Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 633 PM EDT Mon Jun 22 2020 Valid 12Z Fri Jun 26 2020 - 12Z Tue Jun 30 2020 ...Overview and Guidance/Predictability Assessment... Once again the latest ensemble means provide a fairly stable and coherent picture of the large scale pattern evolution through the period while some operational runs develop significant differences. Thus today's forecast maintains an approach that starts with an operational model consensus early followed by increasing emphasis on the ensemble means (reaching 70 percent by days 7-8 Mon-Tue) along with remaining inclusion of operational guidance that is closest to the means. Operational models continue to be quite varied among each other and in some cases consecutive runs for the Arctic upper low and surrounding flow. Some GFS runs including the 12Z version have been trying to split the upper low in a curious fashion, with the 12Z run actually tracking the main low across the northern mainland late in the period. CMC runs have also been straying from the ensemble mean cluster (which keeps the upper low northwest of the mainland) with the 12Z run dropping it into the northern Bering Sea before tracking toward the southwestern mainland. The 12Z ECMWF briefly brings the center close to the northwest mainland but an average of the last two ECMWF runs is closer to the means. Preference is to stay near the ensemble means until they provide a strong trend toward an alternate scenario. Already by day 5 Sat the latest ECMWF runs diverge on details of leading energy shearing eastward/northeastward across the mainland--also arguing for use of both runs as part of the forecast blend. Among latest guidance the notable difference for the system that tracks into the central Pacific/Aleutians during the weekend is that the 12Z GFS is weaker than other guidance with the upper trough/low. In fact by day 5 Sat the GEFS mean is closer to consensus than the operational run so the forecast used the GEFS mean in place of the GFS after Fri. Questionable northern stream evolution in the CMC led to its removal from the forecast after Sat. The UKMET compared well to consensus to the end of its run early Sun. Faster trends that were starting to emerge yesterday have generally followed through, but the 12Z ECMWF has adjusted somewhat slower than the previous run. The ensemble means and an average of the two ECMWF runs provide a good starting point for this system which should rapidly weaken once it passes the Aleutians. Recent continuity has been good for the system expected to reach the Gulf of Alaska by the start of the period early Fri. What scatter there has been in the guidance over the past couple days has generally been gravitating toward the ongoing forecast. ...Weather/Threats Highlights... Panhandle precipitation associated the system reaching the Gulf by Fri will quickly taper off late this week. The mainland will see some scattered convection/rainfall, with increasing uncertainty in specifics depending on the details of the Arctic upper low and possible shortwaves embedded in the surrounding flow. In the early part of the period the best focus should come from shortwave energy near the western coast of the mainland on Fri and then progressing/shearing eastward or northeastward. Assuming the most likely scenario for the system tracking into the Aleutians by this weekend and quickly weakening thereafter--including minimal phasing with flow to the north--expect an area of precipitation to spread into the Aleutians/Alaska Peninsula and then to some degree across parts of the mainland. Some local enhancement may be possible between the Aleutians and southern coast of the mainland but with low confidence at this time. Continue to anticipate below normal highs over much of the state during the period. Coolest anomalies should be over the west and south as well as the Panhandle late this week into the weekend. The southwest may moderate a bit by early next week. The northern coast and northeast corner of the mainland should be closest to normal or even slightly above normal. Low temperatures will likely be more moderate and best potential for above normal lows will be along the northern coast/over the northeast as well as in some pockets over the south, especially after the weekend. 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 Hazards: No significant hazards are expected over Alaska during this forecast period. 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