Archive for October, 2008|Monthly archive page
THIS CHART IS TIME-SENSITIVE,
AS ARE MOST CHARTS PROVIDED IN THIS WEB-LOG SITE.
Vertical wind shear is defined as the change of the wind (velocity or direction or both) with changes in altitude. Vertical wind shear, particularly in velocity, is a significant factor in the probability for tropical system intensification, or weakening. Here is a general rule of thumb on that subject: The probability of intensification increases when vertical shear is 20 knots or less – and when shear exceeds 20 knots there is a decrease in the probability of intensification.
The graphic that I have provided (above) shows the wind shear forecast for tomorrow afternoon – Saturday, November 1, 2008, Eastern Time. I recommend two single left clicks upon the image to enlarge it adequately. I have marked some belts of high shear and low shear and I have also placed a white arrow on the scale at the bottom of the map showing 20 knots which is about 10.29 meters per second.
It is expected that for the first half of the month high wind shear will protect Florida from storms that might develop over the typical late-season hurricane breeding sites. If this forecast pans out, it is not likely that the Gulf Coast states will see tropical action for that portion of the month. However, storm probabilities, with respect to vertical wind shear, may increase during the second half of the month. Of course there are other factors – e.g. – sea surface temperatures. I will address that soon.
Living in Florida, hurricanes are of great concern to me. Members of my family depend upon me to provide as safe a home as possible. Even though a storm threat seems unlikely for at least the next two weeks, and we are well into the period of steady decline in tropical weather activity, I have no intention of letting my guard down any time soon and this is what I recommend for you if you live in a hurricane-prone region. Some very impressive storms have occurred in November.
NOTE: I have tried to help you get your bearings geographically by marking Florida – not because I felt that you couldn’t find it but because the deep color contrast obscure the geographical outlines. I simply wanted to make it as easy for you as possible in the event your eyes are as bad as mine. LOL
A single left click followed by another single left click will enlarge this image significantly.
Altocumulus lenticularis clouds are not rare but, none-the-less, they inspire countless observers and much “camera clicking” occurs when they are present. Forming mostly over mountainous topography, they generally mark the movement of the air at the altitude where they form. Often (as in this case) they form not immediately above the mountain peak (or peaks) themselves but quite some distance higher. In this case it is forming on a “lee wave.” Rising air is what causes most clouds to develop and this is no exception. Envision water flowing quickly in a stream over a boulder that is on the bottom. Not only does the water touching the boulder rise up and over – so does the water above that lowest layer; and if you look closely you might see at least one other standing wave on the downstream side of the boulder. Likewise, a prevailing wind obstructed by a mountain or mountain range is forced to rise up and over and the air above it does the same. If the air aloft contains enough moisture and the lifting cause enough cooling of that air, condensation (and deposition) can occur forming cloud droplets and ice crystals respectively.
I took this photo today (10-29-2008) from the Blue Ridge Parkway. The clouds are forming on the leeward side of the Black Mountain Range, the famous range most notable for the highest peak in the U.S.A. east of the Mississippi, Mt. Mitchell. I was northeast of Mt. Mitchell at the first viewpoint northeast of the intersection of the Parkway with highway 80. The camera faced south-southeast.
The environment was changing quickly while I took several photos. I intend to post more soon in order to demonstrate how quickly the changes were occurring (which was a function of how rapidly the air was moving up there).
In this region the prevailing winds aloft are from west to east (generally speaking) and the Black Mountains trend north-south. The Black Mt. Crest Trail, rated “difficult” runs northward from the top of Mt. Mitchell (which can be reached by roadway) for about 7 miles to Celo Knob. From near the top of Celo Knob the Bowens Creek Trail heads northwestward another 5 miles to the mountain valley below. My youngest daughter hiked the trail with me when she was 11. It was an experience I’ll never forget – nor will she.
LEFT CLICK IMAGE TO ENLARGE.
The entire post beyond this sentence is taken verbatim from this morning’s weblog by Dr. Jeff Masters’ at http://www.wunderground.com/tropical/.
“One of this hurricane season’s biggest disasters continues to unfold in Central America, where the death toll now stands at 39 from ten days of heavy rains triggered by last week’s Tropical Depression Sixteen and this week’s tropical disturbance 91L. At least 10,000 homes have been destroyed and 250,000 people made homeless by the floods. Hardest hit is Honduras, where 23 are dead and 8 missing in flash floods and landslides. Approximately 60% of the nation’s roads have been damaged, and the flooding is the worst since Hurricane Mitch of 1998 killed 10,000 people there. The past week’s flooding has also killed four in Guatemala, seven in Costa Rica, four in Nicaragua, and four in El Salvador. In Belize, damage is at least $15 million from the floods, and some areas are seeing flooding worse than was experienced during Hurricanes Mitch and Keith. Satellite estimates suggest that up to a foot of rain has fallen over some parts of Central America in the past week. The heavy damage to crops across the region will likely cause severe food shortages in coming months, and substantial international aid will be required.
Rains over the hardest hit areas of Central America have eased in the past day, with only 1-2 inches of rain reported. However, visible satellite loops show that heavy thunderstorm activity continues over the Western Caribbean, and has moved into northeast Honduras and Nicaragua this morning. While there is currently little chance that a tropical cyclone will form in the Western Caribbean over the next five days, persistent low pressure and sporadic heavy rains will continue to affect the region. A strong cold front is expected to push southward into the area next Tuesday or Wednesday, and the tail end of this cold front could serve as the nucleus for a new tropical disturbance that will generate another round of very heavy rains for Honduras and Belize late next week.
Elsewhere in the Tropics, no computer models are forecasting tropical storm development anywhere in the Atlantic over the next seven days.”
IMAGES FROM NOAA AND TEXT ARE TIME SENSITIVE
The image above, released 5:07 PM EDT, today, Oct. 22, is a prognostication for 8:00 PM EDT, Friday, Oct. 24. It predicts moderate rain and thunderstorms along a wide expanse of the Florida Gulf Coast. This anticipated weather will be a combination of the former Invest 91L and typical pre-cold frontal weather. Though the tropical component was responsible for at least 13 flood-related deaths in Honduras, great difficulties in Guatemala, and tens of millions of dollars of damage in Belize – it is not expected to be a threat to Florida. In fact, in-so-far as the National Hurricane Center is concerned, 91L no longer exists.
It appears, then, that the forecast posted in this web-log yesterday is now toned down significantly. However, there is a high probability for precipitation in Florida on Thursday and Friday and wind velocity forecasts that I’ve been able to examine this evening suggest winds above 25 mph at and near the Big Bend of Florida’s Gulf Coast peaking around 8 AM EDT Saturday morning. See image below:
I apologize to my few “regulars” (mostly in Central Florida) who rely on my postings of weather tips. My on-line signal has been very weak and undependable up here in Nature’s Wonderland – the Southern Appalachians. But these cool temperatures (below freezing in the early morning hours and with the beautiful Autumn colors make being without the Internet a trivial inconvenience in the whole scheme of things. Conditions are allowing me to be on line this moment so I want to get this information out to you while I can. Almost all of what I have learned about the tropical disturbance centered slightly north of Honduras has come from a quick check with Dr. Jeff Master’s statement found at Weather Underground. Since I’m having difficulty getting on line, I strongly suggest you go there for updates. Here is a link:
Though the weather system has been deadly in Central America, there does not seem to be cause for great alarm in Florida at this time but heavy rains with fairly strong winds (30-35 mph) might be expected for part of the state on Friday night – probably somewhere between Tampa and the Big Bend. Strong wind shear over the Gulf of Mexico will probably prevent the storm from becoming any stronger – in fact, by the time it reaches Florida it will probably have taken on extratropical (mid-latitude) characteristics. An example of such characteristics is the establishment of a front (or fronts). It will be interesting to see if that happens.
My personal favorite with regard to the models is the GFDL which is shown in blue in the image below. Since it is dated, and since nothing new yet appears today, I’m hoping that something has happened in the interim to further weaken the system.
If you would like to view the GFDL model animation and need instructions on how to do so, the last posting on my tutorials page will tell you how. Here is a link: https://cloudman23.wordpress.com/weather-tutorials/
MY NEW TITLE FOR THE IMAGE ABOVE IS – “WILL HE EVER GET BACK ON LINE?”
This post is for the purpose of letting my “regulars,” mostly friends interested in tropical weather or family members looking in to see what’s happening, know that I am currently dependent upon a very weak wireless signal up here in the mountains. On my retirement budget I can’t justify at this time the expense of a more sophisticated and reliable connection. The signal I get, in addition to being weak, is intermittent. This is the first time I’ve been able to get on line since yesterday morning. So – if there is inactivity, please know that all is O.K. I hope that all is O.K. for you too. My only weather source under these conditions is the weather channel but my ability to communicate with you is temporarily very limited. I hope, within the next few days, to be able to improve the situation.
THE IMAGE ABOVE AND THE TEXT BELOW ARE TIME SENSITIVE
Tropical storm Omar is likely to be hurricane Omar in the morning. Conditions are favorable for intensification. Puerto Rico and the Northern Leeward Islands must be alert and ready. I recommend you use these two links to keep updated on the storm’s progress:
There is “action” in the Atlantic basin today in-so-much as multiple systems are concerned. The Gulf of Mexico and the Caribbean are considered to be a part of the Atlantic Basin. The most immediate concerns reside in Puerto Rico, I should think, due to tropical depression 15 nearby. If it moves northeastward, as predicted, the island will no doubt get precipitation that it doesn’t need considering the pelting it has already taken this season.
Storms do not get named until they “graduate” from tropical depressions to tropical storms. But, there is a tropical depression out there today with a name, Nana. She is “named” because she had reached tropical storm status earlier in her history and she retains that name even though she has now deintensified as was predicted.
That is not uncommon but there has been a very interesting occurrence with Nana. Wind shear earlier on broke her into two distinct parts. The northern segment (Nana) is not likely to survive but the southern segment, Invest 90L, stands a chance of intensifying. Both Invest 90L and Invest 99L are tropical disturbances (also called tropical waves). If you didn’t already know you might have guess that numbers are assigned to tropical disturbances and when under “investigation” they are labeled with the abbreviation, “invest.”
So, there are four systems out there being watched.
1. Tropical disturbance (tropical wave) – An area of organized convection, originating in the tropics and occasionally the subtropics, that maintains its identity for 24 hours or more. It is often the first developmental stage of any subsequent tropical depression, tropical storm, or hurricane.
2. Tropical depression – A tropical cyclone in which the maximum sustained surface winds are 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave which has continued to organize. NOTE FROM CLOUDMAN23 – By convention, it has been the practice of the National Hurricane Center to associate the beginning of “rotation” with the transition from disturbance to depression. In fact, in some definitions rotation is an important element. Generally, this causes no problem because once one or more closed isobars are needed to plot the pressure, rotation has almost always begun.
3. Tropical storm – A tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per hour (63 knots). At this point, the system is given a name to identify and track it.
4. Hurricane (or another name depending upon geographical location – e.g. typhoon) – The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean.
(definitions source is the Weather Channel Glossary found at http://www.weather.com/glossary/a.html
During my long college teaching career I was asked from time to time why such a list was not labeled “Classification of Synoptic-Scale Cyclones of Tropical Origin” instead of Synoptic-Scale Lows. The reasoning is straight-forward. The first member of the quartet, the tropical disturbance, is not cyclonic. In order to be cyclonic there must be rotation. Therefore, though all cyclones are lows, not all lows are cyclones.
THE IMAGE ABOVE AND THE STATEMENT BELOW ARE BOTH TIME SENSITIVE – RELEASED AT 5 PM EDT 10-12-2008 IMAGE AND STATEMENT SOURCE = NATIONAL HURRICANE CENTER.
WTNT44 KNHC 122047
TROPICAL STORM NANA DISCUSSION NUMBER 1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL142008
500 PM EDT SUN OCT 12 2008
ALTHOUGH THE CONVECTIVE ORGANIZATION OF THE AREA OF LOW PRESSURE
LOCATED OVER THE EASTERN ATLANTIC HAS DEGRADED SOME THIS
AFTERNOON…THE SYSTEM HAS MAINTAINED SUFFICIENT CONVECTION FOR A
LONG ENOUGH PERIOD TO CONSIDER IT A TROPICAL CYCLONE. DURING THE
AFTERNOON…THE CENTER OF THE CYCLONE HAS BECOME EXPOSED WELL TO
THE WEST OF THE LARGE AREA OF DEEP CONVECTION DUE TO WESTERLY
SHEAR. AN ASCAT OVERPASS FROM 12Z INDICATED MAXIMUM WINDS OF 30-35
KT. GIVEN THE TYPICAL LOW BIAS OF THIS INSTRUMENT…THE CYCLONE IS
DECLARED A TROPICAL STORM…THE FOURTEENTH OF THE 2008 SEASON. THE
LONG-TERM SURVIVAL OF NANA SEEMS BLEAK AS STRONG UPPER-LEVEL
WESTERLIES ARE FORECAST TO CONTINUE. ALL THE INTENSITY GUIDANCE
SHOWS WEAKENING AND SO DOES THE OFFICIAL FORECAST. THE OFFICIAL
FORECAST SHOWS NANA DEGENERATING INTO A REMNANT LOW IN ABOUT 48
HOURS…BUT IT WOULD NOT BE SURPRISING IF IT OCCURRED SOONER.
NANA HAS BEEN MOVING SLOWLY WEST-NORTHWESTWARD OR 290/6 KT. THE
TRACK GUIDANCE PREDICTS A SLOW WEST-NORTHWESTWARD MOTION AROUND THE
SOUTH SIDE OF A LOW- TO MID-LEVEL RIDGE OVER THE EASTERN ATLANTIC.
THE CYCLONE IS FORECAST TO BE NEAR THE SOUTHWESTERN PERIPHERY OF
THE RIDGE IN A COUPLE OF DAYS AND SHOULD TURN TOWARD THE NORTHWEST
AHEAD OF A DEEP-LAYER TROUGH THAT DEVELOPS OVER THE CENTRAL
ATLANTIC. THE TRACK FORECAST IS CLOSE TO THE MIDDLE OF THE
GUIDANCE ENVELOPE…BUT IS SLOWER THAN THE MODEL CONSENSUS.
FORECAST POSITIONS AND MAX WINDS
INITIAL 12/2100Z 16.4N 37.9W 35 KT
12HR VT 13/0600Z 16.7N 39.0W 30 KT
24HR VT 13/1800Z 17.0N 40.4W 25 KT
36HR VT 14/0600Z 17.4N 41.7W 25 KT
48HR VT 14/1800Z 17.9N 43.0W 25 KT…REMNANT LOW
72HR VT 15/1800Z 19.0N 46.0W 20 KT…REMNANT LOW
96HR VT 16/1800Z…DISSIPATED
IMAGES ARE TIME SENSITIVE!
Norbert has reached the mainland of Mexico after recently ripping though the Baja peninsula. It’s strength is likely to diminish quickly but it will maintain tropical storm intensity for quite some distance into Mexico and could easily effect weather in west Texas and New Mexico early this week. Notice that the storm is now elongated (oval, rather than circular). The elongation is along the axis of wind shear which is depicted in the second image which was completed about two hours earlier than the first image. So, the two images are not a perfect match-up but they are close. It looks as though the storm is likely to continue traveling 40 to 45 degrees east of due north along the direction of winds aloft.