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Area forecast discussion 
National Weather Service tiyan GU
845 PM chst Thursday Oct 23 2014

Update...
trough to the northwest was expected to interact with the trough
to the southeast. This would have caused the trough to the
southeast to move farther north...therefore expected to affect
primarily Tinian and Saipan. This interaction either did not occur
or was weaker than expected. This placed better weather over
Tinian and Saipan with the more active weather affecting Guam and
Rota.

&&

Previous discussion... /issued 513 PM chst Thursday Oct 23 2014/

Marianas synopsis...
a weak circulation is centered just southeast of Guam near 12.5n
and 145.5e. Scattered showers over Guam and Rota this morning
this morning dissipated rapidly before noon. Radar and satellite
imagery show more showers developing on the down shear side of
the circulation over the coastal waters east of Guam. Most of the
showers appear to be concentrated on the east side of the
circulation between 10n and 15n from 148e to 150e.

Discussion...
the weak circulation just east of Guam is embedded within the
monsoon trough and is expected to pass over Guam tonight. Felt
GFS initialized well in terms of spatial characteristics of
convection this morning. GFS and higher resolution models
indicate that much of the shower activity associated with the
system will develop to the north of the circulation center over
Tinian and Saipan later tonight so opted for scattered wording
late tonight. This corroborates well with current satellite
imagery showing shower activity increasing east and north of the
circulation. Upper-level dynamics are not ideal for thunderstorm
development but cannot rule out a thunderstorm or two overnight
north of Rota. In the long term...GFS and navgem are still the
drier solutions compared to the European model (ecmwf). European model (ecmwf) increases moisture
and showers and develops a broad circulation south of the
marianas more aggressively than the GFS and navgem over the
weekend. Uncertainty still exists regarding shower coverage
during the 3 to 5-day time frame but maintained the drier
scenario until models come into more of a consensus during the
next several runs.

Marine...
tanapag and ritidian buoys show north-NE swell continuing to slowly
diminish across the marianas. A second...long-period northeast
swell event will commence Friday beginning with Saipan. This will
cause surf to rise to hazardous levels Friday after a brief drop
in surf heights tonight. Surf heights of 7 to 9 feet are expected
through much of the weekend with surf expected to drop below
hazardous levels Sunday afternoon.

Eastern micronesia...
an inverted surface trough is approaching Majuro from the east.
Updated Majuro forecast this morning to increase shower coverage
to scattered...but convection associated with the trough has
begun to stratify. A weak circulation is embedded in the monsoon
trough to the southwest of Kosrae near 4n160e. Convection
associated with this system is loosely organized and primarily
located on the west side of the circulation. Kosrae is located
beneath weak ridging.

Latest forecast issuance follows continuity with a few
adjustments. Reduced first period probability of precipitation for Majuro to isolated
based on warming/ stratifying cloud tops upstream. Despite
this...kept scattered probability of precipitation for Friday based on strong signal in
the GFS for redevelopment of convection within the zone of trade-
wind convergence on the downstream side of the trough. Forecast
for Kosrae maintains isolated probability of precipitation through day 5 as weak ridging
remains the primary player in the forecast. Convection associated
with the circulation southwest of Kosrae has already shifted to
the west of Pohnpei. Made minor adjustments to the wind forecast
to account for the passage of an inverted trough.

Concern continues to revolve around a possible high surf event
from Chuuk eastward to Majuro beginning on Friday night or
Saturday. Wave watch model graphical and tabular guidance both
suggest long period north-northeast swell will be blocked before
they reach Majuro and Kosrae...while Pohnpei and Chuuk remain
fully exposed to swell from that direction. Regardless...high
surf advisories will be needed for at least Chuuk and Pohnpei on
Friday.

Western micronesia...
convection in the monsoon trough to the west of the Kosrae
circulation is passing over Chuuk. Still expect this round of
convection to wind down during the next 3 to 6 hours. Flow
pattern over western micronesia is becoming weak and chaotic.
Monsoon trough passes to the north of Yap and Koror...but
convection developing over and south of Koror is inducing more
troughing. This has resulted in weak ridging forming over Yap.
Otherwise...have kept dry bias in the forecast for all 3
locations as convection across western micronesia is currently
mesoscale and loosely organized... then zone of best low-level
convergence shifts to the north of all 3 forecast locales by
Tuesday. Have adjusted timing of scattered showers for Chuuk to
this evening only based on satellite trends. Added scattered
showers and isolated thunderstorms to the Koror forecast for this
evening only to account for the passage of a small mesoscale convective system.

&&

Gum watches/warnings/advisories...
GU...high surf advisory until 4 PM chst Sunday for guz001>004.

Marianas waters...none.
&&

$$

Stanko/McElroy/Williams

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