Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EDT Sun Sep 25 2022 Valid 12Z Thu Sep 29 2022 - 12Z Mon Oct 03 2022 ...Heavy rainfall expected across southern coastal Alaska and the Panhandle this week... ...Strong storm possible near the Aleutians Friday-Saturday... ...Synoptic Overview... High latitudes will see an active period this week into the weekend as strong upper ridging sets up to the northwest of Hawai'i and, more importantly, over northwestern Canada. This will favor an amplified pattern that may be exacerbated by the extratropical transition of newly formed T.D. 19W in the West Pacific. This may lead to an initial period of modest to perhaps locally heavy rain over coastal Southcentral into the Panhandle followed by the tropically-infused system into the Aleutians then the mainland/Southcentral this coming weekend. ...Guidance/Predictability Assessment... 12Z guidance mostly fell into two camps: quicker GFS/GEFS and slower ECMWF/Canadian/UKMET in two areas. For the extratropical transition of 19W (forecast to become a typhoon before reaching the higher latitudes), the GFS/GEFS were quicker than the other models/ensembles to bring it northeastward toward the Dateline. Though the other models were slower, a northern stream trough exiting the Kuril Islands south of Kamchatka will play a role in the phased trough development and to what extent a more northern low develops and becomes more dominant. With much more uncertainty than normal in the non-linear domino effect, opted to rely on the larger consensus nearer to the 12Z ECMWF but especially its ensemble mean (which typically handles more amplified flow better than the GEFS). This parallels another system downstream south of the Gulf, where part of the cold front will stall as a digging northern stream shortwave slips into the longwave trough, trying to form a closed low in the mid-latitudes well west of the CONUS Pac NW. Again, the GFS/GEFS were more progressive but preferred to side mostly with the slower solutions given the other features that support a more amplified and slower flow. Though the ECMWF could be too slow, a minority weighting of the GEFS balanced that possibility. This would stall the front's movement through the Panhandle Fri-Sat, continuing the heavy rainfall threat. Overall this blend/preference seemed to match fairly well to the latest day 3 point from JTWC for 19W (then forecast to be extratropical). ...Weather/Hazards Highlights... Heavy rainfall (with snow in the highest elevations) is likely on Thursday from southwest Alaska eastward across coastal Southcentral into the Panhandle, with several inches of rainfall expected in some locations (particularly coastal Kenai and the Chugach Mountains of southern Alaska). Rainfall will progress east with the front, but then return in earnest if the sfc wave along the front lingers to the southwest and then moves over at least the southern Panhandle. Locally stronger winds may accompany this system as well, especially Thursday in the Gulf. By Friday, the strong area of low pressure associated with ex-19W may bring a period of strong winds and heavy rainfall to the Aleutians with some possible coastal flooding. Many details still need to be sorted out over the next couple of days. Fracasso 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