Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 358 PM EDT Sat Oct 13 2018 Valid 12Z Wed Oct 17 2018 - 12Z Sun Oct 21 2018 ...Multi-day heavy precipitation possible across southern/southeastern Alaska next week... ...Guidance and Predictability Assessment... A nebulous forecast looms across the state and its periphery as vast model spread plagues the Day 5-8 period. Initially, global models are in agreement with a weakening surface wave spinning over far southwestern Alaska. Amplifying flow underneath this closed low is where the guidance quickly begins to break down. A very impressive low is likely to materialize and lift toward the northeastern Pacific during the Thursday/Friday timeframe. Recent runs of the ECMWF have tended to be on the slower side of the pack. Operational models over the past day or so agree on developing such a system but very little continuity exists with timing and placement. Given the lack of consistency amongst individual models coupled with spatial differences between the global models themselves, confidence is below average until some semblance of clustering is evident. With the potential for a surface low reaching pressures below 970-mb, wind fields will likely be much stronger than advertised in the latest forecast. What ever does materialize should lift sharply toward the north given the potent upper ridge sitting over British Columbia and the Yukon Territory. To complicate the forecast further, another deep cyclone is possible over the far western Aleutians. Ensemble low plots literally show a wave train of cyclones spanning the entire northern Pacific so it may be a while until the guidance resolves these complexities. As expected, the preference was heavily ensemble based from Day 5 onward utilizing a combination of the 12Z GEFS mean and 00Z ECMWF/NAEFS ensemble means. Before then, kept small proportions of the 12Z GFS/00Z ECMWF in the mix to capture the southwestern Alaska low on Day 4/Wednesday. ...Weather Highlights/Hazards... Vast differences in the solutions make some of the impacts more uncertain than usual. Assuming these cyclones emerge as shown, a fairly lengthy period of moderate to locally heavy precipitation would be in store along the south-central Alaskan coast into the panhandle region. Based on the high-resolution 12Z GFS model output, 96-hour precipitation totals would be in the 3 to 6 inch range across the former region while numbers nearly doubled would occur toward southeastern Alaska. Additionally, gusty winds are possible at times depending on the eventual pressure gradients, potentially in the 40 to 60 knot range. At this juncture, it is too difficult to pinpoint where this would even occur along the coast. Regarding temperatures, a cool down is anticipated over the northern section of the state with readings dropping into the low/mid 20s during the day and teens at night next weekend. Rubin-Oster WPC medium range Alaskan products including 500mb, surface fronts/pressures progs and sensible weather grids can be found at: http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html