Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 750 PM EDT Wed May 19 2021 Valid 12Z Sun May 23 2021 - 12Z Thu May 27 2021 ...Overview... Guidance still shows considerable spread and run-to-run variability over how the more blocky regime aloft will evolve, with some significant differences for embedded smaller-scale details as well. The established consensus per recent ensemble means would still have an Aleutians/Bering Sea upper low with associated surface system drifting eastward with time while ridging builds over a decent portion of the mainland. An alternative scenario would keep the upper low farther westward with differing shapes/positions of upper ridging to the north and east. What happens to short-range energy as it reaches the mainland and vicinity by the start of the period early Sunday is also up for debate. ...Guidance/Predictability Assessment... What happens to shortwave/upper low energy passing just north of Kamchatka around Wednesday-early Thursday does not appear any closer to being resolved than it was yesterday. The notable guidance shift today is that the 12Z CMC/UKMET have adjusted closer to the persistent GFS idea of dropping the larger proportion of this feature southeastward into the Gulf to merge with or replace the leading upper low/trough over that area--in contrast to latest ECMWF runs that have a compact upper low drifting over the mainland during the first half of next week. There is an area of potential weakness over the northern/eastern mainland, between a ridge to the northeast of the mainland and the ridge building into the western mainland, so cannot discount the possibility of some energy getting stuck within that area. However the consensus evolution of rising heights aloft over most of the mainland still recommends leaning away from the deep side of the upper low envelope regardless of the model (was the CMC 24 hours ago). Also of note, a more non-ECMWF handling of the energy leads to a somewhat deeper/closed and slightly slower upper feature nearing the Panhandle by Sunday. Farther west, an operational model blend provides a reasonable depiction of detail for the Aleutians/Bering Sea system during days 4-5 Sunday-Monday. By Monday this consensus is a bit south of continuity and the ensemble means. After that time operational runs begin to diverge with the upper low and character of the upper ridge to the north/east, as well as with details of Pacific flow. In varying ways both ECMWF and GFS runs have been erratic and at times very divergent from the ensemble means. Thus prefer to maintain the best continuity possible by trending the forecast more toward the ensemble means later in the period with lingering operational input from solutions that are closest in principle--in today's case the 12Z ECMWF and CMC. From Sunday into Monday the initial blend started with even weight of the 12Z GFS/ECMWF/CMC/UKMET. This helped to temper the lower-confidence mainland upper low in the ECMWF. The resulting blend actually provided fairly good continuity for the Arctic surface low and trailing front extending into the mainland as of early Sunday. Day 5 Tuesday served as the transition toward days 7-8 Wednesday-Thursday that started with about two-thirds total 12Z GEFS/00Z ECMWF means relative to the 12Z ECMWF/CMC. Arrival of the new 12Z ECMWF mean lowers confidence a little more as it retrogrades the Bering upper low in a manner similar to the 00Z operational run (and in contrast to multiple previous ensemble runs). Also it trends a bit weaker with the mainland upper ridge. ...Weather/Threats Highlights... The evolution and path continue to be uncertain for shortwave energy that may be over the mainland from Sunday into next week. Currently the most likely scenario would have a weak form of this energy supporting the potential for some scattered precipitation through Sunday and possibly into Monday over eastern areas. The Panhandle may see light precipitation continue into Sunday as a nearby system weakens. Expect Bering Sea/Aleutians low pressure and a potentially wavy leading frontal system to spread one or more organized areas of rain across the Aleutians, likely into the Alaska Peninsula, and possibly reaching as far east as the southern coast/Panhandle late in the period. Confidence is still low over the precise eastward extent and magnitude of precipitation, considering for example that guidance for the Alaska Peninsula ranges from nearly dry (in the minority) to some degree of front/wave-aided enhancement. Below normal highs are likely to be most persistent over the Panhandle and parts of the far southern mainland. Elsewhere there should be a mix of above/below normal highs on Sunday followed by a warmer trend that would increase coverage of above normal readings, with highest anomalies over western/interior areas. Uncertain details of upper level energy that may be over the mainland could have some influence on high temperatures. Low temperatures will also see a trend toward broader above normal coverage with highest anomalies expected to be over western and northern locations. 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