Talking the Tropics With Mike: Florence is an increasing threat to the U.S. east coast

Helene, tropical depression #9 far E. Atlantic

Talking the Tropics With Mike: Florence is an increasing threat to the U.S. east coast

LOCAL IMPACTS: No direct impacts from the tropics through the upcoming weekend though there will be an increasing easterly swell at our beaches from distant Florence...

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Everyone up & down the east coast of the U.S. needs to stay up to date on the latest forecasts... & should consider spending the weekend working on storm preps......

Though slowly weakening, the notable "spin" between Florida & the Bahamas persists in satellite imagery.  This feature is a tropical upper tropospheric trough or "TUTT" - not the king or old SNL skits but rather an upper level disturbance that will enhance showers & storms over Florida into the weekend.  Once in a while, these kind of features can eventually lead to surface development (low pressure), but such is not expected in this instance.  However, we will need to monitor a cluster of t'storm activity southwest of Bermuda & far to the northeast of Jacksonville that's been rather persistent.

As for "Florence" which became the 3rd hurricane of the '18 Atlantic season Tue.... went "major" (Cat. 3 then briefly Cat. 4) Wed. ...  fell apart in the face of shear & dry air Thu. but continues over the Central Atlantic. Such a rapid weakening that includes the decoupling of the mid & low level centers - due to shear - will likely translate into slower intensification than is indicated in forecasts. The more shallow, poorly organized tropical cyclone will move almost due west steered by the trade winds until & unless there is deepening/re-intensification at which time the trend should be more west/northwest or even fully northwest.

It is becoming more clear that a hit on the U.S. east coast is quite likely.  While a full recurve - or a direct hit on the U.S. east coast - is not a sure thing, don't get too caught up in individual run output of any particular model.  We've seen huge swings on virtually all the models with where Florence might be next week. Bottom line at this point: East Coast needs to stay alert & up to date & one should always be prepared for what mother nature might have in store.

It's become pretty obvious that the next upper level trough / surface cold front this weekend/early next week will leave Florence behind..  Much will hinge on how strong - or how weak - high pressure will be across the N. Atlantic following the trough that will miss Florence.  All indications are that the high pressure will build in strongly - much has been the case in recent weeks.

The GFS & European models have recently come into better agreement which, quite frankly, is meaningless this far out (week or so) since there have been & will continue to be potentially major swings.  The critical point - the fork in the road - will be early next week when Florence is south & southwest of Bermuda with either an opportunity for a tight recurve just east of the U.S. OR a continued move westward that would hit the U.S.  I continue to favor a scenario that takes Florence north of Jacksonville at this time, but that's not to imply such a forecast is set in stone or that the forecast can't change. The trend, afterall, has been to shift west. STAY UP TO DATE!  The UKMET model has generally been farther south & west than either the European or GFS models but has trended a little more north recently.  However, all of the models are doing some flip-flopping & realize - as I've always said - they are just models.

I'm not ready to fully bite on any specific scenarios. I will say that the tendency the latter part of this season has been for a pretty strong upper level ridge + surface high pressure from the upper U.S. east coast eastward into the N. Atlantic.  If this pattern remains in place or even strengthens then Florence will not likely have the escape route to the north.

Now that the weekend is here, it's a good time to review hurricane plans, kits & do some work on & around the house before severe weather moves in (cleaning storm drains, gutters, etc.).  It seems that any direct impact on the U.S. will be at the middle to end of the week - from approximately Sept. 12-16th or so.  However, easterly swells emanating west from the hurricane will start arriving at Jacksonville & other east coast beaches later in the weekend into next week which will coincide with an increasing dangerous rip current risk.

Bottom line: Don't panic but always be prepared.....

On satellite imagery.... it's clear Florence is a shadow of its former self (so powerful Wed.)..... for now:

The map below is a mirror of the above spaghetti plots but shows an ensemble of all the model runs. DEFINITION OF ENSEMBLE MODELING: A set of forecasts that present the range of future weather possibilities. Multiple simulations are run, each with a slight variation of its initial conditions and with slightly perturbed weather models. These variations represent the inevitable uncertainty in the initial conditions and approximations in the models. They produce a range of possible weather conditions.

It's worth noting that there are model runs all the way to the U.S. and there are model runs showing recurvature.  BUT there has been a definite trend west the last several days.  At the moment, a combination of model runs is just about split 50/50 on whether or not Florence makes it all the way to the U.S.

The Saharan dust (courtesy CIMMS) imagery shows just how dry the air is over the Central & Eastern Atlantic making Florence's rapid strengthening Wed. all the more remarkable.  Some of that dry air has been ingested by Florence, & it'll take a while to "shake" that out not to mention the obvious strong shear out of the southwest that is blowing the upper level cloud canopy to the northeast.  It will probably take a while for the low & mid level center to re-align but once that happens, restrengthening may be pretty rapid.  Note the two following tropical cyclones will be up against the dry air as the systems move west & northwestward.

As a whole.... the well advertised Atlantic changes - regarding potential storm development, a mini hyper season of sorts - are underway & should continue into at least the middle of Sept. The uptick in activity coincides with a very persistent pattern of surface & upper level high pressure anchored over or near the N. Atlantic which implies lower pressures to the south (the ol' what goes up, must come down postulate).  The upper level map below from the GFS model for Mon., Sept. 10th shows the upper level troughs far to the north as Florence tries to move west or west/northwest underneath.  The orientation, position & strength of high pressure ridges will help determine how far west Florence & other tropical systems might go.  Combine this set-up with the approach of the peak of the season, less shear overall, warm ocean temps., lots of moisture & a MJO pulse, & you get an active period in the tropics/Atlantic Basin.

Wide view of the active Atlantic Basin:

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Tropical storm Helene & tropical depression #9 are over the far Eastern Atlantic.  T.D. #9 - soon to be "Isaac" -  will move farther west over the Atlantic as high pressure tries to assert itself across parts of the Central/Northern Atlantic & underneath Florence.  Just how strong the Bermuda high is - or is not - will go a long ways in determining whether or not #9 makes it into the Caribbean though such a track seems to be likely with the question being intensity. Helene looks like a full recurve over the Central/ E. Atlantic.

I'm also keeping an eye on the NW Caribbean &/or Gulf of Mexico for possible longer range tropical development.

The map below shows - in simplest terms - rising (green) & sinking (brown) air (related to the MJO).  The green areas - in the tropics - are more favorable for tropical development & this kind of pattern is likely to overspread the Atlantic Basin in the coming weeks.

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Florence followed by t.d. #9 & Helene.  T.D. #9 will likely take a much farther south track while moving just about due west... while Helene should turn rather sharply northward in time....

Tropical depression #9:

Helene:

Mid & upper level wind shear (enemy of tropical cyclones) analysis (CIMMS). The red lines indicate strong shear......

The Atlantic Basin....

Caribbean:

Gulf of Mexico:

Water vapor imagery (dark blue indicates dry air) - notice the dry air right up against Florence:

Deep oceanic heat content continues to increase over the Caribbean & Gulf of Mexico as one would expect now that we're in August.....

Sea surface temp. anomalies show a general recent warming over a good portion of the Atlantic Basin including rather dramatic warming east of the Caribbean & near the NE coast of S. America ....

SE U.S. surface map:

Surface analysis centered on the tropical Atlantic:

Surface analysis of the Gulf:

Caribbean:

The E. Pacific remains a busy basin.  Weakening Norman is turning north which will keep the tropical cyclone far enough north & northeast of Hawaii to only result in some sea & surf impacts on the north facing beaches... while Olivia moves westward & will be near or over Hawaii - likely weakening some - by the middle of next week.

Norman:

Olivia: