Area Forecast Discussion
National Weather Service San Juan PR
403 AM AST Wed May 20 2020
Drier than normal conditions will continue, but are about to end as
we head into the weekend. Isolated to scattered showers are possible
for the next few days, but ample moisture will move in for the
weekend and next week. Coupling the moisture with strong surface
heating will lead to heavy rainfall and the potential for
localized flooding. A weak swell is expected this Thursday and
into the weekend, but seas will still stay below 5 feet.
.SHORT TERM...Today through Friday...
Post Tropical Storm Arthur 1100 miles north of San Juan and a strong
low in the north central Atlantic will force the sub tropical ridge
further south to around 25 degrees north latitude, north of Puerto
Rico and the U.S. Virgin Islands. Arthur will weaken during the
period, but not before it spins a low off of itself that runs
rapidly toward the northeast. These lows will hold the high pressure
closer to the Canary Islands and allow east to southeast flow to
continue across the local area. Moisture is patchy, but increasing
with steadily wetter patches moving through the area today and
Thursday bringing slowly increasing shower activity through
Thursday. Moisture will increase dramatically late on Friday.
500 mb temperatures continue between minus 6 and minus 7 degrees and
capping will not be sufficient to hold back local convection so have
continued with a slight chance of afternoon thunderstorms in the
west northwest portions of Puerto Rico each day. 1000-850 mb
thickness values forecast by the GFS indicate a a slight cooling
today but warming again on Thursday. We will continue to see warmer
than normal temperatures over the area with many locations in the
lower elevations reaching the upper 80s or the lower 90s each day.
Presently it looks like Thursday will be the warmest day for the
Greater San Juan Metropolitan area and the coastal plains to the
At upper levels the weak cut-off low pressure will be 1150 miles
east northeast of Puerto this afternoon and is now far enough a way
as to allow a modest ridge to build over the area. A 60-85 knot jet
is approaching and will arch over the top of the ridge well to our
north. Although upper level moisture will increase somewhat and
bring increasing high clouds, mid levels will continue very dry
until late Friday such that any tall convection will be locally
.LONG TERM...Saturday through Thursday...
Forecast models continue to agree on a trend towards much wetter
conditions as we approach the weekend and into the next week. As
discussed in the short term section, moisture will begin to ramp up
in the latter half of the workweek, and it will continue to increase
during the weekend. This time of year, average precipitable water
values are around 1.75 inches, and by Saturday the foreacst is for
around 1.9 inches, and around 2 inches into the middle of next week.
Just given those values, heavy showers are expected.
The important questions then are, where is the rain most likely, and
what is the potential for thunderstoms. Throughout the longterm
period, somewhat weak southwesterly flow is expected at low
levels, which will favor heavy afternoon rainfall for the interior
and northwestern sectors of Puerto Rico, as well as downwind of
El Yunque over the San Juan Metro area. Nighttime and morning
showers will favor the southeast coast of Puerto Rico and El
Yunque. However, it should not be assumed that these will be the
only locations that have a chance for heavy rainfall, and we will
get a better idea of what is expected as high resolution models
begin to extend into the weekend. As for lightning potential, it
doesn`t look great, but some of the stronger storms have the
potential to surprise. Models suggest that we will be on the
subsidence side of an upper level ridge throughout the long-term
period. However, soundings don`t really show much in the way of
warming aloft or any inversions, so clearly the ridge will not be
very impact. Generally, CAPE values stay above 1500 J/kg each
afternoon, and coupled with ample moisture and very moist
soundings, deep convection and lightning generation is possible.
However, the main story is the threat for localized flooding for
each day in the long term period.
Sct SHRA ovr lcl waters, arnd the USVI and ern PR
are expected thru 20/15Z. SHRA will increase across PR with a slgt
chc of TSRA WNW PR aft 20/17Z. VCSH TJMZ, TJPS and TJBQ and mtn
obscurations in PR between 20/17-23Z. ESE sfc flow of less than 10
knots expected thru around 20/13Z, bcmg ESE at 10-18 knots with sea
breeze variations thereafter. Maximum winds NW 45-55 kt btwn FL 410-
Winds are expected to slow down slightly beginning today, which
will help seas to become more tranquil. Thus, wave heights below 5
feet are expected for the most part. A weak northwesterly swell
is expected to begin Thursday and last into the weekend, but seas
will still be below 5 feet. Thereafter, calm seas will prevail.
However, active weather over the weekend and into the next week is
expected, so scattered showers are expected over the local
Elevated fire weather conditions are expected today, though this
is expected to be a borderline case. Soil moisture remains low at
8% with KBDI values still above 700 at Camp Santiago. Furthermore,
winds will likely exceed 15 mph sustained this afternoon, and at
the same time RH values may drop into the low to mid 40s. There is
a low fire danger for the southwestern plains given low KBDI
.PRELIMINARY POINT TEMPS/POPS...
SJU 91 77 92 79 / 20 40 60 60
STT 88 78 88 78 / 20 40 40 40