A strong deep-layer high pressure system over
the south-central United States helped initially steer Erika westward. On 15
August while Erika's forward speed began to slow. Erika began to outpace the
deep-layer high, which had also been moving westward, and early on 16 August
Erika's forward speed slowed further.
Forecast uncertainty at the observing time was focused in one areas: over the
Caribbean Sea, to the south of Cuba. This area was associated with an upper
level (~300mb) cyclone to the south of Cuba and a tropical wave heading towards
Belize.
The ET KF targeting map valid for taking observations on the 15th at 00Z to
improve forecasts on the 17th at 00Z indicates that observations should be taken
far to the south of Erika, in the Caribbean sea. While the deep layer ridge in
the eastern United States was the feature most responsible for the path of
Erika, it was well sampled by the routine observational network, and this was
not considered by the ET KF. The area in the Caribbean sea which the ET KF
selects to observe is associated with the upper-level cyclone near Cuba and a
tropical wave pushing towards Belize. Both these features cause forecast
uncertainty.
The ET KF also indicated sensitive areas over Mexico, associated with a deep
layer cyclone. While this area is not appropriate for dropsonde observations, it
does indicate that perhaps additional surface launched observations would be
beneficial to the forecast of Erika. Perhaps this is an area where the routine
network is not as dense as it is over the United States, but more likely, it is
the influence of this feature on the slowing down and southerly jog of Erika at
the time of landfall.
A last region the ET KF selected to observe was the area in and around Erika, in
the eastern Gulf of Mexico. At the time this ET KF summary map was produced,
Erika was still developing, and likely did not have a strong signal in the
ensemble members. In more developed storms, the storm itself is usually the top
choice of the ET KF
The flight sampled most of the Gulf of Mexico, as well as near the Yucatan
peninsula, near the region of high ensemble spread located south of Cuba. There
was an impact on the analysis to the south of Cuba resulting from these
observations.
