Tropical storm Emily Forecast/Discussion 3

Emily’s structure has not changed much the past day or so,
with the low level circulation running ahead of the mid level circulation and
associated convection, in fact becoming exposed for a time earlier today. Emily’s
center has occasionally tried to slow down and slip back under the convection
yielding an erratic but generally westward motion today, which is taking Emily
much farther south than originally expected.

The most recent microwave imagery shows that the low level
circulation is still west of the main convection—near 16.4N, 71.4W in this
image.

The 0z NAM initialization shows a good part of the problem—a
northerly upper level flow above a strong easterly flow in the lower levels.
This is resulting in the convection being blown to the south and east of the
low level circulation as it tries to form, which is the cause of Emily’s
current disorganization.

In addition, some dry air is still in place north of Emily
and is being injected into the system by the aforementioned wind shear:

This dry air is slowly mixing out as it moves west, but will
continue to bother Emily in the short term.

With that said, will forecast no intensification before
Emily interacts with western Hispaniola and/or eastern Cuba starting Thursday
morning. Given Emily’s disorganized nature, it is hard to tell what state the
system will come out on the other side of the islands in. Given that Emily is
currently outrunning the mid level center, land interaction may not make things
much worse and may even allow the mid level to catch up to the low level
vortex. It is also possible the low level vortex opens up into a tropical wave
or gets caught up over the high terrain of either of the Islands, and need to
rebuild itself completely after passage.

The global models are essentially split in this situation.
The GFS/ECM are the best global models at handling tropical cyclones, and the
12z ECM run dissipates Emily over Cuba, which it has been fairly consistent in
showing, while the 12z GFS weakens the cyclone over land but shows re-strengthening
thereafter, which it has been showing fairly consistently. The 0z GFS shows
Emily dissipating over land. The 12z CMC dissipates Emily, while the 12z UKMET
does not, although neither model is a great tropical model.

North of the islands, the GFS/ECM both agree in the low
level flow weakening under an upper level anti-cyclone, allowing shear to
decrease significantly. Note the light winds at 300MB over the southern Bahamas
Friday morning as Emily passes overhead, which indicates fairly weak wind
shear.

The 12z ECM also shows about a 48 hour window of low wind
shear over the southern Bahamas and off the Florida/southeast US coast before
re-intensifying upper level winds. Sea surface temperatures are very warm in
this area, so re-intensification would be favored if Emily survives land
interaction.

Emily will encounter increasing upper level winds by day 5
as she begins to run into upper level westerlies off the southeast US coast.

For the intensity forecast, will show Emily holding steady
at 45 knots until landfall within the next 12 hours. Will then weaken Emily
down to a tropical depression by hour 24 and maintain her as such until hour 72.
This is a change from the previous forecast, but takes into account the fact
that the low level circulation may open up into a tropical wave over the high
terrain of eastern Hispaniola/western Cuba. Will then restrengthen Emily into a
tropical storm by day 4, however this is highly uncertain. It is now possible
that Emily’s surface circulation gets destroyed or caught up over Cuba while
the mid level circulation runs off farther east given that Emily is currently
so disheveled.

Emily is being steered by the lower level winds. Emily’s
surface center has tried moving due west today as it has become disjointed from
the convection. However, recently Emily’s center has slowed significantly and
tried turning north. This is likely due to a combination of convection
attempting to re-fire over the surface center and the large weakness in ridging
at all levels north of Emily.

If Emily’s center remains completely expose or opens up, it
may continue to track farther south than model guidance. In fact, model
guidance indicates Emily should be turning more northerly right now; however
this is not the case as Emily is still moving slightly north of due west.

Given that Emily is being steered by the southwestern
periphery of a large subtropical high pressure near the Azores it is almost
inevitable that a turn more towards the west-northwest will begin soon, however
it is not happening as fast as the most recent modeling suggests, so will have
a track that lies just to the left of the model consensus in the near term.

This takes Emily over or near the eastern tip of Hispaniola
and over far eastern Cuba.

After this point, the track forecast is extremely uncertain.

The 0z GFS run shows low level ridging extending west all
the way to Texas, but shows the heart of the ridging off the east coast.

The GFS also shows significantly stronger mid to upper level
ridging over the southern US than any previous runs of most models. There is a
weakness between the two ridges, but it is not very strong, and the ridge to
the west of Emily will control her steering somewhat due to its size. If Emily
is a deep system it may run off through the weakness between the two ridges and
stay just off the Florida coast as most 0z ATCF models show, however the 12z
ECM, CMC, UKMET and now 0z GFS show Emily’s surface reflection being steered by
the low level steering and show either a brush or direct “landfall” in Florida.
Given I am now expecting Emily to possibly become an open wave while it passes
over Cuba, will side with the westerly lying models, as most of the 0z ATCF
models seemed to strengthen Emily faster than will likely happen after land
interaction and seem to follow the track of the mid level center.

Either way by day 3, the global models show the next shortwave
in the zonal flow over the northern US flattening the ridging over the Plains
and allowing the ridge over the Atlantic to control Emily’s steering much more,
which should result in a curve north by that point. This is shown by all global
models, including the ones that dissipate Emily in the near term, so the track
forecast will reflect this.

It should be noted that while outliers, the whole BAM model
suite shows Emily getting well into the Gulf of Mexico which is a small
possibility should the central US ridging continue to be stronger than modeled.

It should also be noted that if Emily is a weak system that
it may be absorbed by the shortwave trough moving over the eastern US in 5-6
days, so will show Emily as an absorbed cyclone by hour 144.

Forecast track:

Intensity:

12 hours: 45 kt

24 hours: 40 kt

36 hours: 30 kt

48 hours: 30 kt

72 hours: 35 kt

96 hours: 35 kt

120 hours: 50 kt…being absorbed

144 hours: absorbed

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