A very interesting AFD from NWS San Juan about future low

[cycloneye]

fxca62 tjsj 071953
afdsju

puerto rico and u. s. virgin islands forecast discussion national
weather service san juan pr
352 pm ast sun nov 7 2004

. synopsis ... at upper levels ... split flow is forming around a weak
low just east of georgia and an 80 knot jet is rounding the trough
over the florida straits. this system will dig southeast and by
tuesday evening the jet will place southwest flow over the windward
passage. models place strong upper level divergence over the area
beginning wednesday and continuing through friday morning. the
entrance region of the jet then passes over puerto rico on saturday
night.

at mid levels ... a ridge of high pressure will give way to southwest
flow on monday as a trough pushes southeast over cuba. during the
middle of the week the trough becomes more positively tilted and
edges over hispaniola. the trough becomes much more rounded and
moves northeast past puerto rico on sunday of next week.

at lower levels ... high pressure is over the west central atlantic
and southeast flow continues over the local area. a trough extends
from the caribbean waters off colombia north to weak low pressure
north of the bahamas. a trough over canada at the moment will move
east toward the atlantic and align with the trough extending north
from the caribbean. this troughing will then move eastward ... more
slowly in the south than in the north and will become more and more
positively tilted as the week progresses. current models suggest
that low pressure will form in the trough south or southwest of
hispaniola early in the week and then move north of puerto rico or
over puerto rico on friday and saturday ... although some models
suggest this may happen as early as thursday. drier air will be
introduced saturday night through monday.

&&

. discussion ... southeast flow in lower levels of the atmosphere has
created convergence over northwestern puerto rico. radar indicates
that thunderstorms have already dropped up to 4 inches of rain over
parts of arecibo and san sebastian and the area is being monitored
for flash flooding. other areas may soon follow. models are
currently indicating that the airmass over the area this afternoon
is drier than what passed last night or what is yet to come monday
morning. with higher moisture moving in tonight have kept pops at
numerous levels over eastern puerto rico and much of the atlantic
and caribbean waters east of puerto rico including the virgin
islands. showers here are also likely to have some embedded
thunderstorms. some urban flooding may be possible tonight on the
east and southeast coasts and adjacent foothills.

an interesting weather pattern is shaping up for the up coming week.
a prominent trough is developing in the western caribbean and
western atlantic. the trough will move more quickly in the north
than in the south ... supply a strong positive tilt by weeks end. some
models strengthen the trough most near 30 degrees north ... while
others develop fairly strong lows at lower latitudes ... which have
the possibility of becoming tropical cyclones or sub tropical
cyclones during the week. one of those centers is being forecast by
the gfs to pass over puerto rico. at present this low pressure is
being depicted with lighter winds around the center and models do
not appear to develop storm force winds. however ... because it is
developing in the very moist air mass now building over the area in
an environment where 1000 - 850 mb theta - e temperatures remain above
340 k and increase to 344 k by wednesday over the forecast area with
higher values in the low itself ... we cannot discount the possibility
that this week will be characterized by heavy rains on and off
throughout the next 5 days. models show moisture developing in a
band that streams across the windward islands from the southeast.
that band is joined over the area by moisture amassing along the
southeast border of the trough to our west on tuesday. these two
bands are nearly perpendicular and tuesday and wednesday place us
near their intersection. then on thursday and friday we are
swept ... according to the gfs ... by low level relative humidities from
the low pressure predicted to pass over the area ... that also moves
northeast along the second moisture band. some areas could easily
see their normal november rainfall this week and if models are
correct some areas will receive more. since winds vary considerably
due to the passage of upper level troughs and the low pressure
system ... it looks like most areas will see significant rain from the
pattern forming and evolving during the week. the extreme
southwestern caribbean which is being shown to be the genesis area
for the low pressure system yet to evolve is now full of clouds and
shows some broad scale turning. as wind and moisture patterns firm
up ... we will have a better handle on how much flooding is likely to
occur and at least a general idea of where.

What a very detailed discussion about the scenarios for this developing low pressure.Let's see what happens in terms of where I am in Puerto Rico.

[msbee]

wow!
I had trouble understainsing all that actually Luis:-)
It is very detailed.
basically it seems to be saying that PR will be having some unsettled weather at best
maybe that goes for us too?
Barbara

[weatherwindow]

my hat is off to nws/sju....those gentlemen can certainly write a discussion, i sent them an email to that effect...everything seems to be falling into place ...the big questions are: when, where, and tropical or not...after staring at models for the past three days...my conclusion is that if the system develops a closed circulation in the sw carib, somewhat protected from shear, the system will be tropical...however, most of the models are suggesting that a closed circulation will not occur until the incipient center is in the vicinity of 18-20deg north....that environment will be more conducive to sub-tropical development...the track would seem to be dependent on the timing of dev...slower(and more southerly) would seem to favor more of a blocking pattern and subsequently delayed recurvature to the northeast. however, most of the models seem to favor a more progressive solution with almost immediate northeasterly bias. the early threat to either the bahamas or florida no longer appears realistic.

[cycloneye]

wow!
I had trouble understainsing all that actually Luis:-)
It is very detailed.
basically it seems to be syaing that PR will be having some unsettled weather at best
maybe that goes for us too?

According to the model scenarios that I haved seen this broad low will move NE from the SW caribbean over PR.However msbee for the area of the Leewards where ST Marteen is some of this may go to your neck of the woods.But the most important thing is that nothing cyclonic meaning a named storm will cross the islands.Maybe when this moves away from us some type of subtropical caracteristic or a gale center one may form well north of the caribbean.

[Anonymous]

Man...what if a subtropical cyclone made landfall....

[cycloneye]

my hat is off to nws/sju....those gentlemen can certainly write a discussion, i sent them an email to that effect...everything seems to be falling into place ...the big questions are: when, where, and tropical or not...after staring at models for the past three days...my conclusion is that if the system develops a closed circulation in the sw carib, somewhat protected from shear, the system will be tropical...however, most of the models are suggesting that a closed circulation will not occur until the incipient center is in the vicinity of 18-20deg north....that environment will be more conducive to sub-tropical development...the track would seem to be dependent on the timing of dev...slower(and more southerly) would seem to favor more of a blocking pattern and subsequently delayed recurvature to the northeast. however, most of the models seem to favor a more progressive solution with almost immediate northeasterly bias. the early threat to either the bahamas or florida no longer appears realistic.

Yes they do very detailed discussions covering all the scenarios about what will happen in the future here in PR.When the hurricane season comes every year I go first to read what the NWS in San Juan have to discuss about the tropics and later to the NHC.

[cycloneye]

W ATLC TROPICAL WAVE ALONG 56W S OF 18N MOVING W 15 KT. THIS IS
A WELL-ORGANIZED WAVE FOR SO LATE IN THE YEAR AND IT IS THE
FOCUSING POINT FOR PLENTY OF RAIN. WLY SHEAR/UPPER DIVERGENCE
SHOULD ENHANCE THE WAVE'S CONVECTION AND HEAVY RAIN WILL MOVE
INTO THE WINDWARD ISLANDS TONIGHT OVERSPREADING THE LEEWARD
ISLANDS BY LATE MON. SCATTERED MODERATE FROM 10N-14N BETWEEN
54W AND BARBADOS.

msbee this wave will combine with what will come from the west this week to create a very wet scenario for us.

[msbee]

Luis
If I understand the discussion properly, this low will be moving NE over the PR area all week then?

[cycloneye]

Luis
If I understand the discussion properly, this low will be moving NE over the PR area all week then?

It will move slowly eastward and with that wave moving from the east wow the net result will be rain,rain,rain........

[cycloneye]

afdsju

puerto rico and u. s. virgin islands forecast discussion
national weather service san juan pr
545 am ast mon nov 8 2004

. discussion ... water vapor imagery showing mid to upper level trough
located over the southwest north atlantic early this morning and it
continues moving slowly east. this trough is expected to move a bit
further east today and tonight and then become located just west
through northwest of the local area tuesday through at least
thursday ... before weakening slightly and lifting northeast friday
and saturday. at low levels ... already have a broad trough developing
from central america northeast across the windward passage and into
the bahamas. a cold front will exit the east coast of the u. s. today
and move southeast ... it is then expected to merge with the low level
trough across the north central caribbean ... and move east into
hispaniola tuesday ... then into puerto rico on wednesday. the gfs
model shows a surface low or a series of weak lows developing over
the central caribbean and then moving northeast near or over the
local islands late tuesday night or more likely wednesday through
thursday.

although fine details are still a bit sketchy and seem to differ
slightly from model run to model run ... it appears that the
combination of this deep layered troughiness ... increasing dynamics
and instability ... good 850 - 700 theta e advection and resultant theta
e ridge aligning over fa and local effects ... should result in a very
active week locally ... with weather becoming more unsettled today and
tonight ... but especially tuesday through thursday. would expect
increasing potential each day ... for showers and thunderstorms with
heavy rainfall and flooding ... especially wednesday and thursday ... as
approaching surface boundary and/or surface lows act to focus and
lift deep tropical moisture that will be pulled into local area from
the southwest and south. followed inherited grids and forecasts
quite closely this morning ... as they already had a very good handle
on expected weather for the next several days.

No big changes from yesterday thinking of a rainy pattern for the northeastern caribbean his week.

[Stormsfury]

Man...what if a subtropical cyclone made landfall....

They have in the past and will do so in the future ...