31L & 33L Expected to Merge
Posted by JAC on 8/19/2010, 10:24 pm




PREDICT Weather Discussion

Date(UTC): 2010/08/19 14:00
Author: Kyle Griffin
Submitted at(UTC): 2010/08/20 00:53
Current Conditions/Review of Yesterday's Forecast:

Several interesting features dominate the map as of 12z 19 Aug.
In the western Caribbean, numerous convective showers are
associated with the presence of PGI27L. The convection
associated with this disturbance is making a resurgence this
morning, although of a less organized nature than the morning
convective bursts observed yesterday. This convection is still
being enhanced by the small upper cold low that is retrograding
westward about 5-7 degrees west of the pouch for the last two to
three days. This feature is filling/shearing out, but the
25-35kt northwesterly upper level winds on the east side of the
system are still serving to create a nice outflow channel for
continued convective activity with PGI27L.

PGI30L resides in a bed of stratocumulus this morning, with no
convection in sight despite analyzed PW values approaching 60mm.
Two main factors can be attributed to this lack of convection.
First, the more obvious feature in PW fields and satellite
observations is the dry mid-latitude air ahead of the system
combined with a new dust/SAL region coming off the coast of
Africa farther north. Because of this, some finer particulate
matter may be intermingling with the stratocumulus and
preventing and development of PGI30L in the immediate future.
The second factor is the relatively cool SST values, which, at
present, are remotely sensed to be equivalent to the
boundary-layer temperatures which then follow the moist adiabat
through the majority of the troposphere: hence no CAPE can be
observed in much of the environment in and around PGI30L.

A bit to the south and east of PGI30L lies the current center
and vortmax associated with PGI31L. This feature lies within an
area of deep moisture and has a well-established and broad upper
level outflow channel extending to the southwest, much like the
circulation with a monsoonal trough might be expected to show.
This system is currently exhibiting cold cloud tops with a
handful of regions breaking the -70C threshold. A broad cyclonic
rotation can be observed in this convection as well. The only
possible hinderance is an area of convection located off to the
southwest of the system that may be limiting the low-level
inflow into the system. However, it seems likely that this
nuisance area will be absorbed by the pouch within the next
12-24 hours.

PGI33L is in a nearly identical environment to PGI31L, but
closer to the African coast. Convective bursts are evident, and
the system is a latitude similar to that of the "nuisance"
system mentioned above. PGI33L was also still carrying remains
of an outflow boundary turned undular bore in the mid-morning
hours (AST), from the outflow boundary that was formed around
10Z yesterday.

PGI32L was dissolved into the dry mid-latitude air, perhaps
mixed with some dust as well. More importantly, it is serving to
enhance convection on the east side of a large upper low
positioned just NE of the Leeward Islands. This pouch was
dropped in this morning's track updates.
DAY 1 (Next 24 hours) Outlook:


24HRS OUTLOOK

Forecast motion of PGI27L will take it further out of range of
the GV based at STX, and thus of little direct concern to
PREDICT operations. Likely tracks will landfall in the northern
Yucatan by 00z/20, emerge by 12z/20, and landfall in northern
Mexico by 12z/21. With no more than 24 hours left over any
single body of water, no multi-flight operations could be
conducted in the system, and thus is of little meaning to the
tri-agency group as a whole.

PGI30L is expected to slowly begin reconvecting as it moves over
progressively warmer waters during the next 24-36 hours. This is
indicated in the ECMWF by a slowly increasing low-level
vorticity signature and a continued WSW motion in the short
range. Any convective activity in the next 24 hours would be a
welcome sight to reassure forecasters that this will be a viable
target for potential flights beginning in 48 hours.

Further south and east, both the ECMWF and GFS show PGI31L
merging with PGI33L during the next 24 or so hours. In Mark
Boothe's pouch-tracking time series (not included below), the
models agree that the two will merge, develop into a respectable
cyclone, and move WNW into the future. The only discrepancy
comes in the form of exactly which pouch is tracked as the
seemingly imminent cyclone. The ECMWF prefers keeping PGI33L as
the dominant pouch (the first time this has been the case in any
model), while the GFS keeps the status quo of PGI31L acting as
the more mature cyclone. Regardless, pouch consolidation should
mostly occur during the next 24 hours. (Image 13, Image 145)
DAY 2 (24 - 48 hours) Outlook:

48HRS OUTLOOK

PGI27L's life cycle is described in full under Day 1, and
terminates at the 48hr mark from this discussion.

PGI30L is forecast to bottom out with respect to latitude during
this period, around the 15N/48W mark by 12z/21. This mark (48
hours out from present) is around the first scheduled flight
into the system on Saturday morning. Both ECMWF and GFS show the
system regaining some weak low-level vorticity by this point,
which is also embedded within a region with PW values of 55-60mm
throughout the 24-48 hour time period.

PGI31/33L is forecasted to complete the merging process by
around the 36 hour mark (00z/21) and evolve similarly to a
monsoonal gyre for the remainder of this period. Distinctive
features include the maintenance of a respectable equatorward
outflow channel and motion along (and eventual separation from)
a long, extended band of low-level vorticity that is present
along the ITCZ to the west and south of the developing system.
The forecasted vorticity fields indicate that the circulation
may be very large and perhaps diffuse in its formative stages.
Extended Outlook:

EXTENDED OUTLOOK

PGI30L is forecast to continue on a solid WNW track from the
position mentioned in the Day 2 portion above, glancing the NE
Leeward Islands (including STX) and progressing onward along or
just north of the Greater Antilles (perhaps even into the Gulf
by Day8-9?).

PGI31/33L has been forecast to recurve since the 12z model runs
yesterday, and each progressive run is continuing to trend so.
00z/19 cycle runs did not bring the center of the cyclone past
60W, and now do not pass 55W with the system. A potentially
powerful ET is possible, with basin-wide data showing wind barbs
of 95kt in the ECMWF in the Day8/9 time frame. CPA to STX is
likely late on the 26th/early on the 27th as the storm's
position is around 24N/55W at 00z/27. Obviously much can still
change with these forecasts, but the one certainty seems to be
that this will be an impressive TC.















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31L & 33L Expected to Merge - JAC, 8/19/2010, 10:24 pm
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