Aug 5, 2014

Tropical systems to threaten Hawaii..while Bertha threatens North Atlantic shipping lanes

Trop. Storm Julio
Tropical cyclone activity in the Pacific Ocean continues at above normal levels with the 10th named system, Tropical Storm Julio  located about 1145 miles to the SW of the Southern Tip of the Baja Peninsula. Julio is forecast to move toward the WNW over the next 5 days and slowly intensify to hurricane strength. Winds are currently near 60 mph this early Tuesday morning and are forecast to increase to near 85 mph by Friday morning then weaken to near 75 mph by Sunday morning.

NHC forecast track for Julio
Currently sea surface temperatures are very warm along the forecast path of Julio which has the storm located to the ENE of the Big Island of Hawaii by Sunday morning.

Major Pacific Hurricane Iselle
 Of greater concern for the Hawaiian Islands is MAJOR Hurricane Iselle. Early this morning Iselle was located about 1055 miles to the ESE of Hilo, Hawaii, moving toward the West at 9 mph. Iselle is a Category 3 hurricane (it was a CAT 4 storm yesterday) with maximum winds of 125 mph. Iselle is expected to weaken very slowly over the next 24 hours with a slight turn toward the WNW. It is possible that tropical weather watches may be required for (parts of) the Hawaiian Islands later today or tonight; for sure ALL interests in the Hawaiian Islands are advised to monitor the progress of Iselle. The forecast track for Iselle is fairly straightforward with the system expected to affect the Hawaiian Island during Thursday through Friday. The uncertainty is in how strong will it be when it does impact the Islands' weather.
Forecast track of Iselle

For now the potential for Iselle to be a strong tropical storm (with winds of 55-73mph) when it affects Hawaii is estimated to be about 70%; there is about a 20% chance that it could be a weaker tropical storm and a bout a 10% chance that it could still be a minimal hurricane.

Over in the Atlantic the second named tropical cyclone of the season continues to churn toward the north-northeast and out to sea away from the U.S. mainland  at a speed of near 22 mph. It is also weakening as increasing wind shear aloft continue to push the strongest convection toward the northeast, away from the lower-level circulation. Loop of Tropical Storm Bertha . The forecast track of Bertha calls for acceleration and continued weakening over the next 36 hours. After this time Bertha is expected to become a post-tropical or extratropical cyclone that will rapidly accelerate toward the east or east-northeast and perhaps affect Western Europe (Iberian Peninsula) and/or  the British Isles by Sunday.
Forecast track for Bertha

Aug 1, 2014

Tropical Storm Bertha forms to the east-southeast of Barbados

IR satellite picture of Trop. Storm Bertha

 An area of convection as flared up to the north and northeast of the low pressure circulation over the Atlantic Ocean located to the ESE of the Lesser Antilles. The organization of the system now warrants classification of the disturbance as a tropical storm - in this case  Tropical Storm Bertha. The NHC is now issuing advisories on this system
The storm is moving toward the WNW at 20 mph with maximum winds of near 40 mph. Environmental conditions currently around the storm's circulation and along its forecast path only favor marginal intensification.
Latest position/strength of TS BERTHA

"Spaghetti" model plots
The "spaghetti plots are tightly clustered on WNW track and the official NHC track is pretty much in agreement. Due to the expect WNW track at a rather brisk forward speed Watches and Warnings have been posted for parts of the Lesser Antilles and also across some islands in the Greater Antilles chain.
Official NHC forecast track for Bertha

Tropical Warnings/Watches currently in effect
A TROPICAL STORM WARNING is in effect for:
* Barbados * St. Lucia  * Dominica 
A TROPICAL STORM WATCH is in effect for:
* Puerto Rico * Vieques  * Culebra * U.S. Virgin Islands
* St. Vincent and The Grenadines

Jul 31, 2014

Potpourri Weather Post

Don't be surprised by today's storms

KENX radar
There could be a few thunderstorms today and they might be locally strong or marginally severe. Main threats for today: hail 1/2" to perhaps 1" in diameter, winds gusting 40-50mph, heavy rain and of course DANGEROUS cloud to ground lightning. One small line of storms is moving out of Eastern NY State (with a warning out for Washington and Saratoga counties in NY) to Bennington county in VT. More scattered cells are "popping" up farther to the west.  

31 July 12z Albany, NY Rawindsonde 
The reason for only a marginal threat 
of severe weather for today is that  CAPE (potential instability) is small, wind shear is moderate and lapse rates are weak as indicated by the morning "sounding" from Albany NY (which I modified for the expected maximum temperature and dew point for today [at the surface]). 

CHAP/RI computations for 31 July 2014 for Eastern NY State
Using the data I then "ran" the Convective Hazard Assessment Program or CHAP (also know as Ricks' Index or RI)  to gauge the probability of severe weather. An RI value of < 110 implies NO SEVERE thunderstorms while a value of 160 implies SEVERE thunderstorms are likely. The RI can also be useful in predicting potential hail size, maximum wind from the storms, radar VIL [storms that reach or exceed this value of the VIL could be severe] and rainfall. Today's RI = 116 so hail size could be around 1/2 or so, winds could gust to 40-45 mph. The Probability of Precipitation (PoP) for today is 40% with the Probability of Severe storms (PoSVR) is only 7%. Rainfall of around 8/10 ( 0.80") is possible in storms with MAXIMUM potential rainfall of 2". Storms will tend to move quickly towards the ENE at 30-35 mph, so I would think that the threat for any flash flooding from today's storms is LOW.

Atlantic Tropical Disturbance to the southeast of the Leeward Islands

From the National Hurricane Center
200 PM EDT THU JUL 31 2014

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

1. A Hurricane Hunter aircraft has recently begun investigating the
low pressure system located about 550 miles east of the southern
Windward Islands.  Preliminary reports from the aircraft indicate
that this system is producing winds to near gale force.  However,
satellite images indicate that the associated showers and
thunderstorms have diminished significantly since this morning.
Therefore the low currently does not meet the criteria to designate
it as a tropical cyclone.  Showers and thunderstorms could
redevelop later today or tonight, which could result in tropical
cyclone formation.  Interests in the Lesser Antilles should continue
to monitor the progress of this disturbance as it moves
west-northwestward at 15 to 20 mph, and watches or warnings may be
required for some of these islands later today or tonight.
* Formation chance through 48 hours...high...70 percent.
* Formation chance through 5 days...high...70 percent.

Forecaster Pasch

Visible Satellite picture of tropical disturbance 93L

Model forecast tracks for tropical disturbance 93L

Jul 20, 2014

Derecho expected for the Northern Plains and Western Great Lakes

This upcoming week will feature some significant and widespread heat and humidity from the east facing slopes of the Rockies to the East Coast. The weather setup for this week  features a sprawling high pressure area surface and aloft , acting like a heat pump, extending from the Southwest States to the Great Lakes.

The image to the left is the 500 hPa level (approximately 18,000 feet above the surface of the earth) from Sunday morning 20 July 2014  8AM EDT. An upper air chart such as this is called a constant pressure chart; on the chart the pressure is 500 hPa but the 500 hPa pressure is "located" at a different "height" above ground.) Think of upper-air maps as a topographic map of the atmosphere. The solid black lines are the actual heights of the 500 hPa and also can serve or approximate the the direction of the the winds aloft at near 18,000 feet. The distance between these contours can also approximate the wind speed -in general the closer the distance between any two contours, the stronger the winds and the greater the distance between the contours then the lighter the winds. The center of the "heat" producing high is located over southwest New Mexico with an elongated axis of higher heights extending to western South Dakota. From the Pac Northwest States to northern Wisconsin, a flow of air bringing some Pacific moisture prevails along with embedded disturbances that will cause ascent or lift.

The combination of all of these above features is expected to cause a significant severe weather outbreak across the Northern Plains to Wisconsin during Monday into Tuesday morning (July 21-22). Here is the severe weather outlook from SPC for Monday 21 July. The severe weather will initially start over the the border region of North and South Dakota with Montana as scattered but intense supercells during Monday afternoon, capable of producing severe hazards of damaging winds, large hail and a few tornadoes. These supercells are then expected to merge into a strong progressive derecho over the eastern parts of the Dakotas and then move rapidly east across and towards Minnesota to Wisconsin by late Monday night into Tuesday morning.

This derecho is expected to cause extremely strong winds of hurricane force (75 mph) or more in places along its path. The remnants of this system may also  redevelop and perhaps affect our area during Tuesday night, though probably not as intense. Often times these derecho are "episodic" in nature, that is they tend to develop on consecutive days and may even form over the same geographic region or move across the same region as the predecessor derecho did. Progressive derechos during the summer tend to form and become quite intense during extreme heat and or heat waves. The pattern for the development of these derechos is often referred to as "ridge-rollers and the ring-of-fire"

Jun 28, 2014

Potential for Tropical system off the Southeast US Coast

An area of disturbed weather off the the Southeast U.S. coast has been designated (disturbance) 91L by the National Hurricane Center. While not a true warm core system - yet - it will develop a warm core over time. Pic 1 is the location of the low and the convection associated with it.
Pic 1
 As you can see the the area of lowest atmospheric pressure is removed from the area of strongest convection. This is a satellite signature of unfavorable wind shear aloft which leads to a very disorganized system.

Pic 2

The model tracks for this nascent system are displayed in (Pic 2). There is a lot of "scatter" (basically in two clusters):  one group/cluster tracking the system toward the SW, while the other is on a track toward the SE. So a forecast S-SE motion is forecast. likely.

Since the motion is toward the south -  and south moving tropical systems tend not develop much or only very gradually (in the northern hemisphere) - only a slight or low probability (Pic 3) for development of this system is expected over the next 48 hours. 

Pic 3

If the system should intensify and attain sustained wind speeds of 40 mph or more then it will be "christened" with the name Arthur.