It has been a strange 24 hours in the Tropics, with the full spectrum of forecast uncertainty on display.
Yet as we roll into the second half of the weekend, Tropical Storms Laura and Marco remain on course to potentially strike the U.S. Gulf Coast just one or two days apart, filling our eyes with that double vision just as Foreigner warned us long ago.
Tropical Storm Marco has already crossed into the far southern Gulf of Mexico as of Saturday afternoon, exceeding intensity expectations as it does. As of 5 p.m., sustained winds are around 65 mph, and Marco could become a Category 1 hurricane by Sunday.
Marco has been continuously jumping well east and outside of the NHC forecast cone for the last 36 hours, tracking much more northward than the anticipated northwest motion. Both the track and intensity surprises are due to Marco’s complex interaction with a trough of upper-level low pressure over the western Gulf, which has been positioned to favorably ventilate strong convection near diminutive Marco’s core.
Trough interaction is a double-edged sword, and today’s outflow may be tomorrow’s 30 knots of unfavorable wind shear. Small storms’ intensities take the elevator up and down, strengthening or weakening rapidly in response to environmental changes. While loss of organization is possible if shear disrupts Marco’s small core, the longer it moves mostly northward, the longer it can remain in a pocket of supportive upper-level divergence.
The NHC’s Saturday 5 p.m. forecast has shifted significantly eastward, but continues to show a northwest turn on Sunday ahead of Monday landfall in eastern Louisiana. While this is certainly a possibility, do not be surprised by additional shifts east in the forecast track.
Central Louisiana east to the western Florida Panhandle should be prepared for a high-end tropical storm or Category 1 hurricane hit on Monday, and Hurricane Watches have been issued for the Alabama, Mississippi, and eastern Louisiana Gulf Coasts.
Outer bands are likely to pick up along the central Gulf Coast overnight Sunday into Monday, with intermittent rainfall further east into the Big Bend and Florida peninsula primarily associated with the upper-level trough persisting for the next several days.
►Track the storm and see the latest model runs for both storms
The unclear fate of Marco has ramifications for the evolution of Tropical Storm Laura as well.
Laura is departing Puerto Rico and negotiating its Hispaniola crossing currently. While convection has increased in association with Laura in the last day, the storm still appears to be a mess of competing low- and mid-level circulations.
Per the NHC’s Saturday 5 p.m. advisory, sustained winds are around 50 mph, and the broader circulation envelope is moving just north of due west at around 18 mph.
Whether Laura crosses the high mountains of central Hispaniola or rides its north coast early Sunday has significant implications for track and intensity down the line. If the former, as represented by the NHC forecast track, more land interaction with Cuba awaits through Monday, and measured increases to low-end hurricane intensity are more likely on a course towards the west-central Gulf Coast midweek.
If the latter, as consistently hinted by some of the hurricane-specific model guidance, Laura could reach hurricane intensity before crossing the Florida Straits or swiping the Keys on Monday. This would set up the potential for continued intensification in the eastern Gulf on Tuesday, where shear, moisture, and SSTs are all very favorable: a major hurricane remains a possibility in that timeframe.
A gradual turn northwest towards the central Gulf Coast is expected as western Atlantic ridging expands west in two or three days. One note of caution: the implications of a further east track with Marco on this steering ridge have not yet been fully digested by model guidance, and large shifts in forecast track remain possible.
In such circumstances, it is important to remember that the NHC forecast cone is delineated by the radius within which two-thirds of errors in the last five years of forecasts fall. By definition, around one-third of tropical cyclone tracks will be outside the cone. Of course, significant wind, surge, and rainfall impacts occur well beyond its limits.