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Severe Thunderstorms in the Upper Midwest; Excessive Heat in the Central U.S.

Severe thunderstorms, capable of severe wind (locally exceeding hurricane force), very large hail, flash flooding, and a few tornadoes, are possible late this afternoon into tonight across portions of the of the Upper Midwest into the southern Great Lakes. Excessive heat covers much of the central third of the country and is expected to persist into the weekend over the south-central states. Read More >

FXZS60 NSTU 280116

Area Forecast Discussion
National Weather Service Pago Pago AS
216 PM SST Tue Jul 27 2021

.Short term...

Plenty of sunshine reigned over American Samoa today as drier low
to mid level air remained over the region. The CIMSS MIMIC
precipitable water imagery shows the American Samoan islands
between two moist regions to the north and south. The latest ASCAT
scan of the area shows wind converging along a trough to create
showers and thunderstorms well to the south of American Samoa.

Overall, the weather pattern is expected to change starting
tomorrow as the dry and mainly clear skies from the past weekend
and early this week becomes more cloudy with an increasing chance
of showers from Wednesday through the weekend.

Models show the aforementioned trough moving northward to reach
the territory by Wednesday, but is expected to weaken as it moves
towards the region. The risk of showers is numerous as the
moisture moves into the region Wednesday and Wednesday night, but
the directional wind convergence won`t be as prevalent.

Another high pressure system to the southwest will strengthen over
the territory on Thursday to suppress the risk of showers to
scattered with the easterly breeze increasing up to 20 mph.

By Friday, models show the potential of a low pressure system to
develop well to the south of the territory. However, winds will
turn from east to northeast throughout the day to increase
relative humidity at the lower to mid levels of the atmosphere to
increase the risk of showers back to numerous.

.Long term...

For late in the week through early next week, rainfall is expected
to be above average for this time of the year. Even through the
aforementioned low on Friday will continue to move away from the
region throughout the weekend, winds will shift to come from the
north and continue bringing in moist air into the region. This
will likely generate showers throughout this time, but not enough
to produce flash flooding in the region. The northerly breeze will
increase to around 20 mph throughout the weekend.



The latest information from the PacIOOS buoy shows that waves have
been around 5 feet this morning and early this afternoon. Long
southerly swell continues to mix with wind driven waves. Easterly
winds are expected to increase throughout Wednesday and to
eventually become a northerly fresh breeze by the weekend. Waves
are expected to reach close to the Small Craft Advisory criteria
by Wednesday night and winds up to 20 knots by Thursday night.