Area Forecast Discussion
National Weather Service San Juan PR
512 AM AST Sun Nov 1 2020
Relatively dry and stable weather conditions are expected today,
though afternoon showers are still possible. Tomorrow, moisture
associated with a tropical wave moving through the Caribbean will
make its way into the area, and an uptick in shower activity is
anticipated, though likely moderated by Saharan Dust over the area.
This is fairly quickly followed by a return to drier air Tuesday
through midweek. A more active weather pattern is currently forecast
for next weekend, but confidence in the forecast beyond midweek is
.SHORT TERM...Today through Tuesday...
Showers overnight were very minor and tended to stay offshore from
Puerto Rico. Several did cross the northeast coast, however, and
left up to four tenths of an inch along the foothills of the
Luquillo range. Shower activity is expected to be very minor today
with dry air now in place. A tropical wave is moving west northwest
through the windward islands and consistent with yesterdays model
runs, moisture is expected to move into the area around 01/22Z.
Hence a considerable uptick in shower activity is expected then.
Moisture will not be able to penetrate Puerto Rico very deeply
overnight tonight, but Monday should have some showers activity
across the entire island. Today, Saint Croix will be the first to
detect this surge is shower activity, and some chances of rain there
persist through at least Monday. Drier air follows the moisture
behind the tropical wave passage today and tonight and by Tuesday
morning precipitable water values are expected to be below 1.25
inches--but only briefly. Moisture recovers very quickly and reaches
1.9 inches by Tuesday evening.
With high pressure just north of the area at mid levels and
northwesterly flow over the area at upper levels due to high
pressure over the Caribbean, little in the mid and upper levels of
the atmosphere is found to enhance the showers born by the mostly
lower level moisture pulse that moves through tonight through Monday.
Saharan dust with peak dust concentrations on Monday morning is
expected, which will also serve as a limiting factor for shower
activity during the period.
.LONG TERM...Wednesday through Monday...
Dry air persists over the region on Wednesday. Aloft, a mid- to
upper-level ridge will continue to provide stability. Therefore,
conditions are not going to be conducive to convective development
and shower activity will be inhibited. However, there will remain
sufficient moisture in the low-levels to aid in the development of
some afternoon showers due to sea breeze convergence. The confidence
in the forecast up to this point is moderate to high. This does not
Tropical Storm Eta is located more than 600 miles southwest of
Puerto Rico, moving away. This system is forecast to intensify;
there is good model agreement on expecting further development, as
well as tracking to Central America over the next few days, which
brings us through Wednesday. The most likely scenario for Eta is
landfall in Central America, likely somewhere around Honduras and
Nicaragua, as is reflected in the most recent forecast from the
National Hurricane Center. The majority of the model guidance
suggests that the system will move inland and weaken
significantly; from there, the tropical cyclone would likely
dissipate or maybe make its way into the Pacific. The GFS
continues to lack consistency between runs as well as much in the
way of support from the other models. Currently, it suggests less
opportunities for troughing to advect or develop in the mid- to
upper-levels late in the week, which could weaken support aloft
for convection; even so, it would bring significant deep moisture
over the area. For the ensemble GFS, the spread itself is fairly
vast, with an array of scenario options to choose from - crossing
Central America, heading north past the Yucatan Peninsula, and
even some ensemble members making their way northeast. A ridge
that is forecast to build into the western central Atlantic
between now and the second half of the week is likely to be strong
enough to keep this system from making any significant headway
towards the northeast. A series of synoptic systems over the
mainland US are likely to keep the system from making much
distance northwestward, either. The spread between ensemble
members also illustrates how sensitive the forecast is to any
relatively minor changes in the environment. So: while a scenario
in which Eta makes a bit of a U-turn over Central America cannot
be wholly ruled out, it being an outlier in model guidance, lack
of good consistency from run to run, large spread in the GFS
ensemble, and just all of the environmental factors that would
need to be -just so- do make this scenario not remarkably
plausible. What currently looks more likely, however, is for
another low to develop in the wake of Eta, over the western
Caribbean, after the system has weakened significantly or even
dissipated. Again it brings the potential inhibition of troughing
aloft that might otherwise occur over the area, though would be
less impactful than what is illustrated by the solution from the
GFS. This scenario has at least some support, in varying measures,
from the German (ICON) and Canadian (CMC) models, and is
currently the scenario given by the most recent run of the ECMWF.
Some troughing in the mid-levels would be likely by the end of the
week, supporting afternoon convection. And the one thing it seems
that the guidance model can very nearly agree on is that moisture
convergence is expected by the end of the week, likely by Friday,
leading to deep columns of increased moisture over the area. The
exact timing is subject to change, as well as the magnitude of
moisture advection and convergence.
To make a long story short (or shorter, anyway), though - the
current expectation for Thursday is slow increases in moisture,
though possibly remaining on the dry side of normal through the day.
More significant increases in moisture are forecast to arrive likely
by Friday, or possibly even early Saturday. Increasingly active
conditions, especially during the afternoons, are expected for next
weekend. Steering flow is also currently expected to subside late in
the week, which has the potential to bring more vigorous afternoon
showers and higher rainfall amounts. Confidence in the forecast for
increasing moisture over the area late in the week is low to
moderate. But confidence in the forecast dynamics is quite low,
associated with Tropical Storm Eta`s indirect impacts; as such,
the confidence in the forecast as a whole is low. Therefore, the
current forecast is fairly near to climatology.
Hazy conditions with 9-15 SM visibilities will persist through the
period. A weak trop wav is moving thru the windward islands with sct
SHRA this morning and will apch TISX late this afternoon. In PR,SHRA
will be few and brief till aft 01/17Z, then sct SHRA expected ovr
nwrn and interior PR. VFR conds expected thru at least 17/00Z, but
mstr incrs aft 01/22Z TISX, and mtn obscurations will likely dvlp
aft 02/00Z in ern PR. Sfc winds bcmg 10-20 kt from the E with ocnl
gusts to 28 kt. Max winds W-NW 25-30 kt btwn FL400-495. Incrg aft
Moderate to fresh winds continue across much of the local waters.
This will promote choppy to hazardous seas, especially in the
offshore waters and local passages. The hazardous conditions will
continue into late tomorrow or early on Tuesday. Small Craft
Advisories remain in effect. Wave heights could increase again late
in the day on Tuesday due to a combination of weak swell and
moderate to locally fresh winds in the offshore Atlantic waters.
Conditions during the day on Tuesday are very borderline for Small
Craft Advisory criteria, and so, for now, the Advisory currently
only extends into Tuesday morning.
For the beachgoers, there is a moderate risk of rip currents across
most local beaches. At beaches on the southern part of the west
coast of Puerto Rico, western beaches of St. Croix, and some beaches
of St. John, the risk of rip currents is low.
.PRELIMINARY POINT TEMPS/POPS...
SJU 88 77 88 76 / 50 60 60 30
STT 88 79 86 77 / 50 60 50 20