Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 749 PM EDT Sun Jul 14 2024 Valid 12Z Thu Jul 18 2024 - 12Z Mon Jul 22 2024 ...Wet pattern for the Alaska Panhandle continuing into late week but with lesser totals... ...Overview... Most guidance continues to show surface low pressure tracking north/northwest over the northeastern Pacific late this week spreading moisture into at least the Panhandle, but latest solutions have delayed this evolution and trended lighter with rainfall totals. Farther north, expect a Bering Sea upper ridge to build eastward over the mainland by the weekend, replacing an initial trough and promoting a warmer trend for temperatures over most areas aside from the Southcentral coast and Panhandle. Along the southern periphery of progressive high latitude flow, a mean frontal boundary may set up near the northern coast from the weekend into early next week. Finally, a deep-layer low will make gradual progress from the northwestern Pacific through the western Aleutians and into the central Bering Sea, spreading a broad area of rain across the region and perhaps eventually into the southwestern mainland. ...Guidance Evaluation/Preferences... Complexity of the evolution involving the initial southern coast upper low/trough and separate dynamics to the south/southeast is leading to a fair amount of variability for surface low details over the northeastern Pacific into the Gulf of Alaska. The most prominent issue over the past 24 hours has been a slower trend by about a day. Also, the 12Z ECMWF has shifted the upper low and overall trough axis somewhat farther east than continuity or most other guidance late this week--ultimately leading to a weaker/eastward depiction of low pressure and less westward extent of rainfall. Most of the machine learning guidance (the 06Z/12Z AIFS, plus other ECMWF-initialized ML models that posted for the 12Z cycle after a gap of a couple runs) generally supports the dynamical model/ensemble mean majority in principle rather than the 12Z ECMWF. The surface system should rapidly weaken and drop southward after early Saturday while the upper low most likely tracks close to Haida Gwaii, with some typical spread for the exact path/timing. By late in the period the 12Z CMC strays to the slow side with the upper low. Meanwhile the ensemble means have provided the most consistent depiction of upper ridging that builds into the mainland from the Bering Sea, with the ECMWF tending to be closest to that scenario among operational models. The 12Z CMC fits consensus better after some earlier runs strayed north with the ridge, while the 12Z GFS appears too quick to lift the ridge northeastward. The 18Z GFS looks considerably better though. latest AIFS runs have compared well to the means. A model/mean average provides a steady forecast with the system drifting through the Aleutians and into the Bering Sea. While well within typical spread/errors at distant time frames, a couple less agreeable solutions by day 8 Monday include the 12Z ECMWF straying a bit east and the 12Z GFS weakening the primary system in favor of dynamics/surface low pressure dropping down the western side of the overall upper low. Again the 18Z GFS seems to be an improvement over the 12Z run. Given the non-12Z ECMWF majority over the Northeast Pacific late this week and then the means providing some useful stability by the weekend into early next week, today's forecast started with a blend of the 12Z GFS/UKMET/CMC and 00Z ECMWF for the first half of the period and then quickly transitioned to 40-50 percent total means (12Z GEFS/00Z ECens) with remaining operational input helping to add a little detail where feasible. ...Weather/Hazards Highlights... The preferred majority of guidance still shows surface low pressure lifting northward and then northwestward over the Northeast Pacific into or near the Gulf of Alaska late this week. However slower trends by about day have delayed the arrival of organized rainfall into the Panhandle (possibly extending as far northwest as the Southcentral coast), and rainfall forecasts are generally lower than in guidance from 24 hours ago. Thus expect totals to be much less extreme than with the ongoing event at the start of this week. The upper low initially near the southern coast and overall mainland upper trough may help to generate some scattered rainfall over the mainland into Thursday before upper heights rise. The Panhandle and vicinity should trend drier during the weekend. A northwestern Pacific low drifting into the Aleutians and Bering Sea along with associated fronts will spread areas of rainfall gradually eastward across the region during the period. The leading edge of this moisture may reach into the southwestern mainland by next weekend. This system (along with the northeastern Pacific low) may produce some brisk winds but with speeds likely staying well below hazardous criteria. Lingering upper troughing will support below normal highs over most of the state into at least Thursday, but with gradual moderation in progress as the upper trough weakens. Then expect increasing coverage of above normal readings by the weekend as an upper ridge builds over the mainland. The Panhandle and parts of Southcentral may see greater persistence of below normal highs though, and southwestern areas may trend back below normal with the arrival of some clouds/rainfall. Negative anomalies for min temperatures should be more moderate than those for highs early in the period, followed by increasing coverage of modestly above normal lows by next weekend and early next week. 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