Area forecast discussion
National Weather Service San Juan PR
516 am AST Thursday may 23 2013
Synopsis...at upper levels...a branch of the sub tropical jet splits off
over the Gulf of Mexico and sends a rippled flow over the local
area through Saturday. Also on Saturday a low moves offshore from
the eastern United States and keeps flow cyclonic over the local
area through Tuesday. Another low forms east of Georgia in the
western Atlantic on Wednesday and deepens causing flow to become
more southwesterly over the area.
At middle levels...light but mostly southerly flow prevails now
through the weekend and carries ample moisture. Flow becomes light
and variable Monday and Tuesday and then gains copious moisture
Wednesday through Friday.
At lower levels...high pressure over the central western Atlantic
fades into a stronger high in the north central Atlantic by
Saturday while low pressure maintains a presence in the southern
Caribbean. The resulting gradient maintains moderate to fresh
trade winds over the area. Moisture continues to be advected from
the east. Troughing develops over the eastern Caribbean beginning
on Monday...drifts north and west through the week and pulls even
more moisture over the local area in southeast flow. A weak low
develops northeast of the Bahamas early next month and connects
with low pressure over Central America to maintain the moist flow.
&&
Discussion...showers gradually faded over the Caribbean waters to
the south...but continue to produce showers south of the forecast
area. The mimic product shows a tight gradient between the
moisture slipping southwest and drier air coming from the
northeast. The precipitable water over Saint Croix has fallen to
below 1.5 inches and sounders reveal that moisture amounts over
San Juan were also falling. Satellite imagery shows cloudiness
following the moisture to the southwest and clearing is taking
place over the U.S. Virgin Islands and Puerto Rico. The GFS also
shows drying between 900 and 675 mb till about 24/06z. Although
this will initially favor improved weather...strong heating from
the sun from nearly perpendicular noon time solar angles will
cause showers and thunderstorms to erupt over the area just inland
from the north coast over the northeast portion of Puerto Rico and
shortly thereafter over the western interior. Some showers and
thunderstorms should be heavy enough to cause Urban and Small
Stream flooding in areas already saturated from the previous two
days of heavy rains. Although models show a nearly saturated
atmosphere from 600-300 mb...limited satellite imagery shows there
may be less middle-level cloudiness than would be expected from this profile.
Moisture returns on Thursday night and Friday and another round of
heavy rains and thunderstorms are expected each day through Sunday.
This time sufficient moisture may be present to cause flash
flooding over the weekend and a Flash Flood Watch may be needed.
Although brief periods of slightly drier air do occur over the
next week...low pressure setting up in the western Atlantic east
of Georgia at upper levels Wednesday will induce a trough at the
surface in the Windward Passage that will make low level flow
southeasterly again...tapping the better streams of moisture out of
the tropics and generating continued scenarios for heavy rains
over the area.
&&
Aviation...
scattered-broken skies across the flying area are expected for the forecast
period. After 23/17z MVFR to brief IFR conditions are expected
across tjmz and tjbq due to rain showers/thunderstorms and rain developing across the
western interior of PR with mountain obscurations...thunderstorms in the vicinity are
expected for tjsj. Winds mainly from the east between 10-25 knots up
to 5 kft.
&&
Marine...with moderate to fresh trade winds across Atlantic and
Caribbean waters...seas of 7 to 8 feet have been able to develop in
the northern Atlantic waters and the Anegada Passage. These will
subside somewhat by Saturday. Small craft advisories are not
anticipated through the end of the month.
&&
Preliminary point temps/pops...
sju 85 76 85 76 / 40 40 50 60
stt 87 77 86 78 / 20 40 50 50
&&
Sju watches/warnings/advisories...
PR...none.
Vi...none.
Coastal waters...Small Craft Advisory until 6 am AST Saturday for Anegada Passage
southward to 17n-Atlantic waters of Puerto Rico and USVI from
10nm to 19.5n-Caribbean waters of Puerto Rico from 10 nm to
17n.
&&
$$
13/12
