Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 745 PM EDT Wed May 26 2021 Valid 12Z Sun May 30 2021 - 12Z Thu Jun 03 2021 ...Periods of heavy rain likely from the Peninsula region to the Panhandle, as well as in parts of the southern Alaska mainland... ...Overview and Guidance Evaluation... There is general agreement on the broad pattern but some important discrepancies exist for the details. From Sunday into Monday the guidance consensus shows two separate upper lows, one retrograding over the southeastern half of the Bering Sea and the other to the south of the eastern Aleutians/Alaska Peninsula. Then solutions diverge regarding their relative strength, track, and how they may interact or merge. By Tuesday-Thursday progressive North Pacific flow may carry a couple waves that could also interact with the overall mean low aloft. Meanwhile most solutions suggest that some degree of upper troughing will extend from the Canadian Archipelago into the far northern/northwestern mainland and Bering Strait (perhaps digging into parts of the Bering Sea by mid-late next week) while mean ridging extends from western Canada into the mainland. The general pattern will support a period of wet weather from the Alaska Peninsula eastward to the Panhandle with some moisture eventually making its way northward as well. After reasonable clustering into early Monday, operational models diverge for specifics of the two upper lows with no common theme. Through arrival of the 12Z ECMWF, that model was on its own in holding the Bering Sea upper low nearly in place through the end of the period. At the same time the 06Z/00Z GFS runs became quick to drop the Bering low southward and merge it into the North Pacific Low, and in contrast the UKMET weakened the North Pacific low in deference to the Bering low. These differences ultimately led to significant spread for details of individual surface lows and fronts from the Bering/North Pacific into the Gulf. Given the low predictability of this evolution, preference followed a 12Z model blend early (splitting the ECMWF contribution between the 12Z and prior 00Z runs) and then transitioned to 70 percent total ensemble weight (12Z GEFS/00Z ECens) by day 8 Thursday. This provided an intermediate approach into Tuesday and then reflected the most common theme of the large scale pattern, along with a conservative reflection of North Pacific waves that may interact with flow to the north. The new 12Z ECMWF mean has hedged westward some with the Bering Sea low but not to the point of the operational run, and at least into Tuesday the 18Z GFS has adjusted favorably toward preference with this low. ...Weather/Threats Highlights... The details remain uncertain but the overall pattern continues to favor southerly upper level flow that should bring a series of fronts into the Gulf of Alaska along with periods of enhanced rain and wind from the eastern Aleutians or Alaska Peninsula to the Panhandle. Models and ensembles generally show the most anomalous moisture remaining along the southern coast into the early part of next week. Then the axis of greatest moisture should lift northward with some disagreement over magnitude. Thus expect the best potential for heavy rain along the southern coast during Sunday-Monday. Totals should be more moderate Tuesday-Thursday but some localized heavier rainfall may still be possible depending on exactly how features at the surface and aloft evolve. Northward progression of moisture after Monday will likely increase coverage and intensity of rain/mountain snow over more of the mainland with time. The forecast pattern should yield below normal high temperatures over a majority of the state, including the Panhandle. The North Slope and Panhandle/Southcentral should see the coolest anomalies relative to normal. Far western parts of the mainland have the best potential to see above normal highs during the period. For low temperatures expect below normal readings to prevail over the northeastern half to third of the mainland while mostly positive anomalies extend over remaining areas. 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: - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat-Sun, May 29-May 30. 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