Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1252 PM EST Tue Feb 11 2020 Valid 12Z Fri Feb 14 2020 - 12Z Tue Feb 18 2020 ...18z Update... No major changes were needed for todays update to the medium range progs valid Friday - Tuesday. The latest model runs since last night have not shown any major shifts so thoughts remain the same as below. Today's progs utilized a blend of the latest runs of deterministic models (the GFS and the ECMWF) for days 3 to 5, increasing usage of the means by days 6 and 7 to mitigate the smaller scale, less predictable, detail differences. Through the whole period, 20 percent of WPC continuity was used, resulting in minimal changes to the previous fronts/pressures progs and the weather grids. Santorelli ...Overview and Guidance/Predictability Assessment... Most guidance maintains variations of western U.S. troughing between one ridge over the eastern Pacific and another that should meander over the Bahamas and Cuba during most of the period. There are some alternative solutions but the majority cluster is suggesting that most shortwaves within the mean flow will be fairly progressive through at least the weekend, followed by some amplification of the western trough by next Tue in response to a strong storm forecast to track into the Gulf of Alaska. The overall pattern during the period continues to favor enhanced rainfall potential centered over the southern half of the east-central U.S. but ongoing uncertainties over individual shortwaves continue to make it difficult to resolve timing/intensity/coverage details of any heavy rainfall that occurs. Meanwhile shortwaves flowing into the Northwest will support periods of terrain-enhanced rain and higher elevation snow from the Pacific Northwest through the central Rockies. Based on the full array of guidance there were plausible attributes in both the operational models and ensemble means, ultimately leading to an even blend of the 18Z GFS/GEFS mean and 12Z ECMWF/ECMWF mean as the starting point for the days 3-7 Fri-Tue forecast. The shortest-term question mark has been what happens to shortwave energy reaching the West by the start of the period Fri. 12Z-18Z guidance and new 00Z solutions that have arrived so far have trended closer together, indicating a fairly progressive shortwave over the lower 48 with only a modest proportion of the energy peeling off into a weak trough/closed low offshore Baja California. This is certainly closer to what the ensemble means have been advertising in recent days versus some earlier UKMET/ECMWF runs that had closed off a fairly deep low near California. There is a decent signal in the guidance that another shortwave will reach the West Coast by around early day 5 Sun, with this feature ultimately supporting central-eastern U.S. low pressure by days 6-7 Mon-Tue. It may take a while for guidance to reach a consensus for details of this system given the difficulty that the models have had in resolving shortwave details in this pattern. Shortwave specifics over the West become even more unclear late in the period. The 18Z GFS/12Z ECMWF show fairly similar ideas for an amplifying/sharpening trough aloft over the West by the end of the forecast but in very different ways, while their corresponding means agree with the general trough evolution--again supporting the model/mean approach. ...Weather/Hazard Highlights... Shortwave/frontal systems reaching the Northwest and continuing inland will bring rain and mountain snow to locations from the Pacific Northwest through the northern and central Rockies. Forecasts have been consistent in advertising highest five-day totals over western Washington/Oregon, along windward-facing terrain in the Cascades and along coastal areas. Expect a secondary precipitation maximum in the northern Rockies. The heaviest event should be during the weekend and Sat-Sat night in particular. Amplification of the eastern Pacific ridge aloft will likely promote a drier trend early next week. So far there is little progress in refining the precipitation forecast over the eastern half of the country from the weekend into next week, with shortwave uncertainties still leading to a significant spread for where and how much rainfall may occur over the South as well as for details of lighter precipitation farther north. Tendencies from this pattern and the overall array of guidance still suggest one or more episodes of enhanced rainfall may occur between eastern Texas and the southern Appalachians. A warm front lifting from the Gulf of Mexico into the South may also help to focus activity for a time. However, again confidence is well below average for specifics. Most snow should be confined to locations around the Great Lakes/Northeast and possibly the central Appalachians, though areas farther west across the northern tier could be in play depending on development of the central-eastern U.S. system early next week. Earlier in the period, lake effect activity late this week should taper off by the weekend. The most extreme temperature anomalies during the period will be over the eastern half of the lower 48--with a dramatic swing from below/much below normal readings Fri into Sat followed by well above normal temperatures by Mon-Tue. Locations in the Midwest/Upper Mississippi Valley may see lows reach 20-30F below normal early Fri, while max/min temperatures 10-25F below normal should be common Fri into Sat from the Great Lakes/Ohio Valley into the Northeast. The return of southerly flow will then bring temperatures to at least 10-20F above normal by next week. Over the West, the Rockies will likely remain below normal most days with some pockets of highs 10F or so below normal, mostly on Sat and Mon-Tue. Elsewhere highs should be within a few degrees of normal while the active pattern should keep lows near to 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