Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Thu Feb 27 2020 Valid 12Z Sun Mar 01 2020 - 12Z Thu Mar 05 2020 ...Heavy rainfall threat over parts of the eastern half of the lower 48 next week... ...Overview... Expect mean troughing aloft to progress across the western and central states during the period, possibly reaching the eastern U.S. by the end of the period next Thu. This overall trough will consist primarily of sharply digging energy over the West which then ejects east/northeast and upstream Pacific flow that should amplify to some extent, along with weaker shortwaves across the northern tier and southern Canada. An upper ridge over the eastern Pacific should build into the West by midweek. The forecast pattern ahead of the leading upper feature that ejects from the West continues to be favorable for a heavy rainfall event over portions of the eastern half of the country. However guidance continues to vary significantly on the specifics of this event. ...Guidance/Predictability Assessment... Over the past day guidance spread has actually deteriorated regarding the ultimate focus for heaviest rainfall over the eastern half of the U.S., due to ongoing differences in how flow around the eastern Pacific upper ridge interacts with the upper trough/low that digs into or near southern California by early Mon. Thus confidence in some details is actually a little lower, while there is still good confidence toward a significant event in general. On one extreme, recent CMC runs have been slower than consensus with the upper low. Plus after ECMWF/ECMWF ensemble trends toward less amplification of lower-predictability Northeast Pacific flow coming into the Northwest, the CMC is now the most aggressive (minus some later-amplifying CMC ensemble members) with troughing behind the ejecting upper low. This leads to a CMC surface low track to the west of most other guidance. On the other hand GFS runs have been inconsistent, but generally indicating the highest heights/fastest arrival of upper ridging over the West. This has led to some GFS runs being on the weak/suppressed side of the spread for the surface reflection over the East, such as the new 12Z run and runs from yesterday. The 00Z/06Z GFS had a more intermediate track closer to yesterday's majority cluster. At the very least, by late in the period the 06Z/12Z GFS seem to become too fast with North Pacific flow while the 12Z GEFS mean leads to southern California upper low ejection slower than the CMC by early Wed. Based on guidance available through the 06Z cycle, the overall preference was to remain within the middle of the broad envelope of solutions. During the first half of the period the initial blend emphasized the 00Z ECMWF and 00Z/06Z GFS with minor 00Z UKMET/CMC input where agreeable. The rest of the forecast transitioned to a model-mean blend with more 00Z ECMWF/ECMWF mean input versus the 00Z-06Z GFS/06Z GEFS mean, including phasing out the 06Z GFS after day 6 Wed. Multi-day means are fairly similar in showing positive height anomalies over the Pacific consolidating and retrograding a bit during and just beyond the forecast period. Teleconnections relative to the core of these positive anomalies by the D+8 time frame favor an eastern/northeastern Pacific trough, a modest western U.S. ridge, and broad eastern trough. The aforementioned blend at the end of the period seems to lead into this teleconnection-favored pattern fairly well. ...Weather/Hazard Highlights... Amidst the ongoing spread and variability for the details, the pattern evolution at the surface and aloft continues to support the potential for a significant rainfall event over parts of the eastern half of the lower 48 next week. Strong low level flow of Gulf moisture should become established for 2-3 days--leading to anomalously high precipitable water values--while at least one leading surface wave lifts northeast from the Southern Plains along a slow moving front. Ejection of the upper trough/low expected to be near southern California as of early Mon should provide added focus/surface development by Tue-Wed. The best clustering of guidance continues to show that best potential for significant rainfall will be from eastern Oklahoma/northeastern Texas to the east or northeast into the Tennessee Valley/Ohio Valley and possibly Appalachians, with decreasing confidence beyond the ArkLaTex region. Some locations may see multi-inch accumulations with this event. Heaviest rainfall should occur to the north of areas that have had the most rain in the past couple weeks but this event could enhance/prolong flooding issues over and downstream from areas that see significant rainfall. The Storm Prediction Center continues to monitor severe weather potential, with the current outlook highlighting an area over and just west of the Lower Mississippi Valley for Tue-Tue night. Refer to latest SPC products for updated information. Continue to expect any winter weather effects to be confined to the far northern latitudes of the eastern states. Leading system affecting the West will bring areas of mostly light-moderate precipitation between parts of California and the central Rockies during Sun and progressing southeastward into Mon. The Northwest should see one or more episodes of rain/mountain snow in association with North Pacific flow aloft and embedded energy. Confidence is moderate to low for intensity and southward extent on a day-to-day basis due to guidance spread for specifics of flow aloft. Ahead of the developing mean trough aloft, above normal temperatures (some plus 10-20F anomalies) over the central U.S. on Sun will progress into the East through midweek. By next week expect decent coverage of lows 15-25F above normal and highs 5-15F above normal. Cooler air reaching the West/Rockies by Sun-Mon will bring highs down to 5-15F below normal and then modify somewhat as it continues across the Plains into the east-central U.S. by late in the period. The West Coast states and eventually Great Basin should rebound to above normal temperatures Tue onward. 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