Alaska Extended Forecast Discussion NWS Weather Prediction Center College Park MD 746 PM EDT Fri Jul 28 2023 Valid 12Z Tue Aug 01 2023 - 12Z Sat Aug 05 2023 ...Pattern overview with Guidance Evaluation/Preferences and Predictability Assessment... Recent guidance runs have been showing significant differences regarding the details of upper ridging that may extend from western Canada across parts of the mainland and possibly into the Bering Sea, along with shortwave energy approaching from the west and then a larger scale upper low/associated trough forecast to cross Kamchatka toward the end of the week. The ultimate track of an upper low to the south of the Panhandle will also depend to some degree on the pattern to the north and west. These differences aloft lead to corresponding discrepancies for the surface pattern and sensible weather specifics over a large part of the forecast domain. Latest guidance has generally sorted itself into two relative clusters. The majority grouping consists of the 12Z ECMWF/CMC/UKMET and ECMWF/CMC ensemble means (with the 12Z CMCens trending somewhat more to latest ECens means, adding to this majority). This scenario would have leading shortwave energy splitting just before the start of the period, with a shortwave brushing the northern mainland (with low amplitude flow persisting across northern areas for a time), and ridging that extends across the southern mainland into the eastern Bering Sea. Approaching upstream energy would be diverted underneath the ridge, taking a track along or south of the Aleutians. On the other hand, GFS runs bring this latter energy and associated low pressure northeastward into the Bering Sea while amplifying the mainland/Canada ridge farther northward. Latest GFS runs are also on the fast extreme with the upper low/trough tracking across Kamchatka late in the period. The 12Z GEFS mean reflects GFS ideas but in less extreme form. For the upper low initially south of the Panhandle, ensemble means are fairly similar in showing only a slow northward drift. Not surprisingly the operational model runs show a lot more variation. The 12Z CMC appears over-amplified with its shortwave coming into the mainland from the west early in the period, leading to undue influence on the upper low of interest. Otherwise the overall average of latest models has trended somewhat slower, a nod to the ensemble means. Based on the above guidance comparisons, and yesterday's continuity being much closer to the majority ECMWF cluster, today's forecast followed that scenario. Days 4-5 Tuesday-Wednesday started with half 12Z ECMWF and 1/4 each 00Z ECMWF and 12Z UKMET to reflect operational model detail without incorporating less desirable aspects of the CMC. Then the forecast incorporated gradually increasing input of the 12Z CMCens and 00Z ECens so that total ensemble weight reached 50 percent by day 8 Saturday. Operational ECMWF input continued to be at least a 2:1 ratio of 12Z to 00Z runs. ...Weather/Hazards Highlights... Some episodes of showers will be most likely across northern, eastern, and southern parts of Mainland Alaska with some local enhancement along favorable terrain and near a wavy front that may persist over or near the Interior. One or more waves that should track along or south of the Aleutians may produce rainfall in their vicinity, while the western mainland remains fairly dry. However note that the minority scenario would have a farther north/northeast progression of moisture that could also reach the western mainland, so continue monitoring forecasts for any changes in forecast preference. The Panhandle may see some increase of moisture as the upper low to the south drifts closer, though most rain that falls should be on the light side. Expect high temperatures to be above normal over the Interior but more below normal over northern areas and in some pockets within Southcentral and the Panhandle. Lows should be above normal over much of the state through the period. Rausch No significant hazards are expected over Alaska during this forecast period. 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