Area Forecast Discussion
Issued by NWS
FXPQ60 PGUM 242207
Area Forecast Discussion
National Weather Service Tiyan GU
807 AM CHST WED MAY 25 2016
PGUA Doppler weather radar shows isolated showers moving through
the Marianas waters. There are also a few thunderstorms south and
west of the Guam waters. The VAD wind profile reveals east-
southeast winds of 7 to 13 knots through the lowest 4 thousand
feet of the air. Last night`s weather balloon indicates that above
25 thousand feet the winds change to the northwest.
The prolonged and deep dry season brought to us by the Post-Peak
El Nino related drought continues. The hoped-for trade-wind
disturbance passed too far south to give any relief. The GFS40
continues to gin up wetting rains out of almost nothing. The
ECMWF-HiRes, which has been more reliable at least in this drought
time, prognosticates no wetting rains through the forecast period.
Winds remain light to gentle so combined seas will continue
between 2 and 4 feet. This will maintain benign marine and surf
conditions into the foreseeable future.
The Keetch-Byram drought index has increased to 787 today, which
equals the peak of the 1998 El Nino drought. Now there are only 2
peaks in my 60 year database that are higher than the current peak
in 2016. These are a 792 from the 1983 El Nino drought and a 797
from the year 1987. Either of these could be in danger as we have
not necessarily seen the final peak of 2016. This is especially
true if the ECMWF-HiRes verifies reasonably well. The KBDI could
hit 790 on Saturday and if no wetting rains happen before then
could reach the maximum possible value of 800 on the 7th of June.
However, the light to gentle winds are expected to remain below
red flag criteria.
Satellite imagery shows a band of trade-wind convergence stretching
from Kosrae to Majuro as drier conditions settle in at Pohnpei. With
convection staying just south of Majuro and model cross sections
indicating drier mid-level air, decided to remove the slight chance
of thunderstorms for today. Showers and thunderstorms seen near
Kosrae will gradually spread westward Thursday and Friday. GFS shows
showers overspreading Pohnpei Friday and Friday night while ECMWF
keeps most of the activity to the south. Decided to maintain
forecast as is with isolated showers and thunderstorms unless models
begin to show more convergence on a wetter outlook. In the long
range, trade-wind convergence will shift northward slightly and will
bring an increased chance of rain to Majuro over the weekend.
A broad trade-wind disturbance continues to generate showers and
thunderstorms near Yap and Palau but heaviest showers are found near
Koror with stratifying clouds nearer Yap. Forecast was left mostly
as it was with some drying weather in the short term followed by
more clouds and showers over the weekend as surface winds range from
east to southeast.
At Chuuk, a weak trade disturbance will begin to dissipate later
today with drier weather continuing through Friday. A convergent
trade-wind pattern will bring isolated showers and thunderstorms to
the area over the weekend. Showers could become scattered at times.