Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Juan PR
1050 PM AST Wednesday Jul 1 2015
Update...very dry conditions have prevailed across Puerto Rico
and the U.S. Virgin Islands. Latest 02/00z tjsj sounding showed
precipitable water values of 1.6 inches which is considerably
lower than this afternoon which was 2.04 inches. Saharan dust is
already affecting the region. Drier conditions expected for
tomorrow. No changes to forecast grids needed tonight.
Aviation...mainly VFR conditions expected across all terminals
through the forecast period. Hazy skies expected due to Saharan dust but
visibility should remain p6sm. Surface east winds 10-15 knots overnight with
land breeze variations. However...winds below fl200 will continue
between 15-25 knots.
Marine...no changes to previous discussion.
Previous discussion... /issued 309 PM AST Wednesday Jul 1 2015/
Discussion...layer of Saharan air between 850 mb and 550 mb had
moved over macm by 12z this morning and helped reduce or end the
showers today sooner than expected. High clouds over western PR
also contributed to very little convection to occur there so far.
This layer continues to progress westward and increasingly will
suppress any convection that occurs the rest of today.
Reduced the showers expected tonight to no more than isolated
though the 1.5 inches of precipitable water we will have will be
mainly concentrated below 850 mb and should be enough for isolated
showers for windward locations.
Next decent chance of showers arrives Friday/Friday night according to
GFS as a wave brushes past but a better chance next Tuesday/Wednesday as
deeper moisture envelope local islands with around 2 inches
Aviation...Saharan air will keep rain showers minimal through Thursday...and cause
some false ceilings at tist/tisx. VFR except possible MVFR in rain showers at tjmz
this after and Thursday after. Wind blw fl180 east 15-25 knots strengthening
16-35 knots Thursday nite/Fri.
Marine...no significant swells are expected in near term but
moderate to fresh trades will likely continue into the weekend.
This will keep seas in the offshore in the 5 to 7 foot range but
greater confidence through Friday. Small craft for offshore continues
at least through tonight.
Preliminary point temps/pops...
sju 79 89 79 89 / 10 10 10 40
stt 79 89 79 90 / 10 10 20 40
Am...Small Craft Advisory until 6 am AST Thursday for Atlantic waters of
Puerto Rico and USVI from 10nm to 19.5n.
Small Craft Advisory until 6 PM AST Thursday for Caribbean
waters of Puerto Rico from 10 nm to 17n-Mona Passage
southward to 17n.