"But seeing they could not See; hearing they could not Hear"
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"From its chamber comes the whirlwind, and cold from the scattering winds." - Job 37:9.

"The wind blows to the south and turns to the north; round and round it goes, ever returning on its course".

Friday, September 30, 2011

Fall Arrives On Cue After Midnight - With a Severe Storm Possible

A Severe Thunderstorm Was Reported near Mims Yesterday with 1" Diameter Hail
Other Lightning Storms Were Subsequently Generated in part due to outflow from this storm in Osceola County toward SE Polk County well after dark  

TODAY: Confessing first off that I've delayed posting today to see how parameters would materialize with some day time heating. For the most part, not many  surprises from earlier. Today's post is based on model trends (although they do not agree at all), SPC mesoscale analysis which updates every 15 minutes (but based in the upper levels on the RUC model), radar and satellite trends, and bit of gut feeling. In short, the final straw drawn was from the Magic Eight Ball app...which read, "A word of the wises, be prepared for surprises". Although, not sure if that meant "Surprise, no rain anywhere except in one spot... or, Go for the Big Rains over South Florida". I chose neither, but sided a bit to the unreliable NAM of summer and latest trends.

NOW: The first cold front of the fall season is moving through the panhandle as we speak, and seems to be draped from near St. Augustine to Gainesville to north of Cedar Key. Elsewhere, a mid-level trough is pulling off the Central East Coast which was responsible for the activity yesterday with a bubble high having filled in behind it and responsible for the nearly cloud free skies central early today with southern branch high clouds and some developing cumulus over South Florida. The vorticity max with the upper level trough is strengthening since sunrise and the upper level trough is aligning with mid-level features as well as surface features which was thought might occur in yesterday's post. 

Heating of the day should slow the boundary down just a bit as the coolest temperature plunge is still a full 24 hours away. Despite unfavorable moisture levels over South Central Florida , we still might be able to squeeze out a storm or two as the vort max crosses Central Florida by mid-afternoon through sunset into South Central Portions.  Vorticity advection is also increasing as the vort lobe is now much less linear than earlier this morning.  Moisture is being drawn up the immediate east coast toward Canaveral and Titusville and might be enough to pull off a shower today if not a storm near or just off shore. It is tempting to say there will be very few showers if any today, and all will hinge on one or two storms over South Central Florida. If so, any such storm could be severe due to 1" or larger hail or strong wind gusts around 58mph. On the otherhand, will there be a storm at all?!! 

Sea breezes today should be quite light and remain close to US1, but I am watching for the west coast sea breeze to start to move eastward toward North Central behind the front itself during the mid-late afternoon, that could be a key and final ingredient if so. Winds aloft are averaging up through the steering level around 15 kts from the WSW. Thus, storm motion will be zero until one that forms can latch on to the upper level winds at which point deviant motion, more likely toward the South or SSW-SSE is possible.
Bulk shear is increasing to 35kts over Central Florida, and upper level temperatures are cold enough to support hail.

This image shows the Vorticity Max and frontal boundary with it across north Florida moving ESE. The blue circle shows where it is advecting toward. No storms are occurring now because it is over much drier air, but as it enters the area of better moisture a storm could suddenly pop up unexpectedly south of the Beachline after 3-4pm. Red shows  preliminary areas thought for a very strong or severe storm. Bear in mind, any storm will be very isolated, so most people will not see rain today, if any one does for that matter.   
     The final ingredient for a severe storm today is the mid-level moisture gradient along eastern portions of South Central Florida ...although I must say that during the past hour some of that moisture seems to be mixing out with daytime heating, thus the very low confidence in storm formation there. All in all, if I had to narrow down the red area even smaller it would be for one lone storm over  Okeechobee or Southern Osceola County advecting toward or forming another in Southern Brevard toward Palm Bay.

Further south, a problem down in this region is the high level clouds. Those didn't seem to stop one lone storm from forming yesterday though in Palm Beach County near the lake/sea breeze boundary. The parameters over South Florida for severe are not as good other than here lies much more  low level instability, thus more showers could form in this area, but again, moisture here is waning with peak heating approaching. Temperatures aloft here were cool this morning per the MIA sounding, but guidance also shows those were from the overnight boundary in place which is pulling out, so those temperatures might be warmer than the sounding indicated here bu later this afternoon  . 

Thus, isolated showers despite the better moisture and with lack of better upper level support such as bulk shear and vorticity which is over Central until late, as well as less dynamic sea breeze/lake breeze interactions. Not sure sea breezes will be as effective today as much either as much as mid-upper level dynamics, and with light Lake Breezes those too might only play a small role in storm or shower formation. If I had to pick 'somewhere' over South Florida it would be near the East Coast from Central Palm Beach County through Southern Dade toward interior Dade/Broward, with a possible storm or two as well in Martin County or St. Lucie. 

For the most part, most people will not receive rain today, but those that get more than 1/4" will likely to so in the form of a very strong or severe storm.

TONIGHT AFTER 10PM: Storms could still form, or a storm could still be in progress, through 9pm in any of the highlight areas in the image above. The cold front proper will arrive like  'fall clockwork', after midnight on October 1st per last year, with frontal passage in East Central between 1AM and 5AM, and southward to South Florida from daybreak through mid-morning. It should progress toward Central Cuba where it could eventually retrograde back north to the northern portion of the Florida Straits by Monday.

Morning lows will be the coolest north and central than in quite some time, with the magic 67F possible along A1A in Brevard but cooler west of the Banana River. Afternoon highs on Saturday over South Florida should reach the mid-80Fs, but even 80F might be a stretch along and north of the Beach Line with increasing NNW-NW winds during the afternoon. The driest and coolest air will arrive Sunday morning and remain in place until day break Monday when morning lows everywhere will be at their coolest.

The only areas that will escape the very driest of air is far eastern South central and South East Florida and around the tip toward south of Naples. This will allow daytime highs to heat up toward 80-85F South Central and South on Sunday after a very cool start, but also will allow a fairly rapid temperature drop shortly near and during sunset. Overnight lows along the east coast should occur in the middle of the night, although one more very cool morning could occur Monday west of US1 before NNE winds set up.

The next chance of showers near the east coast appears to be on Thursday, with some stratocumulus clouds advecting on shore on Wednesday. The only other thing to watch for during this cool down will be high cirrus clouds over South Florida streaming across from the Southern Gulf. If so, daytime high temperatures will not be as warm as the current line of thinking advertises south of Lake Okeechobee.

This second storm, part of a cluster, was observed near St. Cloud yesterday toward sunset. cloud to ground lightning was intense, but not too frequent. There also appeared to be a funnel cloud (upper left) induced by  wind shear outside of the rain foot  visible on the right side of the image

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