Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Sat Oct 26 2024 Valid 12Z Wed Oct 30 2024 - 12Z Sun Nov 03 2024 ...Stormy pattern this coming week into the weekend with multiple threats of heavy precipitation/high winds across southern coastal Alaska and the Aleutians plus adjacent waters... ...Overview... During the past day the models/ensembles have continued to advertise an energetic pattern containing multiple significant storm systems, with typical ongoing refinements in specifics. As of early Wednesday, a developing system reaching the Gulf of Alaska during the first half of the week should be over or near the southeastern Gulf, with lingering enhanced winds and precipitation, while a deeper low will be tracking into the far western Aleutians/Bering Sea with a leading front sweeping into the eastern Aleutians. The Gulf system will then drop south/southeast and an embedded weak wave should form along the Aleutians into Alaska Peninsula front with a track into the Gulf after midweek. Guidance is still in the process of consolidating its signal for strong western-northern Pacific low development by late week, reaching the Aleutians around Friday and then slowly weakening as it reaches somewhere between the northern Gulf and eastern Bering Sea/southwestern mainland during the weekend. This system should bring another period of strong winds and enhanced precipitation. Across the mainland, a ridge moving into the west/south should displace initial trough/upper low energy, with some of this energy potentially lingering over northern areas into late week or the weekend. ...Guidance Evaluation/Preferences... For the system likely to be south of the Panhandle as of early Wednesday, there are still some detail differences within a slowly improving guidance cluster. Recent GFS runs have leaned on the west/south side of the spread for the upper/surface system, though overall guidance trends through the 12Z cycle have nudged toward a compromise relative to the spread from 24 hours ago. Fine scale details of upper low formation are affecting the surface low depth as well, with recent guidance between the 970s and 980s mb (00Z machine learning/ML models averaging around 980 mb). The 12Z GFS was on the weak side of the spread while the new 18Z run is now on the deeper side. An intermediate depth looks most reasonable given the variability in guidance. At least into Thursday, guidance clustering is very good with the initially strong (950s-960s mb) but slowly weakening western-central Bering Sea low. There has been some timing spread for the leading front that likely forms a weak embedded wave as it passes through the Alaska Peninsula late Wednesday into Thursday, but ongoing spread between slower ECMWF/faster GFS runs has narrowed with an apparent trend toward the recently favored compromise speed. After Thursday, the Bering Sea low may wobble back to the west/south behind incoming North Pacific/Aleutians storm (though with enough uncertainty for it to become washed out within a larger scale surface trough for deterministic forecast purposes) while the aforementioned frontal wave tracks eastward into the Gulf and then dissipates. Latest guidance maintains its signal for strong development of western-northern Pacific low pressure that reaches into the Aleutians around Friday and then continues northeastward in some fashion into the weekend. Among operational guidance, the 12Z CMC/CMCens mean are on the west side of the spread especially on Friday, with ML models more in favor of the GFS/ECMWF/UKMET and the GEFS/ECens means. At that time, a number of model runs have been showing a depth into the 940s-950s mb with the average of ML models between the dynamical runs and the weaker ensemble means. Just among the means, the GEFS has finally trended deeper to approach the other means as of Friday but then strays a lot weaker than the ECens/CMCens means as has been the case recently--though model/mean consensus does reflect gradual weakening of the system during the weekend. As for track by Saturday-Sunday, the relatively larger cluster of dynamical and ML guidance shows a path into the southeastern Bering Sea. A couple 00Z ML runs had advertised a North Pacific into Gulf track but those have lifted closer to the Alaska Peninsula/southern coast. Meanwhile 6-hourly GFS runs have been alternating between a northern track (00Z/12Z) and a southern coast track (06Z/18Z). There appears to be some sensitivity to exactly how the supporting dynamics interact with initial Bering Sea energy, with greater interaction/merging leading to the northwestern half of the spread versus less interaction resulting in a farther south/east track. The best compromise among models/ensemble means would yield a track reaching the southwestern coast near Nunivak Island by early next Sunday. This is closest to the 12Z CMC/CMCens/GEFS/ECens and 00Z ECMWF. The 12Z ECMWF strayed somewhat faster into the mainland, though it should be noted that overall consensus trends have been somewhat faster. The 12Z ECMWF appears to be on its own for a trailing North Pacific development that has it affecting the Aleutians by next Sunday. This is a separate extratropical system versus the recurving tropical cyclone Kong-rey which the ML models are best-clustered in advertising well south of the western Aleutians (around 45N latitude) as of next Sunday. The 00Z ECMWF/00Z ECens mean were in that neighborhood as well. With typical detail differences, there is a little better agreement over the mainland in principle, as an incoming western/southern ridge displaces initial energy into the northern part of the state, with some degree of weakness possibly persisting or drifting northward with time during the weekend. Guidance preferences early in the period led to starting the forecast with 40 percent 12Z ECMWF and the rest split evenly among the 12Z GFS/UKMET/CMC for Wednesday-Thursday. Then the blend transitioned to half 12Z ECMWF and the rest GFS/UKMET to reflect the most common theme for the deep low nearing the Aleutians by early Friday. By Saturday-Sunday, with ECMWF runs diverging somewhat for the main storm of interest while the 12Z CMC became more palatable as part of a compromise, and the 12Z GFS strayed northward by Sunday, the forecast adjusted to include 20-40 percent total of the 12Z CMCens/00Z ECens while splitting 12Z/00Z ECMWF runs and reintroducing a little 12Z CMC as well as removing the 12Z GFS (Sunday only). This solution yields relatively minor adjustments to continuity. ...Weather/Hazards Highlights... The storm likely to be south of the Panhandle by early Wednesday should still be producing some strong winds in its vicinity, while precipitation trends lighter along the Southcentral coast and Panhandle as the low tracks to the south/southeast. The deep western Aleutians/Bering Sea storm will bring an area of high wind potential over the Bering Sea/Aleutians into Wednesday along with enhanced rainfall focus into the eastern Aleutians/Alaska Peninsula by Wednesday, with the rain focused by a leading front crossing the region. An weak frontal wave likely to form near the Alaska Peninsula should track eastward into the Gulf before dissipating, providing a brief precipitation focus to areas near the Kenai Peninsula by Thursday. A modest amount of moisture could reach as far east as the Panhandle. The general forecast has been fairly consistent over the past day for a potentially very deep storm to reach near the eastern half of the Aleutians by Friday, bringing another episode of high winds and heavy rain to that region and surrounding waters, plus the Alaska Peninsula. Precipitation/wind effects over the southwestern mainland after Friday will be very sensitive to the surface low track, which currently is most likely to be toward the southwestern mainland. With less sensitivity to exact low track, confidence is higher regarding enhanced precipitation spreading eastward along the southern coast and Panhandle during the weekend. During mid-late week, expect temperatures to be mostly above normal over the western mainland and North Slope, with anomalies for morning lows likely to be somewhat greater than for highs on some days. The eastern mainland and Panhandle will be more below normal for max/min readings. Around Friday and continuing into the weekend, flow ahead of the deep Friday-weekend storm should bring warmer temperatures to much of the state from west to east, increasing positive anomalies over the west and finally bringing most eastern areas to or above normal. Rausch Additional 3-7 Day Hazard Information can be found on the WPC medium range hazards outlook 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