Area Forecast Discussion
Issued by NWS
FXPQ60 PGUM 272151
Area Forecast Discussion
National Weather Service Tiyan GU
751 AM CHST SAT MAY 28 2016
PGUA Doppler weather radar shows isolated showers within the
Marianas waters. Drier weather with almost no showers is seen
east of the coastal waters. Gradually this drier weather is
filtering into the Marianas. The VAD wind profile reveals East-
southeast winds of 4 to 8 knots through the lowest 3 thousand feet
of the air. The weather balloon from Friday evening indicates that
winds shift toward the Northeast above 5 thousand feet then toward
the Northwest above 20 thousand feet.
GFS and ECMWF-HiRes still show a surface ridge sagging southward
over the Marianas and surrounding waters. This will tend to keep
the weather relatively dry and the winds light. The light winds
seem like they might promote island convection, however moisture
is limited so did not write that into the forecast. The expected
dry weather and light winds resulting in low combined seas will
create benign conditions in most forecasts. The principal
exception will be fire weather, highlighted below. This general
pattern is expected to persist for at least next week.
While the light winds will preclude a fire weather watch or red
flag warning, that is where the good news ends for fire weather.
Low afternoon relative humidities and dry fuels will generate
otherwise ideal conditions for spread of grass fires especially.
The Keetch-Byram drought index has risen to 790 this morning out
of a possible 800. The only 2 years it has reached values higher
than this were 792 in 1983 and 797 in 1987. If the models verify
in their forecasts of no wetting rains next week the KBDI will
reach that 792 on Tuesday and make it to 797 next Saturday.
A few showers are seen south of Pohnpei while a wider swath of
showers and thunderstorms is seen over portions of the northern
Marshalls extending southeastward to the east of Majuro. Wet weather
will spread over Majuro tonight and linger through Sunday night
before moving westward. Kosrae will see isolated showers and
thunderstorms by Sunday night. Models show the zone of trade-wind
convergence centered near Kosrae but the shower coverage is a little
more diffuse by Sunday. Similarly, Pohnpei will see increasing
cloudiness and a slight chance of thunderstorms early next week as
the trade disturbance continues westward.
Quiet weather conditions persist across much of the region with
some showers seen south and southeast of Yap and Koror and farther
southeast of Chuuk. The subtropical ridge remains just north of
Yap over the Philippine Sea and an upper-level trough curves
northeastward from Yap toward the Marianas. Divergence aloft is
helping to trigger short-lived thunderstorms in the area but
convection is, at best, isolated. With no large scale features
expected to develop across the area, few changes were needed to
the inherited forecast. Light winds at Koror could set the stage
for afternoon thunderstorms the next day or two.