PREDICT Discussion
Posted by JAC on 8/20/2010, 6:31 pm
PREDICT Weather Discussion

Date(UTC): 2010/08/20 14:00
Author: Kyle Griffin
Submitted at(UTC): 2010/08/20 21:04
Current Conditions/Review of Yesterday's Forecast:

Current satellite imagery over the Atlantic show numerous
convective showers, with focusing features scattered across the
entirety of the basin. In the western portions, PGI27L has
stalled off the Yucatan coast and is responsible for showers
near the Belizean and Honduran coasts. Convection farther west
over the Bay of Campeche are likely driven by the small upper
low that continues to retrograde in front of PGI27L. Today,
however, it has moved westward faster than 27L and no longer
drives much of the convection associated with the pouch.

PGI30L remains anti-convective with a few sporadic convective
creations poised near the pouch centroid. Currently it is still
enveloped in a pocket of high PWAT values, but this pocket is
beginning to erode due to the dry mid-latitude air ahead of the
system and the Saharan Air Layer behind it.

PGI31L and 33L are actively convecting and are likely
responsible for the slow formation of a monsoonal trough in the
eastern Atlantic. Strong low-level southwesterly flow is
enhancing the convection with and around these pouches, which in
turn enhances the upper-level equatorward outflow. The
convection in the immediate vicinity of the pouch centers are
beginning to show signs of cyclonic rotation, and a mid-level
circulation with 33L is quite evident in the morning's visible
imagery. Declared Invest 95L by NHC mid-morning today, using
PGI31L's location.
DAY 1 (Next 24 hours) Outlook:

PGI27L forecast is as before, with the only significant change
being that the initial point has been slowed by about 12 hours.
After crossing the Yucatan today, it should have about 24 hours
out over the water before reaching mainland Mexico.

PGI30L is expected to move a bit south of due west over the next
24 hours steadily away from the SAL behind it and over a region
of slowly increasing SSTs. As such, it is expected to develop an
increasing amount of convection, perhaps some of it sustained by
12z/21 (tomorrow). A good estimate of its 24hr position (at
12z/21) is 19N/48W.

PGI31/33L are forecast to begin a merger process later today and
slowly begin to form a large monsoonal gyre. While this is
occurring, a (relatively) slow motion to the WNW should
continue. Strong outflow will also continue to enhance the
convection in the area.

12Z UPDATE: new 12z ECMWF indicates a small area of convection
on the NW part of the ITCZ (as of writing - 1530z - located near
14N/40W) could break off and form a pouch (translation speed
somewhere between 6 and 8 m/s) that tracks westward into
lower/central Caribbean over the next several days with a
low-level vortmax and PW max to accompany it. Development with
30L is also quenched in this run, possibly because of this new
feature to its south. Monitor in future runs.
DAY 2 (24 - 48 hours) Outlook:

PGI30L is expected to bottom out with respect to latitude during
the early portion of this period and begin moving to the west
just south of 20N. A bit more of an increase in SST is possible
along its track, but convection is likely to remain constant at
best during this time period. 48 hour position (12z/22) is
anticipated to be around 18N/56W, though this is subject to the
12z update mentioned above.

PGI31/33L are expected to finish their consolidation/initial
organization phase, with low-level vorticity signatures becoming
relatively circular by 12z/22. Outflow remains well-established,
and moisture levels only increase with time. As the system
begins to take on a more poleward component to its motion, it
slowly breaks away from its connection to the ITCZ, leaving a
trail of PW and low-level vorticity as it completes the process
by the 72hr time frame.
Extended Outlook:

PGI30L, in whatever state it may be in come 2-3 days from now,
can be tracked in at least the PW fields well beyond a week into
the forecast. After passing over the NE corner of the islands,
the storm tracks along the northern edge of Hispaniola and Cuba,
passing over the Keys and into the Gulf of Mexico by Day 8-9. At
this point, it appears to bring moisture into a potential Gulf
system developing at the tail end of a cold frontal boundary. As
a separate entity, there does not seem to be any chance of TC
development with PGI30L.

PGI31/33L have an apparently clear-cut future for the next
several days, with an increasingly northward component to a
generally WNW heading. After reaching 50W, its track becomes
more uncertain, with the last couple of days worth of model runs
recurving prior to 60W (sometimes even prior to 55W). The nature
of any potential ET is still very TBD. Also: latest 12z ECMWF no
longer develops as strong of a system, tracks it farther south,
and it missed the trof and turns west just north of the islands
with a position not too far east of the Bahamas by Day 10. And
yes, this portion of the discussion was reworded to indicated
lower certainty of the forecast track upon seeing the 12z
update.











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Data coming in on 95L - JAC, 8/20/2010, 8:14 am
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