National Weather Service United States Department of Commerce

Hazardous Heat in the Western U.S.; Heavy Rain and Flooding Potential in the Southern Rockies

Widespread high temperatures in the 90s with heat indices exceeding 100 degrees will persist across the western U.S. this weekend into next week. Some daily high temperature records are forecast to be tied or broken. Monsoon conditions continue to linger across the Southern Rockies posing a heavy rainfall threat which may lead to additional flash flooding concerns. Read More >

FXZS60 NSTU 200128

Area Forecast Discussion
National Weather Service Pago Pago AS
228 PM SST Fri Jul 19 2024

.Short term...

Satellite imagery shows only very few clouds over the American
Samoa islands. The last few balloon launches from WSO Pago Pago
has shown a very strong inversion around 3,500 above the surface.
This indicates a warming trend at that level, which creates
conditions that are difficult for clouds to develop. If a few clouds
are able to develop, these cumulus clouds are much thinner than
normal and are very unlikely to produce rainfall. We expect this
type of weather pattern to continue through at least Saturday

The weather pattern shifts as the high pressure system moves
eastward by Sunday morning. A strong inversion remains, but that
warming trend will not occur until higher up in the atmosphere (to
around 5,500 feet). If clouds develop, the clouds will be slightly
thicker and will increase the risk of a passing shower to around
10% chance (for the rest of the day through Saturday night) to
around 30% chance of a passing showers (from Sunday night through
Tuesday night). If showers develop, the confidence is very high
that the rainfall will not produce flash flooding over the region.

Easterly tradewinds are anticipated to increase throughout the
weekend and early next week due to a trough to the north
interacting to the ridge over the islands. Expect trades to
increase to eventually reach up to 25 mph by Sunday and to remain
around that strength through at least Wednesday morning. At this
time, the wind strength is below Wind Advisory threshold levels.
However, we will continue to monitor wind speeds throughout the

.Long term...

The dry weather pattern through the middle of the week is expected
to end by late week. The aforementioned ridge will move away from
the region and will bring the trough, currently well to the north
of American Samoa, south to move over the islands by as early as
Wednesday evening. Numerous showers are likely late in the week
and will monitor if there is a possibility of flooding with these
showers throughout the week. As the trough moves over the region,
the easterly trades will become northerly.



The latest information from the PacIOOS buoys located to the east
and west of Aunu`u shows ideal conditions, especially for this
time of the year, along the American Samoa coastal waters. The
easterly trades decreased to around 10 knots throughout the day
and only mixing with weak southerly swell. However, easterly
trades will increase throughout the weekend to eventually reach to
around 20 knots by Sunday. This will cause seas to slowly build
throughout the day on Saturday to eventually reach 9 to 12 feet by
Sunday and is expected to remain through the middle of the week.
Even though long period swell is forecast to finish moving through
the coastal waters by Sunday, the wind driven waves are expected
to create choppy and hazardous conditions from Sunday through the
middle of the week.