Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 181250

Area Forecast Discussion
National Weather Service San Juan PR
850 AM AST Wed Jan 18 2017

.UPDATE...Sounding showed much less moisture above 6400 feet than
in the previous sounding and precipitable water dropped
accordingly from 1.22 inches in the 18/00Z sounding to 1.03
inches in this mornings 18/12Z sounding. Revisited most of the
near-term grids and replaced them with a more representative
model blend for both POPs and min/max temperatures. Current GFS
1000-850 mb thickness will fluctuate around 1390 meters through
Friday and then edge up slightly through Sunday night, indicating
little change in layer warmth, but winds will also diminish
considerably near the surface to heighten the diurnal variations.
Therefore highs in the low to mid 80s in the lower elevations will
continue. And lows as low as the mid 50s in the highest valleys
and no lower than the mid 60s around 1500 feet on the slopes
should be expected for the next 3 nights. Showers will become much
less frequent tonight.


.DISCUSSION...A dry slot is evident in the MIMIC product and so
the frequent showers on the north coast have ended and drier
weather should continue for the next several periods.


.AVIATION...Mainly VFR conditions are expected to prevail across
all terminals through the forecast period. SHRA could develop this
afternoon over the interior and southwestern portions of PR.
Creating mainly VCSH periods at TJMZ/TJPS. The latest 18/12z TJSJ
upper air sounding indicated NE winds at 14-22 kts from the SFC to
FL200...becoming NNW and higher w/height. Maximum winds 315/66 at
FL385 and 290/58 at FL460.


.MARINE...Winds through the Mona channel proved to be close to 20
knots, but models insist that winds will relax as the drier air
moves in. Hence seas will also follow. Exercise
prevalent in the forecast this morning should all be dropped by
later tonight. Small craft advisory conditions should not return
before the end of next week--if then.


.PREV DISCUSSION... /issued 602 AM AST Wed Jan 18 2017/

A trough aloft will continue its move across the region. A surface
low pressure exiting the eastern U.S. will continue to weaken a
surface high pressure across the southwestern Atlantic. This
pressure systems will induce a moderate east northeast wind flow
across the local islands and surrounding waters, which in turn
will push patches of low level moisture with embedded clouds and
showers across the U.S. Virgin Islands and the northern and
eastern sections of Puerto Rico at times.

As a surge of moisture moved over the region overnight. This
surge produced scattered showers, but due to their rapid movement
the rainfall totals were minimals. Then, satellite imagery
indicated the intrusion of dry air across the region, producing a
diminish in shower activity across the islands of Puerto Rico and
the U.S. Virgin Islands early this morning. For the rest of this
morning and under the prevailing northeasterly wind, isolated
passing showers can not be ruled out across the windward sections
of PR and the USVI.

A surface low pressure is forecast to move across the western
Atlantic by Thursday. The pressure gradient is forecast to relax
as this low system move eastward across the Atlantic waters.
Also, as mentioned in the previous discussion, winds speed are
expected to diminish considerable becoming more from the east late
in the week and during the upcoming weekend. As this happens, a
frontal boundary is expected to move close to the local islands.
The proximity of this frontal boundary combined with a dry cool
air mass could result in cooler pleasant temperatures by the end
of the week and into the weekend.

Mainly VFR conditions are expected to prevail across all
TAF sites through at least 18/22z. Some passing showers are expected
to affect TIST, TISX, TJSJ and TJBQ terminals from time to time. Low
level winds will be mainly northeast at 10 to 15 kts with higher

Marine conditions are expected to slowly improve across the
region today. Mariners can expect seas up to 6 feet and winds
around 18 knots mainly across the Atlantic waters and the
Caribbean Passages. High risk of rip currents are in effect until
this afternoon.


SJU  84  73  84  73 /  30  10  10  10
STT  84  72  84  72 /  20  10  10  20


PR...High Rip Current Risk until 6 PM AST this evening for Culebra-
     Mayaguez and Vicinity-North Central-Northeast-Northwest-San
     Juan and Vicinity-Southeast-Western Interior.



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