Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
1002 PM AST Sat Jan 24 2015
Update...scattered showers persisted this evening over the
Atlantic waters northwest of Puerto Rico. Doppler radar also
detected light isolated showers over southwest Puerto Rico.
Elsewhere over the region no significant precipitation was detected
as the region continued under the influence of a middle to upper level
ridge. This ridge will remain as the main weather feature over the
region until at least Monday. Subsidence associated with the ridge
will maintain very dry conditions at middle to upper levels...resulting
in an unfavorable environment for development of strong showers and
thunderstorms. Limited shower activity is expected on Sunday and
Monday as the ridge remains established over the region. The ridge
will erode by Tuesday as a frontal boundary approaches to the
region. The frontal boundary will bring an increase in cloudiness
and showers over the local islands late Tuesday and thereafter.
Aviation...VFR conds expected at all taf sites overnight. Brief
MVFR conds will remain possible between 25/17z and 25/20z at jmz/jbq
in rain showers. Light winds expected overnight...becoming east-southeast at 10 to 15
knots with some sea breeze variations after 25/15z.
Marine...not changes to previous reasoning.
Previous discussion... /issued 308 PM AST Sat Jan 24 2015/
Synopsis...middle to upper level ridge is expected to remain in
place over the area through early Monday before eroding gradually due to a
strong upper trough early Tuesday. East southeast winds expected
through Monday...becoming southeast Tuesday and Wednesday as a
shearline approaches the forecast area...staying to the north and
west of Puerto Rico by Wednesday.
Discussion...showers were observed across the local area today
with morning showers affecting the local waters and eastern
PR...then showers developing across the central...north
central...western interior...western and northwestern PR in the
afternoon hours while only brief showers remained elsewhere in PR.
The USVI remained fairly dry today with only partly cloudy skies.
This was due to a patch of moisture that moved through the local
area...combined with local effects and produced these showers.
There is a strong middle to upper ridge causing subsidence so the
upper level conditions were not particularly favorable for strong
thunderstorms...which is what we have observed today. After this
patch of moisture passes...drier air will move in. This drier air
will have precipitable water values of 1.3 inches or less for
Sunday and Monday...increasing moisture on Tuesday as a shearline
closes in. However the latest guidance suggests that the shearline
will stay to the north and west of the local area Tuesday and
Wednesday. Moisture slightly higher than normal is expected across
the local area for the latter part of next week even after the
cold front and shearline dissipates. The latest guidance now does
not forecast a frontal passage across the local forecast area.
Aviation...VFR conds to prevail during prd. Few clouds west passing isolate
rain showers by the way PR and northern leewards. Scattered cloud layers by the way fl020...fl080...clr
above. No sig operational weather impacts forecast. Surface wind lgt/vrb...l/lvl
wind from east-southeast 5-15 kts below fl100...then backing become from NE-northwest increase W/ht.
Maximum wind around 55 kts near fl400.
Marine...seas up to 4 feet and winds up to 17 knots are expected
over the coastal waters today and on Sunday.
Preliminary point temps/pops...
sju 73 87 74 88 / 20 10 10 10
stt 74 84 73 84 / 20 20 20 20