Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 336 PM EDT Sun Aug 11 2019 Valid 12Z Thu Aug 15 2019 - 12Z Mon Aug 19 2019 Today's guidance continues to depict an increasingly amplified/blocky pattern but with meaningful spread for exact placement/orientation of the large scale features not to mention embedded shortwaves. For the strong North Pacific ridge building into the eastern Bering Sea, guidance appears to have stabilized for the day 4 Thu forecast but trends beyond that time extend the multi-day tendency toward a stronger ridge. The most prominent model discrepancy involves the 00Z ECMWF straying east of established consensus for the ridge and downstream amplifying trough (the latter forecast by consensus to extend from northwestern Canada into the northeastern Pacific). Thus the first consideration is to phase out 00Z ECMWF input for the latter half of the forecast period. Teleconnections relative to the ridge's positive height anomaly center suggest that the northeastern Pacific trough may end up being closer to the 00Z ECMWF mean than somewhat farther west GFS/GEFS mean runs. The greatest embedded uncertainty involves an upper trough/possible embedded low forecast to be near the northwest corner of the mainland as of early day 4 Thu. Individual model runs have varied considerably over the timing and track/amplitude as the trough/low drop into the favored larger-scale mean trough and likely support surface low pressure between the Gulf of Alaska and just south of the Alaska Panhandle. Recent 12-hourly runs have oscillated on timing and of particular note latest UKMET runs are on the faster side of the envelope after being on the slower side early yesterday/Sat. Also GFS runs had been mildly fast but the new 12Z GFS adjusted back to the 00Z ECMWF/CMC. The new 12Z CMC supports the 12Z GFS/GEFS and 00Z ECMWF mean cluster in principle. However the 12Z ECMWF has completely changed its evolution, and while its scenario is too different from remaining guidance to recommend for a deterministic forecast, it highlights the continued lack of confidence. By late in the period the details of Arctic flow that may approach/reach the northern mainland become another question mark. The 12Z GFS favorably adjusted to a more conservative solution relative to the 06Z and especially 00Z runs. Thus far ensemble means are inconclusive beyond the theme of northwesterly flow. Meanwhile there is decent consensus on a deep layer low over the northwestern Pacific but with underlying uncertainty over the specifics, some of which rely on the evolution of the eastern Bering ridge aloft. Waviness along the associated front may brush the western Aleutians. A blend approach with steadily increasing reliance on the means will provide a reasonable starting point. Forecast considerations led to starting with primary emphasis on the 12Z GFS/00Z ECMWF-CMC for days 4-5 Thu-Fri followed by fairly rapid removal of the 00Z ECMWF and increase of ensemble mean guidance (more ECMWF ensembles versus the 06Z GEFS). By day 8 Mon the forecast started with a 70/30 blend of the ECMWF mean/GEFS mean. The resulting forecast reflects the stronger ridge trends after Thu but otherwise maintains reasonable continuity for the overall pattern evolution. Based on the most likely forecast scenario, moisture ahead of the shortwave dropping through the mainland may produce areas of locally significant precipitation Thu-Fri. Guidance is consistent in showing the potential for a period of fairly strong winds over parts of the central-western Aleutians with one or more bands of enhanced rainfall as well. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can also be found at: https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html