"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".

Sunday, September 25, 2011

Isolated Thunderstorms East/Sct'd West - Some Strong Late

Active Shortwave trough has developed on the SW Corner of Cut off,quasi-stationary upper level cold core low over Western Illinois. The low itself has retrograded to a small degree in 48 hours toward the SW from Lake Michigan. Strong/severe storms possible along that S/W trough. Pressure gradient aloft over Florida is relaxing due to the now even greater distance from Florida, although minute, effective. TUTT low north of the Dominican Republic and High Pressure over the Atlantic producing sinking motion over a layer of shallow , moist air over the Atlantic east of the Gulf Stream. Disturbance, likely enhanced by the Gulf Stream east of Florida is advecting toward North Carolina Coast today
THE BIG PICTURE, SYNOPSIS: The upper level, cold core low over Lake Michigan yesterday (which resulted in a number of waterspouts due to the warmer lake waters and very cold air aloft) has retrograded into West Central Illinois. This depiction was indicated by some of the models yesterday to occur. Meanwhile, a frontal boundary has evolved arcross Arkansas into Southern Illinois, which will gradually wrap into the parent low pressure system tonight and become a part of the occluded boundary further north.

Further South, as a result of the upper low retrograding, the pressure gradient aloft has weakened as if it wasn't weak enough already the past two days, although the overall 'troughing' is digging deeper into the Northern Gulf to the west of the state. Showers have erupted as result over the Gulf and are moving across Coastal SW and West Central Florida at noon time, with additional showers along the east coast. Otherwise, moisture amounts remain high at 2.00+" universally, which is well above the norm for this time of year, as sensed by the muggy conditions outside. Instability is highest just off shore the Cape to JAX with most areas fairly uniform in that department. 

There seems to be a bit of mid-level drying late this morning along the east half of the state north of Lake Okeechobee, possibly due to some subsidence on the backside of the offshore trough.

 As a result, cumulus cloud lines forming along the intracoastal are not gaining as much height as they would otherwise. SW Steering toward the NE seems much stronger in satellite animations on the west side of the state, but dwindles to  nearly neutral in the interior and east side.  Otherwise, conditions would be favorable for waterspouts along the east coast through 2pm, but so far that possibility seems all but naught due to slightly drier layer aloft, as observed by the downdraft CAPE portrayed on  the SPC mesoanalysis page selection for that parameter.

Latest Mesoscale Analysis from the SPC website shows Downdraft CAPE on the east side of the state. This parameter is a good indicator for strong winds in thunderstorms in downdrafts, but is also an indicator depending on other parameters that storms might have a hard time getting started without a trigger, of which in the case of today there does not appear to be one other than lake and sea breeze mergers. 
TODAY: Activity should have no problem initiating as already observed from SE Florida and up the west coast. However, the further North and East one goes, so does the steering currents weaken to almost zero in the area of downdraft Cape. That factor, combined with the presence of some drier air aloft should inhibit thunderstorm (showers still possible though along the sea breeze) development through early-mid afternoon along the east coast except perhaps toward Lake Okeechobee/Martin/St. Lucie/Interior Palm Beach Counties. Showers are already becoming scattered South, but thunder remains isolated,  and with temperatures aloft no different than yesterday expect that persistence should rule the roost today. 

Thus, like the past two days, despite forecasts of model guidance and numerical guidance for scattered thunderstorms, that has yet to be the case. Thus, persistence seems the most appropriate approach until proven otherwise. 

It should be noted that the Mesoscale analysis page for parameters aloft is only as good as atmospheric soundings taken every 12 hours across the state at TPA/MIA/XMR  (KSC)/TLH/JAX/and EVW (Key West). Otherwise, the parameters portrayed are based largely on, you guessed it, a Model (the RUC). Thus, any inconsistencies with the RUC model are reflected in graphics such as that above. I have seen values vary considerably over a 3 hour window of time, but as of early this morning the downdraft CAPE has done nothing but that seems to be the trend for the first 1/2 of the day at least. Note how all the showers on this radar over lay (above) are occurring where there is no down draft CAPE.

THIS AFTERNOON: Sea breezes will set up along both coasts. The east coast sea breeze combined with a lake breeze (Lake Okeechobee) could trigger a strong storm or series of propagating storms either to the north or south toward the East Side of the Lake with lightning and wind gusts being the main danger factors, although a funnel cloud or even landspout could occur with any of that activity as well.

Elsewhere, showers/Storms will form over the SW/W side of the state along the incoming west coast sea breeze. Showers and a few thunderstorms could form anywhere over South Florida and move very little. Again, propagation along the sea breeze and lake breeze boundaries, as well as shower/storm outflow boundaries, will dictate the erratic storm motions in South Florida.

Further north, any activity along Lake Okeechobee could send outflow northward in the very light steering which nearly parallels the coast today, which too can work north with time along the east coast sea breeze. 

However, by that time the sea breeze could be well west of I95 north of Vero Beach or Ft. Pierce. There is one big question though concerning this one tiny but important factor. 

That being, SW to NE steering might increase later today as the shortwave over Arkansas digs deeper toward Northern Louisiana. Granted , that is a long way away, but there is a domino affect which could translate east and south with time. As of now, it appears that will be the case on Monday with all things else remaining the same with the upper low still in place in generally the same location...including moisture over Florida...storms might be more active along the East Coast sea breeze. It is worth noting though with cut off lows, the models as has been cited time and time again, inevitably try to move them out too quickly. 

Thus, Monday/Tuesday's forecast despite what morning guidance reads (which has not all come out yet), could easily be incorrect when it comes down to the finer, more critical factors in the rain bucket department. Thus, I didn't bother waiting for it to become available, it has proven worthless.

Best Guess at this point today. Realizing for most of these early in the day forecasts one has to second guess the model guidance and remain one step ahead of what they actually show, one must project and a bit of a 'means of experience'.  Afterall, ccording to the RUC,   showers  would already be in progress around the Space Center. Not going to happen any time soon...with nearly clear skies at noon.

Showers/storms/and outflows should spread west and north with time, eventually meeting up with the east coast sea breeze west of I-95 (if not further west) after 3pm, and perhaps much later.

The other problem at this time of year is that during peak heating some of the low level moisture mixes out in the absence of any convective inhibition which is the case right now. Thus, the storms do not really 'jack up' until the sun gets low in the sky and dewpoint depressions can lower, or rather, the ambient outside air temperature can lower to meet the dew points. Thus, the most strong activity outside of near Lake Okeechobee and maybe a storm or two over South Florida will be delayed until after 4-5pm...perhaps later. 

Outflows both from the west coast and up the east coast (toward extreme SW Brevard and Osceola County) will possibly foster more outflows to drive the east coast sea breeze further yet inland.

Peak of the strongest activity should be toward Northern Interior as was the case the past 3 days (there's that word 'persistence' again), in or near locations since as the Ocala National Forest where dew-point depressions recover the quickest due to all the vegetation there), and toward Gainesville. Upper level steering should be a bit stronger further north as well, so although not expecting a big coverage of thunder, it could be strong.

The least  chance of seeing rain today will be far East Central, however, with the steering potentially increasing very late today into tonight  combined with remaining boundaries around ..with the factor that the east coast sea breeze will be easing off and and dissolving , some showers/inland storm debris light rains/or even a thunderstorm could impact the near coastal areas near Rockledge/Titusville/Palm Bay and north toward DAB near or after dark through 10-11pm. Very isolated.

TUESDAY: This now seems to be the better day for the East Coast anywhere to receive rain. But then again, that was what the case was previously thought to be for yesterday and today.

BEYOND: High pressure is thought to build eastward behind the shortwave trough toward Florida, putting much of the state in sinking, more northerly component steering ahead of it, and also squeezing out atmospheric moisture, thus lower rain chances. We will have to see if this actually comes to fruition though, since much will depend on the unreliable motion of the already noted hard to forecast Cold Core Cut Off Low over the upper Midwest. The trend has been for decreasing rain chances through Friday after Monday, with little change in temperatures, although inland highs might not be quite as warm. I'm hedging toward keeping better rain chances for Tuesday at this point.

It appears that either solely South Florida or there and South Central will have the best chances of rain by next weekend, mostly showers with a deep , persistent onshore flow, thunder toward the west side.

There are no tropical threats with the two storms well out in the Atlantic. There is a trough now half way between the U.S. and Africa which could pick up Phillip, and Ophelia is having a  hard time with the TUTT low and high pressure north and ahead of it, which is creating shear winds against that system.    

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