National Weather Service San Juan PR
509 AM AST Tue Mar 31 2020
.SYNOPSIS...
.A dry and stable airmass will persist through Wednesday, as deep
layered high pressure ridging will hold across the forecast area.
This will promote subsidence aloft and a strong cap inversion which
will limiting convective development. A gradual shift in low level
winds is expected through Thursday becoming more southerly while
decreasing. This will lead to warmer temperatures especially along
the north coastal areas. A frontal boundary and weak prefrontal
trough is to approach and set up across the region by Friday and
into the weekend. By then a gradual erosion of the mid to upper
level ridge is forecast,as a short wave trough will cross the
western Atlantic. Consequently, increasing low level moisture
convergence and instability aloft is so far expected by Friday
and into the weekend. This will result in better potential for
shower development across the islands and regional waters.
&&
.SHORT TERM...Today through Thursday... A dry weather pattern
will prevail across the local region as a high pressure dominate
the region through at least Wednesday. This drier air mass is
expected to dominate, with precipitable water forecast to drop
below one inch. As a result, only passing brief showers are
expected to affect eastern and northern Puerto Rico through the
nighttime and early morning hours, and with the support of local
effects, additional showers could develop across western Puerto
Rico. The high pressure is forecast to move east with the pressure
gradient becoming less tight, therefore relaxing the wind velocity
across our local forecast area through at least Thursday.
On Wednesday, winds are expected to shift to a more southeasterly
flow, generating warmer temperatures across the area, with highs
climbing into the upper 80s to low 90s across the northern half of
Puerto Rico. Highs will remain in the mid-80s across the U.S. Virgin
Islands through mid-week as well.
.LONG TERM...Friday through Wednesday...
On Friday and into the weekend, the combination of the approaching
frontal boundary along with the proximity of the upper level trough
and a fairly light southerly flow, will result in increasing instability
aloft and low level moisture convergence. The slightly warmer than
normal high temperatures is also forecast to continue for the north
coastal areas at least until Friday.
Moisture pooling will return over the weekend as the frontal boundary
and associated shear line is forecast to stall and drift southwards
across the northeastern Caribbean by Sunday. This along with the
proximity of the upper level trough will increase the potential
for shower development across the islands and coastal waters each
day. Some of the shower activity will be enhanced and therefore
urban and small stream flooding will be possible in isolated areas
over the weekend and into early next week. Under this expected
moist and unstable airmass, and based on latest model guidance,
the best potential for convective development will be on Saturday
and continuing through Sunday. Improving conditions is expected
on Monday as the wind flow will shift once again and become more
northerly. This is in response to a surface high pressure ridge
which will build and spread across the west Atlantic. This will
however be short-lived as low level moisture from the frontal
remnants will return and be lifted back across the area Tuesday
through Wednesday. Therefore this fairly moist and unstable
pattern during the period should result in potential for early
morning and afternoon shower development in and around the islands
each day.
&&
.AVIATION...Mainly VFR conditions are expected to prevail across all
terminals during the next 24 hours. -SHRA over SW PR should result
in mainly VCSH at TJMZ thru 31/22z. East-northeast winds will
continue at 10-15 kt, becoming light and variable overnight.
&&
.MARINE...Fairly tranquil marine conditions is expected today
through Wednesday, with a light to gentle east to south wind to
prevail across most of the coastal waters. Seas will range
between 2 to 5 feet across the regional waters and passages.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 72 88 74 / 30 10 10 0
STT 85 73 84 74 / 30 10 10 0
&&