Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 755 PM EDT Sun Oct 27 2024 Valid 12Z Thu Oct 31 2024 - 12Z Mon Nov 04 2024 ...Late week into weekend storm likely to bring strong winds and heavy precipitation to the eastern Aleutians and southern coast/Panhandle plus adjacent waters... ...Overview... Guidance is advertising an active pattern from the Aleutians eastward after midweek. A leading weak wave initially embedded within a front anchored by central Bering Sea low pressure will track into the Gulf of Alaska late this week. Then strongly deepening low pressure should reach near the eastern Aleutians by Friday with a northeastward track into the weekend as it slowly weakens. There is still uncertainty with the exact track but expect an area of strong winds and heavy precipitation with this system. Models and ensembles are signaling potential for another system to reach the North Pacific/Aleutians/Alaska Peninsula region toward the end of the period next Monday. Solutions disagree as to whether this feature will represent the extratropical reflection of a recurving Tropical Cyclone Kong-rey or be a purely mid-latitude development supported by shortwave energy carried along by progressive upper flow emerging from Asia. As for Mainland Alaska flow, expect an upper ridge to pass over the southern two-thirds of the region Thursday into Friday, followed by a tendency for an upper weakness to the north and southerly/westerly flow depending on specifics of the late week storm and energy over the Bering Sea. ...Guidance Evaluation/Preferences... Clustering and continuity have held up fairly well for the slowly weakening low over the central Bering Sea into late week and the weak Alaska Peninsula/Kodiak Island frontal wave forecast to track eastward into the Gulf of Alaska Thursday-Friday and then dissipate. Beyond early Friday some guidance continues to suggest that what remains of the Bering Sea low may loop around to the west/south, with enough model agreement now for a blend to reflect some definition through most of the weekend. In principle there is still good agreement in principle toward strongly deepening low pressure to reach just south of the eastern Aleutians by Friday, with a central pressure in the 940s-950s mb range, followed by northeastward progression and gradual weakening. There are important track and timing differences though, with some apparent trends. ECMWF runs have generally been leading with what trends have been developing over the past few days, and have recently been advertising an Alaska Peninsula/southern coast/Southcentral track. Latest GFS runs are a little east and eventually south of the ECMWF while the UKMET supports the Alaska Peninsula track to the end of its run early Saturday. In varying ways the 12Z CMC/ICON and all of the ensemble means show a more westward track of the parent low--into the southeastern Bering and eventually near the vicinity of Nunivak Island/Bethel/Norton Sound. 00Z/06Z machine learning (ML) models captured this full spread (while trimming the southern side of the envelope 24 hours ago). It appears a primary sensitivity for system track will be whether the supporting dynamics push east fast enough to maintain the southern coast surface track versus interacting some with energy to the north/west to yield a mean upper low over the Bering Sea/western coast with a surface low in its vicinity. Preference leans mostly in the 12Z ECMWF direction (with relative support from the GFS/UKMET) for the parent low given the ECMWF's recent trend-leading status. Small inclusion of the 12Z CMC and eventually more ensemble means would yield a separate weak western coast low that is simply a reflection of lingering upper energy after the parent low's passage. Review of 12Z ML models after forecast preparation reveals a shift toward greater clustering near the favored southern coast track, aside from the AIFS that remains farther northwest. Behind this system, there is a decent signal for low pressure development reaching the North Pacific/Aleutians around next Monday. However guidance behavior has been very diverse and erratic with respect to what proportion of this evolution may be from a recurving Tropical Cyclone Kong-rey (after reaching Taiwan late this week per the current JTWC forecast) or supported by mid-latitude energy carried along by progressive flow emerging from Asia. Earlier, the combination of 00Z ECens/CMCens and 06Z GEFS means plus the 00Z/06Z ML models were surprisingly agreeable in showing a somewhat cleaner recurvature scenario with a fairly strong system reaching just south of the Aleutians as of next Monday. However the 12Z ensembles and MLs are much messier with respect to the details of system evolution, while 12Z dynamical models focus more on mid-latitude extratropical development. Whatever the details, the 12Z GFS strayed north of nearly all other guidance while the GEFS mean was quite weak per recent behavior. This left the 00Z ECens and 12Z CMCens/ECMWF as the best blend components for next Monday's forecast. The new 18Z GFS surface low compares better to preference. ...Weather/Hazards Highlights... A developing weak frontal wave likely to be near the Alaska Peninsula and Kodiak Island by early Thursday should track eastward into the Gulf late this week before dissipating, producing some light to moderate precipitation across areas from the Kenai Peninsula eastward to the Panhandle. The forecast remains on track for a very deep storm to reach near the eastern Aleutians by Friday, bringing another episode of high winds and heavy rain to that region and surrounding waters, plus the Alaska Peninsula. There is still some track uncertainty after Friday but recent trends are nudging the more likely scenario toward the Alaska Peninsula/southern coast rather than farther northwest. This would temper effects along the Bering Sea coast while maintaining confidence in a period of strong winds and significant precipitation along the southern coast into the Panhandle later Friday into the weekend. Another developing North Pacific system may affect portions of the Aleutians into Alaska Peninsula with enhanced precipitation/wind by next Monday, but confidence in specifics is low at this time. Overall expect temperatures to trend warmer during the period. Some below normal readings will be likely over eastern/north-central parts of the mainland primarily Thursday into Friday followed by a trend toward above normal readings. Most other areas should start out already above normal with anomalies increasing thereafter. The Panhandle may prove to be the exception, with central-southern areas potentially staying below normal through the weekend. 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