Atlantic/2018/02L/Archive/4

Tropical Storm Beryl Discussion Number 4

Wiki-Hurricanes Forecast Center

11:00 PM AST Thu Jul 5 2018

Since the previous advisory, Beryl's core convective activity has waxed and waned with the approach and passage of dusk. The small eye observed earlier disappeared under a transitioning convective structure. The convection associated with the eyewall around the detected eye collapsed somewhat during dusk, though whether this was due to subsidence or diurnal minimum is a bit unclear. Most likely it was a combination of both factors as some dry air was observed wrapping around the periphery of the small CDO and injecting itself into the northeast. Convection has regrown though since nightfall, and we've again returned to the small dense cloud cover we've come to associate with Beryl these past 24 or so hours. A central overcast of cloud tops cooler than -60°C now covers the miniscule circulation. Assessing Beryl's current intensity continues to be complicated as microscale tropical cyclone analysis is still somewhat of a mystery. Using conventional infrared Dvorak analyses, SAB estimated T3.5/55kt while TAFB came a bit behind at T3.0/45kt. UW-CIMSS ADT is estimating T2.3/33kt but given that we've already seen a complete eyewall from Beryl today, it seems safe to discard this estimate. For this advisory, I have opted to initialize Beryl at a very uncertain 50 kt, blending SAB and TAFB.

Beryl's westward motion continues tonight as the Bermuda-Azores subtropical ridge maintains the 20-30kt tropical easterly conveyor belt across the Atlantic Main Development Region. It appears that Beryl has finally gained some latitude after a whole day of subverting statistical model guidance by maintaining a westerly heading as opposed to a west-northwesterly one. Slow northerly gains from the Coriolis effect will continue to occur over time as Beryl spins away from its intertropical convergence zone nursery. This slight west-northwesterly inflection may be enhanced by a reshuffling of the subtropical ridge prompted by a passerby trough digging around the Azores in the short-term, drawing Beryl a bit further north in the general direction of the Leeward Islands. The track forecast for this advisory has more or less remained the same as the various steering parameters are largely unchanged.

As my forecasting colleague noted earlier, intensity forecasts for storms like Beryl are liable to large errors because of how unstable they are. I need not discuss too much at length the sheer number of conflicting signals that were brought earlier, but to elucidate on a few things that we're monitoring this evening: small systems such as Beryl are heavily are highly influenced by diurnal processes. Convective diurnal maxima and minima, which occur in the twilight hours of dawn and dusk, respectively, may also cause the system's intensity to fluctuate noticeably in comparison to its larger counterparts which, if impacted by diurnal instability, would primarily be steady-state. Broad anticyclonic flow continues over the small circulation, dragged over by the associated mid-level trough which has incubated and protected Beryl since it emerged off the coast of Africa. Beryl has been over some cooler waters below 26°C, but has managed to fire off convection just as well as it did over warmer waters as upper-level temperatures have been quite cold, allowing for unstable vertical conditions. In fact, the latest SHIPS analysis showed an 80.2°C temperature differential between the sea surface and the 200 hPa level, with cold air at the upper-levels being tapped from subtropical Africa.

SHIPS and LGEM show rather smooth sailing for Beryl in the next 24 hours, which is the window of opportunity for Beryl to intensify. Relative humidity, sea surface temperatures, and wind shear should all be at least favorable out to 24 hours after which TUTT-related wind shear begins to cycle dry air and unfavorable winds over the system, blowing the circulation apart over time. The current forecast shows strengthening out to 24 hours before weakening begins to set in, eventually degenerating the system. Even as a weakening system, winds associated with the low-pressure area should still be fairly strong given the fast tradewinds in the tropics.

FORECAST POSITIONS AND MAX WINDS

INIT 06/0300Z 50 KT 60 MPH

12H 06/1200Z 55 KT 65 MPH

24H 06/0000Z 65 KT 75 MPH

36H 07/1200Z 60 KT 70 MPH

48H 07/0000Z 50 KT 60 MPH

72H 08/0000Z 40 KT 50 MPH

96H 09/0000Z ... DISSIPATED