Atlantic from 19N to 22N between 55W and 65W Offshore Forecast
|Today...E Winds 15 To 20 Kt. Seas 7 To 10 Ft In E To Se Swell.|
|Tonight...Ne To E Winds 15 To 20 Kt. Seas 6 To 9 Ft In E To Se Swell.|
|Sat...Ne To E Winds 15 To 20 Kt. Seas 6 To 9 Ft In E To Se Swell.|
|Sat Night...Ne To E Winds 15 To 20 Kt. Seas 6 To 9 Ft In E To Se Swell.|
|Sun...Ne To E Winds 15 To 20 Kt S Of 23n, And Ne To E 10 To 15 Kt N Of 23n. Seas 6 To 8 Ft In E To Se Swell.|
|Sun Night...Ne To E Winds 10 To 15 Kt. Seas 6 To 8 Ft In E Swell.|
|Mon...Ne To E Winds 10 To 15 Kt. Seas 6 To 8 Ft In E Swell.|
|Mon Night...Tropical Storm Conditions Possible.|
|Tue...Tropical Storm Conditions Possible.|
| Area Forecast Discussion|
National Weather Service San Juan PR
1134am AST Fri September 29 2023
Mostly sunny to partly cloudy skies prevailed with only a few showers noted over the coastal waters of the Atlantic and northeast PR coastal areas earlier this morning. Recent TJSJ 29/12Z upper air sounding suggests precipitable water at 1.96 inches while that of TNCM (St Maarten)was at 2.17inches, suggesting a fairly moist environment still lingering across the region. That said afternoon convection will be likely across portions of Puerto Rico and mainly downwind and over the coastal waters of the USVI this afternoon. Low level winds continued from the east northeast between 10-15 mph below 700 millibars. Otherwise no change to the inherited short term forecast and reasoning at this time.
.Aviation...Prevailing VFR durg prd. SCT lyrs nr FL025...FL050 en route btw islands. Mstly Isold SHRA/TSRA en route. Aftn convection over the central interior and southwest PR will lead to Isold-SCT SHRA/isolated TSRA (thunderstorms with rain), with VCSH possible at TJSJ/TJBQ and VCTS at TJPS. Few SHRA (rain showers) possible en route to USVI but no sig weather impacts anticipated.
.From previous discussion issued at 441am AST Fri September 29 2023
A variable weather pattern is expected today, with periods of sunshine and a few clouds in the morning, followed by afternoon convection. Another hot day is expected with heat indices from 108-111 degrees for all the coastal areas and the vicinity of Caguas, where a heat advisory is in effect from 10am to 5 PM. An increase in moisture from late tonight into Sunday from the external bands of Philippe will increase the frequency of pesky showers. Choppy to hazardous marine conditions would prevail across the offshore Atlantic waters with seas up to 7 feet trough at least Tuesday.
.SHORT TERM...Today through Sunday... Today's main hazards revolve around the risk of excessive heat, excessive rainfall, and lightning, with threat levels ranging from limited to elevated, and these threats are expected to persist throughout the weekend. From Friday night into Saturday night, there is a chance for increased excessive rainfall and lightning hazard risk levels.
An analysis of overnight radar and satellite imagery indicated that northern Puerto Rico and the U.S. Virgin Islands experienced showers during the evening hours, resulting in radar-estimated rainfall accumulations of up to a quarter of an inch over northeastern Puerto Rico. However, conditions improved significantly after midnight, with isolated showers moving over these areas at times. Overnight temperatures varied across the region, with higher elevations in the interior dropping to around 70 degrees while lower elevations maintained temperatures at approximately 82 degrees. Winds were generally light and variable.
Influenced by Tropical Storm Philippe, located about 640 miles east of Saint John, local winds will remain northeasterly throughout the cycle, peaking at 5-10 mph today and 10-15 mph on Saturday and Sunday. Based on satellite-derived Total Precipitable Water (PWAT) data and following recent global models guidance, typical moisture levels, around 1.8-2.0 inches, will hold across the forecast area today. Despite Philippe's expected distance, an outer band will likely spread across the local islands from tonight into late Saturday night, causing a gradual increase in tropical moisture to about two standard deviations from the typical seasonal threshold, around 2.4 inches by Saturday afternoon. As drier air moves in and moisture content decreases, the likelihood of showers developing will decrease, leading to a weather pattern that relies more on heat and the influence of sea breezes for localized convective development.
Expect a typical seasonal weather pattern today, influenced by a prevailing northeasterly steering flow. The day will start with morning showers moving in from the local waters towards the northern sections of Puerto Rico and the U.S. Virgin Islands. In the afternoon, expect showers along with isolated thunderstorms development to cluster primarily across the interior and extend towards the south-southwest sections of the islands. These showers may bring moderate to locally heavy rainfall, which could pose risks of excessive rainfall, ranging from urban and small stream flooding to localized flash floods and mudslides. Due to abundant tropical moisture and unstable atmospheric conditions, shower frequency will increase, with the possibility of isolated to scattered thunderstorm development from tonight into late Saturday night. This increased rainfall activity will sustain the risk of excessive rainfall hazards, even during nighttime.
Despite northeasterly winds and the potential relief this flow usually represents, heat index values could still exceed 108 degrees, particularly across prolonged rain-free areas across the western, northern, and eastern Puerto Rico, Vieques, Culebra, and the U.S. Virgin Islands, where Heat Advisories are in effect.
Monday through Friday... A variable weather pattern is anticipated across the islands due to the influence of high-pressure systems over the western Atlantic Ocean and the presence of tropical systems, Philippe and Rina, for most of the long term. As per the latest update from the National Hurricane Center, these systems are currently positioned several hundred miles east of the islands and are expected to move northeastward into Central Atlantic waters. This movement will bring changes in the prevailing wind patterns across the region, becoming more from the northeast until Tuesday and then veering winds. Within this altered wind pattern, we can anticipate the arrival of patches of low-level moisture from the outer bands of the tropical systems, particularly during the upcoming Monday and Tuesday.
So far today, meteorological models suggest that the shower activity during this period may result from a combination of local effects, and it will not last due to a mid-level ridge in place. By Wednesday, as Philippe and Rina progress northward and potentially merge into a single system, a weakening in the pressure gradient in the region is forecasted, leading to light and variable wind conditions. During this time, lower-level humidity and local factors, particularly orographic influences, will lead to shower activity focused on the interior sections. The speed of these winds may contribute to the accumulation of rain, potentially resulting in ponding water on roadways and areas with poor drainage. Similar weather conditions would prevail on Thursday.
Weather models indicate an increase in humidity and instability across the region as we approach Friday. As of today, the forecast suggests an unsettled weather pattern for the final day of the extended forecast period.
A surface high pressure over the western Atlantic and the tropical cyclones Philippe and Rina will continue to result in a moderate northeasterly wind flow pattern across the region. A NE swell from tropical storm Philippe will continue to result in hazardous seas across the offshore Atlantic waters, with seas up to 7 feet. Therefore, the small craft advisory is in effect and could be extended for most of the weekend. For the rest of the local waters, seas will remain up to 5 feet. There is a high risk of rip current across the north-central and the San Juan area, at least through 6pm today. The risk will remain low to moderate for the rest of the exposed beaches.
.SJU WATCHES/WARNINGS/ADVISORIES... PR...High Rip Current Risk until 6pm AST this evening for PRZ001-005.
Heat Advisory until 5pm AST this afternoon for PRZ001>005-007- 008-010>013.
Heat Advisory until 5pm AST this afternoon for VIZ001-002.
AM...Small Craft Advisory until 2pm GMT Saturday for AMZ711.