Tropical Storm Cristobal Slowly Intensifying – Very Little Threat to US

(By Steve Gregory - Substituting for Dr. Masters who is on Vacation.)

Edited for latest Fix data, VIS image and Naval FCST Chart - shown in BOLD

Slowly intensifying Tropical STORM Cristobal with MAX sustained surface winds near 45Kts was near 23.2°N / 72.8°W at 18:00Z,or about 320NM E of Nassau, Bahamas now appears to be heading just west of due North (350°) at around 5Kts as it has become caught up in the generally southerly flow just east of the weak, N-S orientated upper level TROF. This TROF off the east coast is located between the sub-tropical High over the central Atlantic to the east, and another sub-tropical high over the Gulf coast states. This synoptic scale environment that we’ve been discussing for the past 2 days, has apparently ‘shown it’s hand’ already – and has GREATLY lowered the chances that CRISTOBAL will have any significant impact on the US mainland.

Additional RECON data also shows a better defined core with a 'thermal eye wall' Temp differential of 4°C - although no eye wall has been reported or seen on imagery. In addition, most significant convection remains to SE and south of the center.


Although wind shear has lowered considerably during the past 18 hours in the immediate vicinity of the vortex center to under 5Kts(!) – normally quite conducive to intensification – the outflow pattern for the storm that was quite symmetrical during the past 2 days has now degraded somewhat, with ‘good’ outflow to the east and far to the south, but poor to fair outflow in the west/northwest quadrant. In addition, there still does not appear to be a well defined jet outflow channel, though this may yet 'connect up' with a fairly strong northerly jet far to the east of the storm. And while dry air is not an immediate problem for the storm, water vapor imagery loops clearly show drier air starting to impinge on the overall circulation field just to the west (near Florida) and further east (northeastern CARIB) of the storm related to a closed upper LOW approaching the NE CARIB. In addition, a weak east-west frontal band along 32N is still sinking very slowly southward, adding yet another potential complication for the intensity forecast, and to a lessor degree, the track forecast. Bottom line, unless / until CRISTOBAL develops a stronger and far better organized core, the areas of drier air will may soon impinge on the storm's inner circulation which will impede development of the storm, or halt it altogether during the next 12-48 hours.

On the ‘plus’ side (depending on your point of view) SST’s are very warm (near 29°C / 86°F) and should remain above 28°C over CRISTOBAL’S projected track for the next 2-3 days - which is quite supportive of intensification, though the very slow movement of the storm may induce some upwelling of cooler sub-surface water, which in turn, would slow the positive impact on convectively driven intensification.

The latest global and specialized hurricane forecast models, while still showing significant spread in forecast track solutions, have all shifted much further to the east (even further eastward then most of the models originally depicted just a couple days ago), with all solutions showing the storm tracking far to the east of the US mainland. The only significant model divergences of note at this time is a clustering of several model solutions that show the storm taking a more North-Northwest track Monday thru Wednesday before finally turning northeastward and then out to sea about 300NM or more east of Cape Hatteras later in the week. This is a reasonable solution and is due to the initial weakening of the TROF now off the east coast, which allows the sub-tropical ridge over the Atlantic to build westward, forcing a more northwesterly track early in the week ahead. But by late Wednesday and Thursday, a mid-latitude TROF now in the PAC NW will be advancing to the Northeastern US, turning the steering flow to the southwest, which will force CRISTOBAL to turn northeastward and head out to sea.

Unlike the track forecasts, the intensity forecasts have remained very consistent over the last several days, with most calling for a very slow intensification to CAT 1 intensity in a couple days. While there is no clear reason to disagree, the above mentioned ‘negatives’ may tilt the odds to keeping CRISTOBAL just below hurricane strength.



Fig 1: Early morning VIS imagery shows a significantly more consolidated and classical looking storm system, with most of the stronger convection (and higher winds) to the east of the storm center.



Fig 2: Water Vapor imagery shows dry air approaching from the west, and to a lessor degree, from the east, associated with an upper Low near the Leeward Islands. Unless CRISTOBAL intensifies soon, the drier air may become entrained in the inner circulation - hindering intensification.



Fig 3: The area of very low winds shear that has consistently been to the S-SW of the developing system for the last 2 days has now orientated itself over the center of the storm with very low values near or even below 5Kts. This is one of the most 'positive' metrics, along with the very warm SST's, for the intensification of the storm.



Fig 4: Outflow associated with a very high level anti-cyclone is quite good in the eastern semi-circle, and may soon be able to 'latch on' to the northerly jet further to the east of the storm. However, outflow is relatively poor to the west and NW. Outflow is a critical factor that determines storm intensity.



Fig 5: The steering level winds clearly show the break in the east-west orientated ridge line that extended from the High pressure center in the central Atlantic to the second center over the SE U.S. The southerly winds ahead of the weak TROF that 'broke through' the ridge are steering the storm poleward. This TROF is expected to weaken on Monday, allowing the ridge to build westward, steering CRISTOBAL on a more Northwestward course for a couple days, before the winds back around to the SW ahead of the next mid-latitude TROF.



Fig 6: Early cycle model runs are in good agreement that CRISTOBAL will NOT significantly impact the US mainland - and confidence in this outlook is now much higher than yesterday.



Fig 7: Intensity forecasts have remained very consistent over the past few days, with slow intensification expected during the next 72 hrs to CAT 1 Hurricane force in about 48-72 hours - though the approaching drier air may prevent or delay this.



Fig 8: Latest NAVY Track/Intensity Forecast for Cristobal


ELSEWHERE in the Tropical Atlantic - a fairly strong disturbance (97L) now S/SW of the Cape Verdes with isolated convection is westbound - but is unlikely to show significant development for at least the next 4 days as the system is embedded within the dry and warm Saharan Air Layer (SAL), By Friday, the system will have a notable chance for development. An even stronger wave now over Africa should emerge off the coast next weekend.

Dr. Masters will return tomorrow with the next full update; but if there is a significant change, I’ll issue another brief update late today.

Note: For those interested , I will once again be providing my own Weather Bog Updates starting tomorrow located HERE



Steve Gregory