textproduct: Anchorage

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)

Key Messages:

* New: A Wind Advisory has been issued for Anchorage and the Hillside from 8PM this evening to 10AM Tuesday. 40 to 50 mph gusts are expected for west/south Anchorage, while the Seward Highway near the Turnagain Arm and the Hillside will see gusts up to 60 mph.

- Heat Advisory remains in effect for the Copper River Valley through this evening.

- Flood Watch remains in effect for the eastern Kenai Peninsula from Monday night through Thursday morning, due to heavy rain.

- Flood Watch remains in effect for Susitna and lower Matanuska Valleys from Tuesday morning through Friday morning, due to heavy rain.

Discussion:

Enjoy one last relatively quiet evening for Southcentral as windy and rainy conditions return to the region for the start of the work week. Diving into the details, a broad, occluded, and anomalously "wet" (high precipitable water values) ~988 mb low pressure system currently located over the northern Bering is sending in some upper level clouds over Southcentral this afternoon. Despite the clouds and the marine layer, temperatures have warmed today into the low to mid 60s for the Cook Inlet region, with predominant low 70s for the Matanuska and Susitna Valleys. The warmest portions of Southcentral today have been located across the Copper River Basin from Glennallen to McCarthy, where temperatures have topped out in the mid to upper 70s. Due to these warm temperatures across the Copper River Basin, and the fact that the minimum relative humidity values have dipped into the 30 to 40% range with winds picking up as well, there is a short lived, low (but non-zero) fire weather concern for this area before temperatures drop overnight.

Surface pressure analysis depicts a coastal ridge developing and beginning to nose in from the south and east, which has tightened local pressure gradients ahead of the low pressure system across Southcentral through typical gaps (Turnagain Arm, Knik, and Copper River). First, a southeasterly Turnagain Arm wind has developed this afternoon from an east to west oriented pressure gradient from Whittier to Anchorage. Due to a negative pressure gradient between Anchorage and Homer, this will soon allow winds to start to bend into the Anchorage Bowl. While winds are just on the breezy side currently, expect these winds to steadily increase through tonight with gusts in the 30 to 40 mph range (with higher gusts to 40 to 50 mph for west/south Anchorage, and up to 60 mph along the Seward Highway near the Turnagain Arm and with elevation on the Hillsides) as the coastal ridge strengthens and the pressure gradient increases further. The Knik Gap wind is on track to ramp up as well due to the prevailing southeasterly wind direction. The Copper River Basin is also seeing winds become breezy this afternoon as the coastal ridge continues to set up shop, though these south-southeasterly winds are getting an "assist" from a weak surface low quickly skirting across the northern most part of the state, which has somewhat enhanced the north to south pressure gradient across this stretch of Southcentral for the near term.

Precipitation chances are on track to lag behind the uptick in gap winds across Southcentral by 8 to 12 hours. Thus, as the Bering low moves inland across the northwestern portions of Alaska, moderate rainfall begins to overspread the Susitna Valley early Monday morning. For Seward, a southerly wind develops Monday morning as it becomes nestled in between the incoming front/low and the coastal ridge to the east. Models are trying to initiate precipitation for Seward and the eastern Kenai Peninsula as early as Monday morning. For Anchorage, initial downsloping will delay the arrival of rainfall. While there is some variation in the model guidance, rain likely begins to fill in for the Anchorage Bowl from west to east as early as late Monday morning. By Monday afternoon, due to the close proximity of the strengthening coastal ridge and the incoming front, gap winds, in particular for the Turnagain Arm, will most likely peak in intensity.

Looking aloft at 500 mb, the trough associated with the surface low digs over Southwest Alaska through Tuesday afternoon and extends as far south as the North Pacific. The trough then takes a neutral tilt Tuesday morning, giving upper level synoptic support to enhance rainfall along the southern and eastern coast of the Kenai Peninsula... in particular for Seward - where we are monitoring flooding concerns. A stout southerly to south- southeasterly flow will continue for this area, and will be aimed towards Seward/Resurrection Bay. The good news for Seward is that models are trending slightly drier than what has been shown in previous runs, but 2 to 3 inches of rain is still expected for this area through late Tuesday. The flood watch for the Susitna Valley remains on track, with no major changes to the amount of rainfall expected. With the surface low stalling and becoming quasi-stationary, the wet pattern continues through the rest of the week, though gap winds look to decrease heading into mid week as the attendant front also stalls and begins to slowly dissipate. Finally, models do show a surface low forming along the front's triple point, but confidence as of now is low on how this feature may impact Southcentral. Stay tuned.

-AM

AVIATION

PANC...VFR conditions will likely prevail through this evening. However, there is a slight chance for lingering patches of lower clouds to move over the terminal, bringing occasionally lower ceilings to IFR or MVFR.

Otherwise, a shift from light northwesterly winds to gusty southeasterly winds later this evening, with gusts as high as 30 to 35 kt tonight through much of tomorrow. Then, rain and lower ceilings will move into the terminal Monday afternoon/evening between about 21Z-06Z. The timing of rainfall and when ceilings will drop to MVFR is uncertain; with the strong southeasterly winds mixing in drier air, this could delay the onset of rain and lower ceilings.