly below normal. In contrast, an area of well above normal temperatures will extend from the central-southern Plains through much of the East during the latter half of the week. Thursday-Friday will be the warmest days for highs with plus 10-20F anomalies and some morning lows that could be as much as 20-25F above normal Thursday into early Saturday. Frontal passage across these areas will bring a cooling trend to near or slightly above normal levels. 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