Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sat Feb 08 2020 Valid 12Z Tue Feb 11 2020 - 12Z Sat Feb 15 2020 ...Heavy rain threat between the Southern Plains and Tennessee Valley/southern Appalachians next week... ...Overview and Guidance/Predictability Assessment... The forecast of the large scale pattern aloft has changed little over the past day, with eastern Canada into western U.S. mean troughing sandwiched between one ridge over the eastern Pacific and another ridge that should settle over Cuba and the Bahamas by late week/next weekend. This pattern will maintain the threat for episodes of heavy rainfall over portions of the southeastern quadrant of the country while occasionally bringing precipitation of more moderate intensity to parts of the West. For temperatures, the Rockies/Plains should see the most persistent below normal readings during the Tue-Sat period. The East/South will see above normal temperatures into Thu before a cooling trend Fri-Sat. While there is good agreement/consistency for the mean pattern, individual embedded features and periodic splitting of flow/stream interaction continue to provide some challenges in the forecast. The most significant issue during the period involves the interaction of the upper low that should start the period near the western U.S.-Mexico border and northern stream flow rounding the Pacific ridge and eventually helping to eject the upper low. Through the 12Z/18Z cycles there were two distinct clusters, a slower majority including the ECMWF-ECMWF mean/CMC-CMC mean and GEFS mean versus the faster GFS/UKMET. Some other global models also fell in line near the slower scenario. Preference for the updated forecast remained with the slower 12Z ECMWF cluster. Upstream shortwave details still have fairly low confidence given their small scale, and ongoing forecast uncertainty is highlighted by 00Z model trends--the UKMET trending slower and the ECMWF adjusting faster (though still slower than the GFS). The dominant solution would yield a surface wave over the Lower Mississippi Valley/Tennessee Valley on Thu with rapid northeastward progress thereafter. Differences for timing of the ejecting upper low as well as for flow to the north would influence the specifics of this wave. Interestingly there was better agreement among pre-00Z guidance for the next trough expected to drop into the West by Fri-Sat, with GFS/ECMWF/CMC runs and the ensemble means all similar aside from the operational runs tending to show more potential for an embedded upper low. Some earlier GFS runs had been underdone with this troughing and the new 00Z run goes astray after Fri due to being extremely amplified versus other guidance with another shortwave immediately upstream. The consensus among pre-00Z guidance allowed the late-period forecast to transition to an even model/mean blend that included the 18Z GFS and 12Z ECMWF/CMC along with the 12Z GEFS/ECMWF means, after the questionable GFS evolution to the east exited the picture. The 18Z GEFS mean trended weaker with the western trough, in contrast to consensus and teleconnections relative to the upstream positive height anomalies, so it was excluded from the blend. The 00Z run looks more similar to preference. Shortwave uncertainties in the Canadian/northern U.S. stream are producing some detail differences/adjustments at the surface. In particular the latest runs are showing a stronger cold front pushing south from Canada into the northern tier by midweek with stronger Canadian low pressure anchoring the front, while trending weaker with low pressure just to the south. ...Weather/Hazard Highlights... Rainfall over the Southeast should briefly trend lighter early in the period as a wavy front settles near the Gulf Coast. Then expect the potential for another significant event around Wed-Thu as the upper low ejecting from northwestern Mexico/southwestern U.S. and interacting northern stream flow support northeastward-tracking low pressure that would enhance focus activity north of the leading Gulf Coast front and along a trailing cold front. The best clustering of guidance continues to highlight an area from the southern Plains into the southern Appalachians but there is still enough spread in system details/timing to temper confidence in specifics. Any rain that does fall may lead to flooding/flash flooding concerns given significant totals observed over recent days. Uncertainty in specifics of this system also causes difficulty in resolving the coverage and intensity of winter weather threats on the northern side of the moisture shield. The initial upper low over/near the Southwest will produce some rain and high elevation snow in its vicinity, mainly over Arizona and New Mexico. Shortwaves dropping southeastward around the Pacific ridge aloft should bring one or more areas of rain/mountain snow across parts of the West, progressing from northwest to southeast. Most activity should be of light-moderate intensity. Within the area of below normal temperatures over the Rockies/central U.S., some locations in the southern High Plains may see highs 10-20F below normal Tue-Wed and parts of the Northern Plains/Upper Mississippi Valley could see one or more days with readings 10-20F or so below normal. Some of this cold air should reach the East by Fri-Sat with northern areas possibly declining to at least 10F below normal--displacing well above normal temperatures from Tue into Fri morning. Before the cooling trend some areas may see one or more days with lows 20-25F above normal and highs 5-15F above normal. 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