Scientific Forecaster Discussion
fxca62 tjsj 221504 aaa
Area forecast discussion...updated
National Weather Service San Juan PR
1104 am AST Mon may 22 2017
Update...sounding from 22/12z came in a little warmer between
6300 and 13000 feet today but columnar moisture was better than
yesterday at 21/12z, coming in more than one tenth inch higher at
1.74 inches. The biggest surprise, however, was that cape values
came in just over 3200 which is a big jump over yesterday.
Therefore remain optimistic that some convection--and perhaps
quite vigorous--could develop over western Puerto Rico this
afternoon, especially since the weak and shallow inversion will
offer little resistance once heating is in full swing.
Main changes to the forecast were to bring down temperatures over
Puerto Rico at mid and high elevations by 2 to 6 degrees today
through Wednesday. Some models are forecasting record
temperatures for which conditions do not exist.
Saharan dust will continue to increase. Geos-5 model shows best
dust over local area to occur 23/15z, but diminish only slowly
thereafter with another pulse later in the week. This will also
make showers less pervasive.
Aviation...VFR prevailing. As heating mixes down drier air only
expectation for MVFR conds is under iso-sct rain showers/thunderstorms and rain in wrn or
nwrn PR till 22/23z / 23/02z respectively. Visibilities will
likely remain less than 15 miles most areas due to Saharan dust.
Sfc winds east-northeast 5 to 15 kt with sea breeze influences and higher
gusts. Maximum winds west 50 to 70 kts from fl380-500 thru 24/00z.
Marine...marine conds are relatively tranquil, but will increase
in the western Caribbean and western Atlantic on Wednesday. Seas
still expected to reach 7 feet there then.
Previous discussion... /issued 503 am AST Mon may 22 2017/
Synopsis...fair weather conditions will prevail most of the week.
Pulses of Saharan dust will reach the northeast Caribbean over the
next several days reducing somewhat the visibilities. A wetter
pattern is possible during the weekend and early next week as an
induced trough and a tropical wave moves across the region.
Short term...today through Wednesday...
Doppler radar indicated isolated shower activity mostly along the
northern sections of Puerto Rico as well as across the
surrounding waters overnight and early this morning. The induced
surface trough northeast of the region will continue to drift
northeastward and away from the region today as a surface high
develops north of the local islands. A weak easterly wave over the
central Caribbean will continue to move westward and also away
from the local area today. Model guidance suggests that Saharan
air layer is expected to increase considerably over the region
today through midweek. This will maintain hazy skies and warmer
than normal temperatures across the region for the next several
days. GFS guidance indicated precipitable water values at near 1.60 inches
today, decreasing below 1.50 inches Tuesday and thereafter as a
relatively dry and hazy air mass encompass the region from the
east. However, residual moisture will combine with strong daytime
heating, sea breeze convergence and mountains lifting to induce a
new round of showers and thunderstorms over central interior,
western and northwest Puerto Rico each afternoon. Not significant
precipitation is expected to affect the U.S. Virgin Islands.
Long term...Thursday through Tuesday...
Upper level ridge will dominate the northeast Caribbean Thursday
and Friday while an upper level trough amplifies east of the
Lesser Antilles. At the surface...high pressure over the central
Atlantic will continue to push Saharan dust particles over the
local area. Therefore...conditions will remain hazy, dry and
relatively stable through the end of the work week. Then...an
induced surface trough will promote moisture convergence and
thunderstorm activity during the weekend. A tropical wave will
follow the surface trough keeping the conditions somewhat unstable
through at least early next week.
Aviation...mostly VFR conditions expected across the local flying
area for the next 24 hours. Vcsh/thunderstorms in the vicinity is possible across tjmz and
tjbq after 22/17z as afternoon convection develops over western and
northwest PR. Winds will increase at 10-15 knots after 22/13z with
sea breeze variations. Latest tjsj sounding indicated an easterly
wind flow up to 15 knots from the surface to 6k feet, becoming light and
variable between 6k-30k feet and then westerly and stronger above
Marine...high pressure will build over the central Atlantic by
midweek resulting in seas building to 5-7 feet across the Atlantic
waters. Marine conditions will gradually improve through the weekend
when seas will subside to 2-4 feet across most of the waters.
Preliminary point temps/pops...
sju 90 79 90 80 / 20 20 20 20
stt 89 80 89 80 / 20 20 20 20