textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key message:

* Flood Watch is now in effect for the Susitna Valley for areas upstream of Lake Creek and Yentna Rivers through Wednesday afternoon. Rainfall totals of 2 to 4 inches expected. Near the Parks Highway, from Willow through Talkeetna, 1 to 2 inches of rainfall expected.

Discussion:

The large low that has been tracking through the Bering Sea is now in Southwest Alaska. The main front associated with it has swept through Southcentral and is in the Interior of the state as of early this morning. The result is that there are strong pressure rises behind this front that are spurring gusty southerly winds up Cook Inlet and also though the Copper River Basin gap areas. Southcentral is also in the showery sector of the low with multiple shortwaves kicking off periods of showers. Throughout the day there should be a slow diminishing of both the winds and in shower activity for most of the area. The Susitna Valley is the one region that will likely really hold onto the rainfall as the south-to-southwest flow aloft is favorable for upsloping along the Alaska Range and Talkeetna Mountains. In addition, the Susitna Valley is closer to the upper low center which is keeping it more unstable as well. This is a major reason there is a Flood Watch in effect for some portions of the Susitna Valley.

By Tuesday, the upper level low will ave tracked northeastward though the Interior. However, as it usually the case in August, the large longwave trough remains over the Bering Sea and another upper level low embedded in this trough is ready to replace the old low. This will allow less shower activity for tuesday as the second low is sill out in the Bering Sea, but by Wednesday it should be moving into Bristol Bay and bring more widespread rainfall into Southcentral.

LONG TERM FORECAST (Days 4 through 7: Thursday through Sunday)

The long-term period starts with an upper-level low situated over Bristol Bay and a surface low over the Alaska Peninsula or extreme western Gulf of Alaska. These features will bring rain across the Southern Mainland through Thursday.

A stronger and potentially more impactful storm for Friday and the weekend across the Southern Mainland begins to take shape upstream in Kamchatka Thursday. A strong shortwave digs south from Siberia to the northern Bering Sea by Friday morning. A gale- force surface low Will move into the northern Bering around St. Matthew Island to St. Lawrence Island by Friday morning. Moderate to heavy rain is likely to overspread the Southern Mainland, starting first with Southwest on Friday, and working east to Southcentral through Saturday. Showers continue across the Southern Mainland on Sunday as the low works farther into Western Alaska.

Trends with this system, deviations in track and strength, will have to be watched closely when it comes to determining coastal impacts along the Kuskokwim Delta Coast. Taking a quick look at ensemble data, the GFS and Canadian ensemble suites are more in line with each other with a slower and stronger solution. The European ensemble guidance is a little faster with the storm track which has the system moving inland of Western Alaska by Friday; this solution also features a weak system. Regardless, there will be a push of on-shore flow with this system, it will be a matter of its magnitude which will be ironed out in due time.

Farther out west, embedded shortwaves moving from west to east atop a strong North Pacific ridge will keep the Aleutian Chain showery at times for Thursday. The northern Bering low's front will also sweep across the Chain for Friday and make it to the Alaska Peninsula (AKPEN) for Friday night into Saturday. The southern AKPEN will see the wettest conditions with this front Friday night into Saturday as it only grazes the Aleutian Chain Friday. The AKPEN will remain showery for Saturday and Sunday as flow turns northerly behind the northern Bering low which is moving inland by then. A surface ridge works in across the Central and Eastern Aleutians Saturday and Sunday as shortwave and another round of showers approaches the Western Aleutians from the North Pacific.

AVIATION

PANC...VFR conditions are expected to remain through the TAF period, though periods of showers may causing ceilings to dip to MVFR at times. Gusty southerly winds will persist through the day and start to diminish some overnight. As the surface winds and winds aloft are both from the south, low level wind shear is no longer expected.