Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Wed Feb 26 2020 Valid 12Z Sat Feb 29 2020 - 12Z Wed Mar 04 2020 ...Heavy rainfall threat most likely centered over the east-central U.S. early-middle part of next week... ...Overview... Expect mean troughing aloft to move into the western-central U.S. after the departure of a cold amplified upper trough from the East Coast states this weekend. As this occurs, upper ridging will build over the Caribbean/western Atlantic while another ridge over the east-central Pacific should drift eastward to 130W by next Wed. Embedded details remain uncertain but the overall pattern should be favorable for a heavy rainfall event from parts of the Southern Plains northeastward. The primary question at this time is the path of highest totals beyond the Plains, with significant spread in latitude. ...Guidance/Predictability Assessment... The primary overall forecast challenge continues to involve the evolution of sharply digging energy near the West Coast, possibly closing off an upper low for a time near southern California by around early Mon, and upstream Northeast Pacific energy whose specifics will have increasing influence on ejection of the Southwest feature. Some GFS runs have been a bit on the western side of the spread for the path of the upper low that may close off over/near California though not to an extreme enough extent to preclude from a multi-model blend. The 00Z UKMET was also on the western side. New 12Z GFS/UKMET runs have adjusted to the established model/ensemble mean consensus for the overall trough--providing some increase of confidence at least to about Mon-early Tue as the feature cross the southwestern U.S./northwestern Mexico. Meanwhile guidance is still quite diverse for the relative strength/timing of low-predictability upstream energy that should flow into the northwestern U.S. and reinforce the overall mean trough, along with details of the Pacific ridge aloft. As has been the case in recent runs, the GFS/GEFS are slower and flatter than most other solutions with the shortwave energy, corresponding to a broader upper ridge. The guidance majority is sufficiently great to continue favoring some shortwave energy reaching the Northwest by day 6 Tue and amplifying into the Interior West/Rockies by day 7 Wed. Some GEFS mean input currently provides the best hedge to account for alternative ideas, as its recent runs have been much closer to other non-GFS solutions for low pressure reaching near the Great Lakes by early day 7 Wed in spite of the differences upstream. This is in contrast to GFS runs that depict a slower and more detached southern stream wave late in the period. During the weekend and into early next week there is still some spread and variability for southern Canada/northern tier U.S. flow and corresponding surface features. Latest majority cluster has adjusted faster and in fact is closer in principle to what some earlier GFS runs had advertised. Recent UKMET runs have been even faster, enough to require exclusion from the deterministic blend but not of the realm of possibility given recent trends. The updated forecast started with primary emphasis on the 00Z ECMWF/00Z-06Z GFS/00Z CMC during the first half of the period followed by a trend toward more 00Z ECMWF mean/06Z GEFS weight with some lingering input from 00Z ECMWF/CMC runs (and a little 00Z GFS into early day 6 Tue). ...Weather/Hazard Highlights... Expect portions of the West to see two primary episodes of rain and mountain snow during the period. The first event should be in progress over the Northwest as the weekend starts, with mostly light/moderate activity dropping south-southeast into early next week as flow aloft sharpens/amplifies near the West Coast and possibly closes off an upper low for a time. Highest totals should be along southwestward-facing terrain. Upstream energy heading into the Northwest and then amplifying to some degree should bring another area of moisture into the West Mon-Wed. Depending on specifics, favored Pacific Northwest terrain may see a brief period of heavier precip in this event. Guidance spread for the eventual amplitude of flow aloft keeps confidence low regarding the southward extent of moisture across the Great Basin and Four Corners states. Thus far guidance has been consistent in principle for highlighting the potential for a heavy rainfall event most likely to be centered over the east-central U.S.. The combination of a couple days of fairly strong low level Gulf inflow along with strong dynamics in the form of the ejecting southwestern U.S. trough/upper low seem to support the idea of a significant event. There is decent agreement on an area including northeastern Texas and eastern Oklahoma as being on the western side of the threat area. However there is considerable spread for the axis of highest totals farther east--anywhere between the Tennessee Valley/southern half of the Appalachians and Ohio Valley/Lower Great Lakes. Current forecast preference leans somewhat more toward the northern half of this range. The Storm Prediction Center is also monitoring the potential for strong to severe convection. Details are too unclear to specify any favored area at this time so check future SPC outlooks and discussions for updated information. Guidance continues to indicate that any winter weather with this system will be confined to fairly far northern latitudes in the eastern half of the lower 48. Low pressure that may track from the Southern Plains northeastward around Tue-Wed could provide some added focus for snowfall where sufficiently cold temperatures exist but again confidence is fairly low in the details at this time. Western/central U.S. warmth during the weekend, with some plus 10F or greater anomalies, will continue eastward with time and lead to a pronounced warming trend over the East next week after a chilly start to the weekend (highs 5-15F below normal). Especially by next Tue-Wed the eastern half of the country should see one or more days of lows 15-25F above normal and highs 5-15F above normal. Mean troughing aloft that moves into the western-central U.S. should bring moderately cool readings across the West and into the Rockies/High Plains from late weekend into next week. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml