Area Forecast Discussion
Issued by NWS
FXPQ60 PGUM 252159
Area Forecast Discussion
National Weather Service Tiyan GU
759 AM CHST THU MAY 26 2016
PGUA Doppler weather radar shows isolated showers moving through
the Marianas waters. Most of these showers will pass south of
Guam. However, a few will pass near enough to Guam and Rota to
possibly accumulate a small amount of rain. The VAD wind profile
reveals east winds of 8 to 14 knots through the lowest 12 thousand
feet of the air. The winds then turn northeast up to about 22
thousand feet. Last night`s weather balloon shows that the winds
then turn toward the northwest above 35 thousand feet.
The Tropical Upper Tropospheric Trough (or TUTT) is becoming
active between 20 and 25 north, producing scattered showers and
isolated thunderstorms north and west of the Marianas. The deep
moisture and trade-wind disturbances are confined south of 10
north for the time being. A hybrid of the monsoon trough and the
Mei-Yu front is bending the subtropical ridge over the forecast
zones. For the time being that will keep the Marianas fairly dry.
Also light to gentle winds will maintain 2 to 4 foot seas, leading
to benign marine conditions.
While the ongoing dry weather is resulting in fire weather
conditions that are much less benign, the low winds mean that at
least they will not reach red flag criteria. It will be of the
greatest importance to put fires out quickly before they can
spread and become too large to control. The Keetch-Byram drought
index has risen to 788 today. There is a slight chance of wetting
rains through Friday evening. If these do not materialize, the
drought index may have an uninterrupted run through the rest of
the forecast period. This could have it reaching the 1983 peak of
792 on Monday and the 1987 peak of 797 next Saturday.
Showers and thunderstorms are seen near Kosrae and south of Pohnpei.
Drier weather has settled over Pohnpei overnight with convection
pushing southward while Majuro has stayed mostly dry. The trade-wind
convergence zone will stay focused near 5N as weak disturbances move
westward. A trade-wind trough east of Majuro will shift westward and
bring isolated thunderstorms to the area tonight. Both ECMWF and GFS
show a broad area of disturbed weather as the trough pushes westward
and another zone of trade-wind convergence becomes better
established over the weekend and early next week.
Rather benign conditions persist at Yap and Koror with isolated
showers and thunderstorms found farther to the west and northwest
where upper-level divergence is more supportive of deep convection.
Models maintain the trend over the weekend of the monsoon trough
extending out over the Philippines and the northern Philippine Sea
with a low-level ridge building in over Yap and Koror. This will
help to keep winds reduced and the weather quiet. However, should
there be enough atmospheric moisture, Koror could see some brief
island thunderstorms. Chuuk will see continued dry weather as
showers stay to the south until the weekend. GFS, and ECMWF to a
lesser extent, show a surface trough passing over Chuuk Saturday
that will bring an increase in showers and isolated thunderstorms
before dry weather resumes on Sunday.