textproduct: San Juan

KEY MESSAGES

Issued at 1249 PM AST Tue Dec 9 2025

* Marine and beach conditions will deteriorate beginning Thursday. Expect choppy to rough seas and life-threatening rip currents.

* Winds increase late in the week, leading to breezy to windy conditions across coastal and elevated areas. Light-weight outdoor items may be blown around in exposed locations.

* For Puerto Rico, showers are expected to continue during the morning hours across the east/southeastern portions. Over west/northwest PR, afternoon showers are expected each day. Peak heat indices to around 100 deg F are expected near the coast.

* For the U.S. Virgin Islands, passing showers will continue throughout the morning, becoming frequent later tonight and early tomorrow.

Short Term (This evening through Thursday)

Issued at 1249 PM AST Tue Dec 9 2025

SYNOPTIC OVERVIEW: Puerto Rico and the USVI reside on the southwestern periphery of a stout (592 dam) H5 ridge. Proximity soundings reveal a dry, subsident regime in the mid to upper troposphere. In the lowest 10 KFT, light to moderate SE flow persists. Weak vorticity maxima are embedded in the SE flow, accompanied by brief surges of low- level moisture. This overall pattern is forecast to continue through the period. H85 Ts are projected to remain near to slightly above normal.

SENSIBLE WEATHER/IMPACTS: At least two weak disturbances are forecast to pass over the district over the next couple of days, the first of which is marching its way westward across PR this afternoon. Showers are already in progress, with an initial windward focus shifting toward the interior and western half of PR. Satellite imagery shows this progression well, with skies clearing out over the USVI. Sufficient instability is in place, but cloud towers will ingest very dry air above 10 KFT, leading to mostly low-topped downpours void of lightning. Briefly drier conditions are expected area-wide through the evening as we sit between disturbances.

After midnight, hi-res guidance suggests the next wave of energy approaches USVI, leading to scattered shower redevelopment. This feature appears to have slightly better moisture and instability, so additional showers and a low chance for lightning continue into Wednesday. Relatively speaking, the lightning potential is highest (20%) as the overall coverage slowly shifts to western PR tomorrow afternoon. A third, and even weaker, disturbance is slated to pass overhead by Thursday, preserving the oscillation between sunshine and scattered showers. Any excessive rainfall / flooding threat appears limited to especially sensitive locales in higher terrain: the experimental REFS shows only spotty chances (10-20%) for in excess of 2" of rain through Thursday.

On Wednesday, expect a few gusts to around 25 mph on the east-facing coasts, Vieques, and the USVI. Elsewhere, moderate SE breezes should keep temperatures near to slightly warmer than normal, with peak heat indices along the coast from 96-101 deg F each afternoon.

Long Term (Friday through next Monday)

Issued at 1249 PM AST Tue Dec 9 2025

SYNOPTIC OVERVIEW: The anomalous upper ridge over the Central Atlantic is anticipated to retrograde slightly and expand across the NE Caribbean late this week, deflecting a deep trough on the US East Coast well north of the islands. H5 heights are projected to remain above normal through the period. Surface high pressure also noses westward, enhancing the pressure gradient from Friday through the weekend.

South of the ridge axis, guidance suggests that additional perturbations of energy and moisture should clip the district as they translate westward across the Caribbean Sea. These features appear to be of a lower amplitude than those passing by during the next 48 HR, which makes sense considering the large-scale subsidence. Ensemble members hint at an approaching front around next Tue/Wed, but it is currently forecast to stall and dissipate somewhere near the Turks & Caicos and/or Hispaniola.

On the whole, moisture profiles remain consistent throughout, characterized by seasonable low-level RH and very dry parcels above H7. Due to the ridge and invariably warmer-than-average SSTs, PBL temperatures should continue to run on the warmer side of seasonal averages.

SENSIBLE WEATHER/IMPACTS: At best, scattered (30-50%) coverage of showers is forecast through the period, with coverage highest over eastern PR to USVI overnight/morning before shifting to the western half of PR in the afternoon/evening. The exact timing of these showers will be modulated by the passage of the weak disturbances. Still, we expect quite a bit of dry time through at least the weekend. As the ridge weakens and a weakening front draws closer early next week, rain chances subtly increase (40-60%).

Breezy conditions are anticipated over the weekend. East winds gusting to 20-25 mph are forecast, particularly at the coast. This may give slight relief from the heat, but max heat indices still approach 100 deg F over lower elevations.

AVIATION

(18Z TAFS) Issued at 1249 PM AST Tue Dec 9 2025

Weak disturbances will occasionally produce scattered showers, with activity favoring the USVI and San Juan area terminals in the morning hours before shifting toward the interior and west of PR in the afternoon (JBQ). The chance of lightning activity is non- zero but small, generally 20% or less. Prevailing VFR, except for brief categorical restrictions within showers. Broad east to southeast flow persists through the period, becoming somewhat gusty at IST and ISX on Wed.

MARINE

Issued at 1249 PM AST Tue Dec 9 2025

(modified previous discussion) A broad surface high pressure over the Central Atlantic will promote east to southeast gentle to moderate winds through Wednesday. The high then strengthens, tightening the pressure gradient and increasing winds, allowing seas to build across the Atlantic waters and exposed passages. Additionally, a long-period northerly swell arriving early Thursday will further worsen conditions. Choppy to rough seas are expected, with Small Craft Advisories likely by the end of the workweek. A gradual improvement remains possible by the weekend as winds and swell ease.

BEACH FORECAST

Issued at 1249 PM AST Tue Dec 9 2025

(previous discussion) A moderate risk of rip currents will continue over the north and east-facing beaches of Puerto Rico and the U.S. Virgin Islands through at least Wednesday. Beachgoers are encouraged to exercise caution as life-threatening rip currents are possible along the surf zone. A long-period northerly swell is expected to arrive early on Thursday and spread across the local waters and passages. Combined with increasing winds, beach conditions are very likely to deteriorate and become dangerous, mainly along north-facing beaches of the islands. Residents and visitors are urged to check the beach forecast before going out, and heed the advice of the flag warning system. As the swell eases and winds weaken, beach conditions should improve by the latter part of the weekend.

SJU WATCHES/WARNINGS/ADVISORIES

PR...None. VI...None. AM...None.