815 
fxca62 tjsj 200230 
afdsju 


Area forecast discussion 
National Weather Service San Juan PR 
1030 PM AST Tuesday Aug 19 2008 


Update... 
afternoon convection across the northwest and north central 
sections of Puerto Rico moved offshore and dissipated during the 
late evening hours...leaving variably cloudy to partly cloudy 
skies across the west sections of Puerto Rico...and mostly clear 
skies across the eastern half of the islands...as well as over 
Vieques and Culebra. Mostly clear skies prevailed across the U.S 
Virgin Islands. A few passing showers were noted moving across the 
surrounding coastal waters but no precipitation was detected over 
land during the late evening. 


The weak elongated tropical wave to extended across the northeast 
Caribbean from the southeast Caribbean and will continue westward 
overnight through Wednesday morning. Latest satellite imagery showed 
some cloudiness along the trailing portion of wave axis...with most 
of the shower activity now crossing the northern leewards and the 
Windward Islands. Expect some of these clouds and passing showers 
to move over the regional waters during the overnight hours...and 
reaching the coastal sections of eastern Puerto Rico...as well as 
parts of the U.S. Virgin Islands during the early morning hours. 
No widespread rainfall is however expected. 


Fairly tranquil seas will continue across the local waters with 
no significant marine hazards forecast at this time. 


No major changes were made to the inherited short term forecast 
which had a fairly good handle on the expected weather conditions 
during the next 24 hours. Some minor changes were however made to 
the long term forecast to adjust for the approach of the next 
tropical wave now expected to enter the eastern Caribbean and 
approach the region late Friday into Saturday. 


Aviation update...VFR conditions are expected across all taf site 
during the overnight hours. Light passing showers are possible 
across tisx...tist...tncm...and tkpk. Latest tjsj 00z sounding 
indicated mainly east winds flow...up to 15 knots...from the 
surface to around 15k feet...becoming south southeasterly near 25k 
feet. 


&& 


Previous discussion... /issued 223 PM AST Tuesday Aug 19 2008/ 


Discussion... 
satellite analysis reveals a tropical wave now passing through the 
forecast area coupled with an elongated inverted middle-level trough in 
the central Atlantic. This will create enough instability for 
additional showers and thunderstorms across the central interior of 
Puerto Rico this afternoon and early this evening as diurnal and 
sea breeze effects dominate the pattern. Precipitation chances will 
wax and wane across Puerto Rico and the U.S. Virgin Islands over the 
next few days as pockets of low- to middle-level moisture pass through 
the region. 


The next big question mark for the weather into the first half of 
the weekend remains the proximity of the first of two large areas of 
convection in the tropical Atlantic. Equatorial and monsoonal 
troughs along the inter-tropical convergence zone are currently setting up a mini Nursery in 
the eastern tropical Atlantic facilitating the preservation of new 
waves exiting off the coast of Africa. Over the next few 
days...increased convection is possible with the first wave 
/designated al94 currently near 40w/ especially in the overnight 
hours as evaporative cooling effects take over near cloud tops. Per 
NHC guidance...upper-level winds may become unfavorable for deep 
tropical development if the wave moves more to the northwest with 
time. If however the system takes a more southerly track of the 
upper low...it could encounter a more favorable upper-level wind 
pattern. That...coupled with climatology /mid-to-late August/ and 
warm SSTs...gives this wave a better chance for tropical 
development. At this time however...the medium-range GFS tracks 
the wave more west northwest...placing the system in the path of 
more unfavorable winds and keeping it as an open low. Latest al94 
early-cycle invest runs show an ensemble mean track taking the 
system just to the northeast of the forecast area very near the 
northern Leeward Islands...however this forecast is well into the 
future and depending on where the model initialized its center 
position...we will probably see future track changes with time. 


Given this uncertainty...stay tuned to your National Weather 
Service for the latest information. 


Aviation... 
expect brief MVFR conditions across tjmz and tjbq between 
20-24z...with VFR conditions expected elsewhere. MVFR conditions 
will then be possible across the vi and eastern Puerto Rico after 
00z...when the approaching tropical wave reaches the area. The 
winds will mainly be out of the east at 10 to 15 knots this 
afternoon. 


&& 


Preliminary point temps/pops... 
sju 79 89 79 89 / 40 40 30 30 
stt 80 87 80 87 / 40 40 40 40 


&& 


Sju watches/warnings/advisories... 
PR...none. 
Vi...none. 
&& 


$$ 


71/09