Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 749 PM EDT Tue May 18 2021 Valid 12Z Sat May 22 2021 - 12Z Wed May 26 2021 ...Overview... Latest ensemble means show the pattern becoming more blocky with time as an upper low and associated surface system drift across the Bering Sea/Aleutians (CMC mean a little north of the GEFS/ECens) and upper ridging builds over much of the mainland. Multi-run trends in the means generally lean toward a stronger depiction of both features. Ahead of the building upper ridge, a weakening wave will track across the northeastern Pacific/southern Gulf of Alaska during the weekend while shortwave energy farther north could push a front across northern/central areas. Operational models also indicate the evolution toward a more blocky pattern but already diverge for leading details over/north of the mainland during the weekend and then for what becomes of the Aleutians/Bering Sea upper low. ...Guidance/Predictability Assessment... Before the start of the forecast early day 4 Saturday, there is already a divergence of solutions for energy that tracks just north of Kamchatka around Wednesday-early Thursday. The result is that the energy either feeds into the northeastern Pacific/southern Gulf low (GFS) or tracks over/north of the northern mainland (most other guidance). Alternatively, the 12Z CMC tracks the upper low over the central mainland and then drops it through the Southcentral region. The 12Z ECMWF trended toward less northward deflection than the prior run, tracing a wider arc than the CMC. Ensemble means have been quite weak with this energy so far. Latest model trends balanced by guidance spread and variability recommend adjusting to reflect somewhat more shortwave/surface front definition but in a conservative fashion. Farther south, guidance is better behaved for the weakening system that tracks south of the mainland. Addition of extra upstream energy in the GFS leads to a slightly deeper system but it is sufficiently within the realm of plausibility to be included as part of a favored non-CMC blend. From the weekend into very early next week the guidance is showing improved clustering for the Aleutians/Bering Sea system, with a composite of latest operational runs providing a good representation of the most likely evolution. However after early Monday the operational models rapidly diverge for what becomes of the initial upper low, ranging from varying degrees of elongation and possible splitting to maintenance of a consolidated system (12Z CMC). The 12Z ECMWF becomes the most extreme solution as it splits into separate upper lows that track into Siberia and the mid-latitude Pacific (ultimately building a ridge where the ensemble means have an upper low). For the latter part of the forecast prefer to guide the pattern toward the more stable and similar 12Z GEFS/00Z ECMWF means with some lingering input from the 00Z ECMWF (more compatible than the new run) and 12Z GFS, as well as the 12Z CMC once its mainland upper low issue becomes a minimal factor. ...Weather/Threats Highlights... Shortwave energy and associated surface front that may cross northern/central parts of the mainland during the weekend could produce scattered precipitation with a drier trend progressing from west to east. Specifics are fairly uncertain due to guidance differences with details of the energy aloft. Meanwhile some light to moderate precipitation associated with weakening low pressure south of the mainland should extend into the first part of the weekend over the southeastern coast and Panhandle. Bering Sea/Aleutians low pressure and potentially wavy leading frontal system will likely spread one or more organized areas of rain across the Aleutians and into the Alaska Peninsula and nearby areas. Precise system details that have low predictability at this time frame will determine the magnitude and eastward extent of rainfall over this region. The Panhandle and parts of the far southern mainland should see below normal highs for most of the period while the rest of the state should see max temperatures rise from near normal values on Saturday to above normal after the weekend as upper ridging builds overhead. Uncertain details of shortwave energy that may cross portions of the mainland during the weekend temper confidence in temperatures somewhat early in the period. Expect much of the state to see above normal morning lows Saturday-Wednesday with perhaps just a few pockets of slightly below normal readings. Rausch Hazards: No significant hazards are expected over Alaska during this forecast period. 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