DocNDswamp's WunderBlog

Wx Obs... Tropical Musings...

By: DocNDswamp, 8:03 PM GMT on June 10, 2007

Latest W Atlantic Basin AVN / IR Satellite Image

W Atlantic Basin AVN Satellite Image

(Note: Additional updates, info and thoughts may be included in comments section... as I often do...)

Sun June 10 2007 2:30 PM CDT:
SE LA: May observations from Bayou Cane / Houma... June heating up!

LOL, June has arrived... along with rising temps and humidity... and heat advisories for SE LA as we go into the 2nd week with high pressure in control aloft and at the surface - hit 94 locally Sat June 9, our warmest yet and may top that next few days... Yep, cool, dry air has retreated to the northlands on schedule, so no imminent ice age appears on the horizon (!) ... In review, May 07 turned out fairly typical in this locale... perhaps the only oddity being the strong ELY winds last two weeks of the month after a relatively calm spring period... And I suspect the averaged humidity / dewpoint values were lower than normal... Temperatures for the month averaged cooler than climo normals from both sources I use... Compared to the NCDC avg of 75.8, the official HUML1 station registered a mean temperature of 74.7, or 1.1 cooler... and nearby WxBug stations averaged 73.9, 1.9 degrees cooler... Had a few days reach into the low 90's (12th-14th) but offset by cool near-record Canadian airmass (17th-21st) and a number of cloudy rain days... Meanwhile, parts of W and N LA averaged 1 degree above avg, so statewide was right at normal... But for the last 17 days in a row, most of SE LA failed to reach it's avg high temp... So far, only March has been above norm temp-wise this year... Probably wishful thinking, but nice if that would continue as we head into summer's hot months - fat chance! ... Rainfall here? LOL, well this went exactly as expected... After watching nearby areas getting deluged off/on thru the month while my gauges tallied a meager 1.75" thru the 3rd week, the last 4 days of May brought respectable rainfall spawned by that large, slow moving mid-upper trof over TX / LA and N Gulf, combined with persistent high off E coast brought in plentiful supply of deep moisture from the Carib and E / S Cen Gulf (and nearly 2 weeks of steady 10-20 mph ELY winds)... Totaled 5.62" rain, a .27" surplus over NCDC avg of 5.35"... bringing Year To Date total of 21.68", but still 3.11" below YTD avg of 24.79" thru May... Rainfall in LA was generally above norm in S LA - with some locations receiving 10" to 12" amts - while below avg in N LA... Yet, a lot of contrast with the hit / miss nature - official HUML1 station (10 miles WNW) had 4.53", a .82" deficit... private station in Gray (6 miles NNE) had 9.1"...

***********

Tropical Observations / Analysis: Nice start - TS Barry, our second Atlantic tropical cyclone on June 1st!


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Now wasn't that right on cue? ...After a week of speculation on possible development in the W Caribbean / S Gulf (including the SW Carib low that croaked inhaling dry air on May 27), a well-defined low level circulation formed in the S Gulf on Friday June 1st along that ever-persistent surface trof to become Tropical Storm Barry... Thanks to having the Hurricane Hunter Recon flight mission over the disturbance at the optimal time, their data confirmed what was easily seen on visible satellite - a 1005 mb low had closed off and deepened to 1000 mb from late morn to mid aftn... Flying at 1000 ft, they found a non-frontal, shallow, warm core surface low with a near 2C temp differential...or 75F within the center, 72F outside of it... and surface winds up to 45 kts... Despite being in a high shear environment from mid to upper level winds which displaced convection to the N and E of center, the recon, sat and buoy data warranted upgrading the disturbance to Tropical Storm Barry by the NHC with sustained average 45 mph winds... and a subsequent flight found BP down to 997 mb and supporting 50 mph winds... This was the cyclone's peak intensity and not surprising given the hostile upper environment TS Barry was on the edge of, and quickly moving into...

A quick side note: Guidance from most global models was overall good on expectations, with med-long range from ECMWF and GFS indicating large plume of tropical moisture was gonna migrate from W Caribbean / heading toward Florida in that time frame since over 10 days out, although not much TC development was expected - close to what we got... The CMC / GEM model did quite well, locking in on the surface low's formation and general track by 72-96 hrs out, after it's usual initial overblown scenarios - first as a stronger hurricane crossing the tip of S FL, then as a strong TS going into the FL Panhandle... The N Hemisphere's upper flow pattern into the lower latitudes / interaction with surface features in the spring - summer transition period can be difficult to resolve, leading to forecast uncertainties... meaning, fuzzy guidance from the models on specifics past 3-5 days... LOL, I'll probably be saying the same thing in August!

Subtropical Storm Barry? ...and a couple comparisons.

IMHO... no... I think the NHC got this one right... (now watch 'em change it later as Dr Jeff has suggested, LOL)... Too much of the data supports a warm core tropical cyclone, despite the lack of convection wrapping around it's LLC... This is exactly what one would expect in a high shear environment and why lot of meteorologists and global models did not think a tropical cyclone would form in this instance... TS Barry did not form beneath a transitioning cold core mid-upper low as we often see, such as STS Andrea - the influential mid-upper low / trough was positioned well away to Barry's NW below the S LA coast at that time, IMHO other than introducing zonal SWLY shearing winds and drier air to the system's west, I can see no evidence of coupling of the features until the extratropical transition took place over 24 hrs later... Certainly we have seen many examples in the past of weak, warm core tropical cyclones having their convection sheared away from the LLC by strong mid level winds, such as Tropical Storm Chris '06... and perhaps a better example - TS Hermine '98, which was maintained for 2 days as a TD with it's convection displaced to the east / NE (similarly as TS Barry by a mid-up low to it's west)... and soon fell apart upon being upgraded to a Tropical Storm...


Image of TS Hermine near 28.5N 91.2W on Sept 19 1998 2115Z from NRL Monterey's Storm Archive...

TS Hermine has to rank as one of the most pathetic tropical cyclones ever, LOL, as it passed directly over my SE LA location with observed winds up to... 12-15 mph! Truly a ghost swirl by then, as it had quickly decayed... In contrast, TS Barry did produce TS force wind gusts as it's center came ashore in W Cen FL, with 2 Clearwater Beach stations reporting peaks of 46 and 54.8 mph and MacDill AFB in Tampa with a 44.7 mph gust along with that hvy convection that fired early morning pre-landfall... Highest winds in distant rain band squalls away from center of circulation were either similar or less, not significantly stronger as often seen in subtropical storms... Regardless, Gulf lows and TC's of all types often exhibit strongest squalls / highest winds far away from it's low pressure vortex and designating storms within a specific classification is highly subjective ... One of the strangest I remember was - same year - 1998's Tropical Storm Frances... Oil platforms in the Gulf of Mexico just below Louisiana's coast reported winds sustained over 75 mph gusting from near 90 to 105 mph while over 200 miles distant from Frances' 60 mph core low off the S TX coast! ...Although it seemed a subtropical storm of the greatest magnitude, this storm is described by the NHC and Monthly Global Tropical Cyclone Summary author Gary Padgett as "a rare Atlantic Basin example of a monsoon depression", a sprawling tropical cyclone with a very broad, ill-defined low pressure center having relatively lower winds and often with multiple or transient, reforming low level vortices, while surrounded by outer bands of higher gales... Monsoon Depressions are common in the W Pacific / Indian Ocean Basins, serving as the initial basis of stronger, consolidated tropical cyclones... TS Allison of 2001 was another difficult animal to class, with the NHC defining it as both a TS / TD and later, a subtropical depression / STS... Once again, this a tough call and I may have to disagree with the NHC's assessment on when Allison was a TS / STS as it appeared to be more subtropical in it's early-mid life when over Texas, then drifted SWD back into the Gulf and faded... while a new low pressure center formed below S Cen LA off the E tip of Marsh Island, LA by midday on June 10th and moved ENEWD into SE LA as what I think was truly a tropical storm, not subtropical... I also had the distinct "pleasure" at the time of driving back from E TX / NW LA late that aftn and right on a collision course into that newly-formed low center and the blinding heavy convection wrapping around it... causing horrific driving conditions on last leg of trip from near Franklin to Houma... TS Allison, Part 2 continued strengthening over SE LA that night, I noted a S to WNW to NE windshift as the low center passed nearby and off to my east... LOL, as I've digressed again as usual... We'll see what they decide later on Barry... ;-)

Also of interest - Invest 92L on May 31 2007: During TS Barry's formative stages, a large T-storm cluster just to the northwest of the Yucatan, rotating southwestward toward the Bay of Campeche in the cyclonic circulation of the developing low, entrained mid-level dry air layer coming from the west causing the thunderstorms to collapse, sending out a massive arc cloud... or low level thunderstorm outflow (LTO) boundary, as seen in this image sequence on the morning of May 31...







As we learned from Hurricane Research Divisions' Jason Dunion last year during Hurricane Florence's slow cyclogenesis and struggle with dry air entraining, such events indicate that mid-level dry layer's presence and a hindrance to TC development... LOL, I guess similar to the SAL, we've seen a MAL (Mexican Air Layer) in the early season being advected into the E PAC and W Gulf / W Caribbean... as this dry air from the Pacific and Mexico - pulled EWD / SWD across by the same Mid-ULL trof that was over the NW Gulf then - hindered development of the potential low off Nicaragua the week prior, TS Barbara in the E PAC and TS Barry... However, unlike the SAL, that MAL effect has been temporary and covering a much smaller region...

*****

Looking ahead - A few possibilities in the near future and it's a familiar theme... We'll have to watch for chance of tropical / subtropical low developing beneath a large ULL / upper trof off the SE Atlantic states coast later this week... as several global model runs indicate we could have another Andrea type system... Caribbean will stay in the spotlight as a potential main development region here in the early half of season... Current large Mid-ULL near tip S FL / NW Caribbean helping to enhance convection and pull a disturbance NWD from SW / Cen Carib that is bringing hvy rain to Jamaica... May have to watch this - if ULL keeps moving WWD and/or weakens could allow favorable conditions for development in W Carib as anticyclonic upper ridging bulges over the disturbance with low shear... Very healthy African tropical wave for the early season spinning SW of Cape Verde Is likely to encounter high shear, cooler SST's and dry air next couple days as it tracks W/WNW... Will have to monitor wave once it crosses ocean if holds together, as these features can often find more favorable conditions in the Atlantic W Basin region... Elsewhere - in the EPAC, another wave / low taking shape in same area Alvin formed... should remain away from coast, tracking to the W as it looks now... EPAC remains a typical climatologically favorable region for more development ATT...

Doc's 2007 Atlantic Tropical Season Outlook:
12 - 15 Named Storms...
7 - 8 Hurricanes...
4 Major Hurricanes...


...and like others, I reserve the right to alter this outlook as the wind blows... ;-)

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