308
FXCA62 TJSJ 011611 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
1211 PM AST Tue Oct 1 2024

.UPDATE...As the sun rose, most of the showers in the forecast
area were over the local waters especially from between Saint
Thomas and Saint Croix and south southwest to the Caribbean outer
waters. By noon showers were still present over the waters, but
fading, and new showers were developing over the western half of
the Cordillera Central and just upwind of El Yunque. Flow is still
east northeast in the lower layers and actually becomes northerly
from 10 kft to 30 kft in the latest sounding from San Juan, Puerto
Rico at 01/12Z, although no speeds were above 15 knots in that
layer this morning and most speeds were less than 10 knots.
Moisture in the mid layers was improving. However, TPW satellite
data shows a drier slot of air approaching from the north
northeast at least as long as the forecast area, but the center of
it is being overrun by more moisture behind it, so it will be
short lived as the GFS suggest. This will make the overnight
period tonight quite a bit drier, save for local surface moisture,
and the following day and evening much wetter.

&&

.AVIATION...
(12Z)

VFR conds will prevail over land thru 02/15Z except over land
under sct SHRA/TSRA til 01/22Z, where mtn obscurations and MVFR
CIGS and MVFR/IFR vsbys will exist. SHRA are moving 225/10 KT,
however the cells over the Cordillera Central were moving due
south toward TJPS. SO far development is minor with tops to only
20 Kft, however TSRA are expected to dvlp alg the mtns and in wrn
PR aft 01/17Z. Some land and sea breeze influences will be seen
at the sfc. Aft 02/08Z sct SHRA will dvlp ovr NE PR to yield MVFR
conds and mtn top obscurations in the Luquillo range.

&&

.PREV DISCUSSION... /issued 707 AM AST Tue Oct 1 2024/

SYNOPSIS...

Variable conditions will prevail in the next few days into the
weekend. There will be a drying trend early next week as a high
pressure moves and settles over the forecast area late Sunday. In
the tropical Atlantic, the NHC is monitoring the remnants of
Joyce, Tropical Storm Kirk and a tropical wave with a 90%
formation chance through 7 days. Swells from these systems will
cause a deterioration of marine and coastal conditions late this
workweek and into the weekend.

SHORT TERM... Today through Thursday...

Overnight, calm weather conditions with a light northeasterly wind
prevailed. The low temperatures were warmer than normal across the
islands. The Doppler Radar detected a few showers across the local
waters, and some moved inland across the U.S. Virgin Islands and
the windward sections of Puerto Rico.

A mid-to-upper-level ridge building across the northeast Caribbean
will produce stable weather conditions in the short term. The day
will begin with warmer temperatures, which, combined with the
available moisture, will result in muggy heat indices across the
local islands, especially in areas without rain activity.
Therefore, a Heat Advisory is in effect for St Croix.

The interaction of a surface low pressure (remnants of Joyce)
northeast of the islands and a surface high pressure to the
northwest will result in a light north to northeasterly wind flow
across the islands. This wind flow will bring patches of clouds
and moisture later today, resulting in some showers across the
windward sections. Then, the excessive diurnal heating, local
topography, and sea breeze fluctuations will promote the formation
of afternoon convection along the Cordillera Central and southern
slopes of PR, where we can anticipate urban and small stream
flooding due to thunderstorms.

Although a stable weather pattern will evolve in the short term due
to the presence of the mid-to the upper-level ridge, excessive
heating combined with the sea breeze variations and local topography
could promote the typical afternoon convection across portions of
the islands each afternoon. However, model guidance suggests the
arrival of a surge of moisture by Thursday that could promote
showery weather. At this time, Wednesday seems to be the driest
day of the short term. Hot heat indices will continue each day
across the islands.

LONG TERM... Friday through Tuesday...

On Friday, winds at the surface will remain light from the ENE
until Saturday due to a ridge to our west/northwest and current
Tropical Storm Kirk, forecast to become a major hurricane on
Thursday, over the Central Atlantic. A high level trough in the
north will enhance thunderstorm activity. Precipitable Water
(PWAT) values will range between 2.0 to 2.1 inches, which is near
to above the climatological normal. Additionally, colder 500 mb
temperatures and Galvez-Davidson Index suggest isolated to
scattered thunderstorms over the Cordillera Central and southwest
of Puerto Rico enhancing rainfall accumulations across the area.
Therefore, a limited to elevated flooding risk can be anticipated.

Available moisture and light winds, and slow moving showers will
enhance rainfall accumulations across the area. Therefore, expect
shower activity across interior Puerto Rico with isolated
thunderstorms. Winds will become light from the ESE on Sunday. A
mid-level ridge will start to settle over the forecast area by
Monday. Additionally, relative humidities at mid-levels will
plummet below 20 percent. However, latest model guidance shows a
deeper layer of low-to-mid level moisture reaching our forecast
area than the previous model cycle. Despite this surge of
moisture, this weather set-up will inhibit deep convective
activity for the most part. Still, isolated to scattered showers
can be expected particularly in the afternoon hours. The 925 mb
temperatures will also be higher than the daily mean (21.4
degrees). The combination of limited shower activity and clear
skies in coastal areas with subsidence aloft will promote hot
temperatures for early next week. Limited to elevated heat risk
will prevail for coastal and urban areas of PR and USVI.

The National Hurricane Center public advisory #6 of
Tropical Storm Kirk suggests that the system will strengthen and
become a major hurricane and turn northward by Thursday. Although
Kirk is not expected to impact PR and the USVI, recommendations are
to keep monitoring tropical activity as the hurricane season
continues through November 30.

AVIATION...
(06Z TAFS)

VFR conditions will prevail throughout the forecast period. However,
SHRA/TSRA will develop across the interior and southern slopes of
PR, which may affect TJPS between 01/17 and 23z. Passing showers
could affect TJSJ/TIST at times, but we are not anticipating
significant impacts. Winds will prevail calm to light and VRB
through 01/13z, then from the NNE to N at 10-15 with gusts around 20
kt, but with sea breeze variations.

MARINE...

A high pressure system over the western Atlantic will maintain
light to gentle northeast to east winds. Seas will remain around
3 to 5 feet over the next few days. Showers with thunderstorm
activity will create locally hazardous marine conditions for small
craft. A combination of northeast to east long-period swells will
reach the regional waters this weekend bringing hazardous seas.

BEACH FORECAST...

A moderate risk of rip currents will be in effect today for the
beaches of northern PR, and northern Culebra; low risk elsewhere.

In the tropical Atlantic, the NHC is monitoring the remnants of
Joyce, Tropical Storm Kirk and a tropical wave with a 90%
formation chance through 7 days. Swells from these systems will
deteriorate marine and coastal conditions late this workweek and
into the weekend.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001-003-005-
     007-008-010-011.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ002.

AM...None.
&&

$$

SHORT TERM...WS
LONG TERM....LIS
AVIATION...99