Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service San Juan PR
915 PM AST sun Sep 21 2014
Update...the tropical wave to the east is still strongly
negatively tilted and is at about 59 west degrees at our latitude.
Extrapolation of moisture movement suggests a sharp rise in
moisture over the U.S. Virgin Islands around 22/10-12z...and over
San Juan by 12-18z. Some moisture also appears to be tumbling over
the top of the vertex which would hasten the entry of moisture
into the area. Therefore have increased probability of precipitation and chances of
thunderstorms for the morning period.
Discussion... the sounder in Silver Hills Montserrat is showing
a check rise in moisture now so moisture arrival at 62 degrees
west can be confirmed. Strongest convection is now moving across
60 degrees west longitude. WRF and NAM 5 models all bring in
fairly heavy rains across eastern Puerto Rico before
22/18z...though WRF rainfall amounts are generally unreliable.
Also notable is the northeasterly flow at the surface in the U.S.
Virgin Island and Leeward Islands. This northeasterly flow would
not end before the wave passage and very likely not before maximum
heating. Therefore maximum temperatures on the north coast Monday
should be cooler than today. If showers arrive by middle morning only
one degree cooler may not be enough of a reduction from the
mex maximum temperature forecast ... will hold it there for now to
see where the 00z run takes US. Only weak support at upper levels
but 600 mb Omega values show a maximum at 22/12z over Culebra so
will look for most rapid shower formation then and there. Current
European model (ecmwf) forecast shows continued moisture through Wednesday but even
GFS shows very good convection in a favorable environment on
Tuesday afternoon mainly in western Puerto Rico.
Aviation...VFR conditions expected across the local terminals until
about 22/10z... rain showers/thunderstorms and rain activity is expected to
start affecting the local area as a tropical wave approaches the
local area. Thunderstorms in the vicinity/vcsh likely and possible MVFR conds as the numerous
rain showers/thunderstorms and rain affect the local area through the day. Surface winds expected to
be from the NE to east-northeast at around 10 kts or less overnight...turning
more easterly after 22/18z and increasing slightly to 10-15
kts...with higher gusts near thunderstorms.
Marine...no significant changes to marine grids other than the
addition of more weather Monday and a slight increase in seas in
outer Atlantic waters. Winds should remain below Small Craft
Advisory conditions for the rest of the week with seas generally
below 5 feet except in the vicinity of thunderstorms.
Previous discussion... /issued 233 PM AST sun Sep 21 2014/
Synopsis...TUTT low across the central Atlantic will amplify
into the north and eastern Caribbean by midweek. Tropical wave
will move across the area Monday night into Tuesday.
Discussion...model guidance continues to indicate near to below
normal precipitable water across the northern half of the forecast
area through Monday afternoon with higher values across the southern
portions. However moisture advection is expected across the region as
soon as Monday afternoon into Tuesday morning as tropical wave now
located near 59 west moves across the eastern Caribbean. GFS and WRF
show precipitable water persistently near 2 inches through Thursday.
Under this evolving pattern....expect passing showers and isolated
thunderstorms across the Caribbean waters and windward areas tonight
and Monday morning with showers and thunderstorms developing across
the Cordillera and west Puerto Rico Monday afternoon. Monday night
and into Tuesday and possibly Wednesday...frequent passing showers
with thunderstorms could be expected across USVI and the eastern
half of Puerto Rico. Convection highly dependent on sunshine.
As TUTT axis moves to the southeast of local islands late Thursday
and into the upcoming weekend...low level moisture will slowly erode.
Shower and thunder activity still expected during the period but
coverage/intensity will decrease.
Aviation...shra/tsra activity is expected to continue over western
PR until around 21/22z...with MVFR conds and mountain obscurations.
Thunderstorms moving west over the Caribbean waters are expected to continue
during the evening hours. During the overnight hours VFR conditions
are expected across all terminals. Low level winds will continue from
the east at 10 to 20 kts...with higher gusts near thunderstorms.
Marine...seas 2-4 kts and winds near 17kt but higher near thunderstorms.
Preliminary point temps/pops...
sju 79 88 79 90 / 60 60 50 50
stt 79 88 79 88 / 60 70 50 50