VISIT: Meteorological Interpretation Blog

The June 2008 copy of “The Front” newsletter highlights upcoming changes to the new Terminal Aerodrome Forecast (TAF) which is scheduled to go operational this November (2008).  See this site: www.weather.gov/os/aviation/taf_testbed.shtml for more information. 

And speaking of TAFs, want to know just what happens to those TAFs you write?  Just what do the Center Weather Service Units (CWSU)s, Air Traffic Control Centers (ARTCC)s, and the airports themselves do with those forecasts? Well, they turn them into valuable graphical displays that help reduce weather related airspace congestion, that’s what.  See the story starting on page three.  For a good “live” example, click here. 

Finally, this season’s newsletter ends with a bit of research coming from the Aviation Weather Center (AWC) regarding the prediction of thunderstorm movement throughout the seasons.  More than 27,000 Convective SIGMETs were analyzed for this study whose details begin on page ten.  Good way to categorize and address the “climatology” of thunderstorms in your area throughout the year. 

(Courtesy NASA/MODIS/TERRA - July 26, 2008)

Currently, California has 26 fire incidents…mostly across the northern half of the state.  Fourteen of these fires are considered large at the moment (= or > 100 acres…see following map).

To date (this fire season - through July 30, 2008), over 750,000 acres of California have burned. Most of the current fires started as a result of lightning strikes between June 20th and June 28th…with a couple of them going back to the last week in May!  Several thousand firefighters from across the country have been deployed to the region over the last couple of months, with additional fire specialists from Canada, Australia, and New Zealand also helping out.  Many Incident Meteorologists are also working these fires this summer.  Check out this recent visible image loop that shows the region between the evening of July 29th and through the morning of the 30th. For more information on these fires as well as others across the country, please go to these sites:  http://gacc.nifc.gov/oncc/, http://www.nifc.gov/, and http://www.inciweb.org/ .

This is just a brief reminder that it is monsoon/(FLASH) flood season here in Colorado and the rest of the Rocky Mountain West and adjacent High Plains.  While this region is no stranger to flooding conditions…particularly in the late spring and early summer when combined severe weather threats often aggravate the ongoing snow melt, a secondary, and often much more dire, flood season often accompanies the arrival of the North American Monsoon (see July 15, 2008 blog). 

(From:  Petersen, W. A., and Coauthors, 1999: Mesoscale and radar observations of the Fort Collins flash flood of 28 July 1997. Bull.Amer. Meteor. Soc., 80, 191–216.) The problem is two-fold, as the increase in subtropical moisture via the monsoon is injected into a rather “quiet” mid-upper level flow pattern (typically associated with a mid-upper level ridge - see 500mb analysis above).  If the cap can be broken (which is most often accomplished in the mountains and foothills due to the increase in elevation), the convective storms are most often quite slow to move…sometimes remaining terrain tied to the same area for a period of (several) hours.  Even when there is net movement in one direction, it is relatively slow and new development tends to replace the older storms almost immediately…so storm propagation opposite of the storm’s forward motion gives the illusion of remaining in place and going nowhere. This is a most dangerous situation. Flash flooding is always a concern in mountainous terrain anytime of the year where steep valley walls can contain rainwater…forcing it into gullies, creek beds, streams and rivers in a very short period of time. You can be many miles away from the actual storm…not even able to see or hear it…and get yourself in a great amount of trouble in short order. However, this time of year when you add the additional conditions of increased moisture with slow, or nearly “stationary” storm motion - the problem becomes hugely magnified when the rain falls over these same areas.  Just a few major flash flooding events of interest that have occurred this time of year “out west” follow:The Big Thompson Flood - July 31, August 1, 1976: During the evening of the 31st, over 4 inches of rain fell across a large portion of the Big Thompson basin in less than 6 hours, with over 12 inches falling in a smaller area containing the western third of the Big Thompson Valley.  Much of the canyon was devastated with a 20 foot plus wall of water - killing 139 people!  See these links for more information: http://www.coloradoan.com/news/thompson/ , http://www.reporterherald.com/webextra/1976flood/ , and The Big Thompson Fact Sheet .

The Cheyenne Wyoming Flood - August 1, 2 1985: During the afternoon and evening of August 1st, 1985 a nearly “stationary” thunderstorm produced over 6 inches of rainfall in just under 3 hours.  Add to this, large quantities of hail (three to four feet deep in some slide areas) and a tornado and you have the makings of a disaster.  Twelve people died and over 70 were injured in this mess.

The Fort Collins Colorado Flood - July 28, 1997: The late afternoon and evening of the 27th of July began as a relatively typical thunderstorm event for this time of year with storms blossoming and putting down heavy rain off and on through the overnight hours.  It was a little heavier than normal, with a gradient of precipitation from east to west across the city of Fort Collins lying between 0.75 inches and 4 inches…and higher amounts up to nearly 7 inches just to the northwest of town (near Laporte, Colorado).  This would only herald the beginning as the next day would be the straw that broke the camel’s back.  By noon on the 28th, and under very similar meteorological conditions as the previous day, imbedded thunderstorms once again erupted. In the following six hours anywhere from 1 inch (far eastern Fort Collins) to around 10 inches (far western Fort Collins) of precipitation fell…with rain rates at times as high as 4 to 5 inches per hour.  Spring Creek which runs west to east just south of the center of town exploded from its banks, killing five and causing another 200+ million dollars in damage. For more information see the following links:  http://ccc.atmos.colostate.edu/~odie/rain.html, http://fcgov.com/oem/historical-flooding.php, http://media.www.collegian.com/media/storage/paper864/news/2007/08/01/News/Video.Fort.Collins.Flood.Of.97-2927035.shtml, http://rammb.cira.colostate.edu/resources/docs/Two_floods.pdf, and http://olympic.atmos.colostate.edu/flood97.html.

The Las Vegas Flood of July 8, 1999: The valley that Las Vegas (”The Meadow”) resides in is a primary reason that rainfall events can get out of hand fairly quickly.  Las Vegas is nestled between mountains on nearly all sides…is built over an area that drains these mountains toward the Colorado River…and the composition of the of all this runoff soil/silt is relatively impermeable to water (i.e. runoff).  Add to this the thousands of miles of asphalt and cement from all the new building and you have a recipe for disaster.  The usual Saving Grace is that the region “normally” only receives about 4 inches of rainfall per year, which if stretched over an entire year is not of concern.  However, thunderstorms during the monsoon season can easily put down over a half an inch of precipitation inside of an hour…which is about all that it takes to get flash flooding going in this drain-less oasis in the Mohave. 

On this day, rain rates were somewhat higher…bringing between and inch and a half and three inches of rain inside of 90 minutes.  There were two fatalities and over 20 million in damages with this “little” storm.  Just goes to show how relative conditions are from place to place.  Please click here for more info concerning this event. 

If you have any interesting season events/phenomena in your region of the world, please let us know and send us the info and images (if you have them)…and we will post them for the interest of others. See “Contact Us” or “Topics, Ideas and Questions” to the right under “Pages.”

As Hurricane Dolly made its way into southern Texas July 24, 2008 with 100+ mph winds, drenching a 40 mile wide and 100mile long stretch, along and north of the Rio Grande River, with anywhere between 8 and 22 inches of rain, it heralded the true beginning of tropical cyclone season here in the lower 48.  Yes, the “official” season starts on June 1st and we have already had named storms in both the Atlantic and Pacific, however, this was the first direct hit that the USA has taken in the new year.  Tropical Storm Arthur formed near coastal Belize and then immediately tracked westward into Mexico.  Hurricane Bertha started out as a wave off the coast of Africa, reaching hurricane strength (as high as a category 3 for a short time) northeast of the Windward Islands and then weakened back to tropical storm intensity a hundred miles or so south southeast of Bermuda.  Several days later, after passing almost directly over the island of Bermuda, it slowly headed off to the northeaast where it regained category 1 intensity briefly before heading into the much cooler waters of the North Atlantic.  Of note:  While Bertha did not bother too many interests other than Bermuda and the shipping industry, it did wander the waters of the Atlantic for well over two weeks.  Just before the appearance of Dolly, Torpical Storm Cristobal formed near the gulfstream waters just east of Savannah, Georgia.  Cristobal tracked to the northeast, along and away from the eastern coast of the USA,  for the next 5 days - becoming extratropical on July 23rd.  Cristobal provided areas of much needed rain to the southeastern USA in addition to some good breakers for the surfers up the coast.   In the Pacific, there have been 7 named storms from Alma to Genevieve, however, most of those have had little direct impact (again, other than the shipping business) on the USA.�

(Courtesy NASA/MODIS/TERRA - July 23, 2008 - at landfall)

The week of July 27th is turning out to be on the slow side  - northern hemisphere, tropical cyclone speaking - so I thought, during this lull, that I would mention what the primary missions of this year’s (2008) Hurricane Field Program are …in the case that you haven’t already heard about them (from the 2008 Hurrcane Field Program Plan, signed June 30, 2008):

(1) Three-Dimensional Doppler Winds: This is a multi-option, single-aircraft operational missiondesigned to use the NOAA P-3 to sample TCs ranging in intensity from tropical depression to a major

hurricane. The definition is meant to separate this category from tropical waves and disturbances that have yet to develop a well-defined warm-core circulation. The main goals of these missions is: 1) to improve understanding of the factors leading to TC intensity and structure changes, 2) to provide a comprehensive data set for the initialization (including data assimilation) and validation of numerical hurricane simulations (in particular HWRF), 3) to improve and evaluate technologies for observing TCs, and 4) to develop rapid real-time communication of these observations to NCEP. The overall experiment is comprised of two parts:  one designed to obtain regular 12- or 24-h resolution airborne Doppler-radar observations of hurricanes, with optional dropwindsondes, and one, the National Environmental Satellite, Data, and Information Service (NESDIS) Ocean Winds and Rain Experiment, designed to improve understanding of microwave surface scatterometery in high-wind conditions over the ocean by collecting surface scatterometery data and Doppler data in the boundary layer of hurricanes.

(2) Tropical Cyclone Landfall and Inland Decay Experiment: This is a multi-option, single-aircraft experiment designed to study the changes in TC surface wind structure near and after landfall. It has several modules that could also be incorporated into operational surveillance or reconnaissance missions. An accurate description of the TC surface wind field is important for warning, preparedness, and recovery efforts. It addresses IFEX Goals 1, 2, and 3.

(3) Tropical Cyclone Unmanned Aerial System (UAS) Inflow/Eyewall/Eye Experiment: This is a multioption, multi-aircraft experiment that uses the Aerosonde UAS and dropwindsondes or aircraft expendable bathythermographs (AXBTs) launched from the NOAA P-3 to fully demonstrate and utilize the unique capabilities of the Aerosonde platform to document areas of the TC environment that would otherwise be either impossible or impractical to observe. It is planned that this effort will be based in Barbados. The immediate focus is to document and significantly improve understanding of the rarely observed TC boundary layer and undertake detailed comparisons between in-situ and remote-sensing observations obtained from manned aircraft (NOAA P-3 and Air Force Reserve (AFRES) C-130) and satellite-based platforms. In addition, a primary objective of this effort is to provide real-time, near-surface wind observations to NHC in direct support of NOAA operational requirements. These unique data will also be made available to EMC for both model initialization and forecast verification purposes. This addresses IFEX Goals 1, 2, and 3.

 (4) Tropical Cyclogenesis Experiment: This multi-option, multi-aircraft experiment is designed to study how a tropical disturbance becomes a tropical depression with a closed surface circulation. It seeks to answer the question through multilevel aircraft penetrations using dropwindsondes, flight-level data, and radar observations on the synoptic, mesoscale, and convective spatial scales. It will focus particularly on dynamic and thermodynamic transformations in the low-and mid-troposphere and lateral interactions between the disturbance and its synoptic-scale environment. It addresses IFEX Goals 1 and 3.(5) Nadir Off-set SFMR Experiment: This is a single-aircraft experiment designed to obtain measurements off nadir of the sea surface to help develop retrieval models for the HIRAD.

(6) Tropical Cyclone/AEW Arc Cloud Experiment: This is a single-aircraft experiment, designed to collect observations across arc cloud features in the periphery of an AEW or TC using aircraft flight-level and dropwindsonde data to improve understanding of how these features may limit short-term intensification. Observations could be made using either the P-3 aircraft conducting another experiment, or the G-IV during a synoptic surveillance mission.

 (7) Saharan Air Layer Experiment: This is a multi-option, multi-aircraft experiment which usesdropwindsondes launched from the NOAA G-IV and NOAA P-3 to examine the thermodynamic andkinematic structure of the SAL and its potential impact on TC genesis and intensity change. The dropwindsonde release points will be selected using real-time GOES SAL tracking imagery from UWCIMSS and mosaics of SSM/I total precipitable water from the Naval Research Laboratory. Specific effort will be made to gather atmospheric information within the SAL as well as regions of high moisture gradients across its boundaries and the region of its embedded mid-level easterly jet. The goals are to better understand and predict how the SAL dry air, mid-level easterly jet, and suspended mineral dust affect Atlantic TC intensity change and to assess how well these components of the SAL are being represented in forecast models. It addresses IFEX Goals 1 and 3. 8) Sea-Salt Aerosol and Cloud Base Number Concentration Experiment: This single-aircraftexperiment is a downwind flight leg outside the eyewall in relatively clear air, or just inside the inner edge of the eyewall. It will measure the concentrations of sea-salt aerosol and CCN concentrations below cloud base (1200- to 2000-ft flight levels are likely) in tropical storms and category 3 or less TCs, as well as approximately 200 ft above cloud base.

(9) Eyewall Microphysics Experiment: This is a single-aircraft, high-altitude penetration of eyewall convection, designed to document the ice-phase microphysics of the eyewall better than ever before, to benefit microphysical parameterizations in simulation of TCs. This could improve modeling of precipitation production, thus accurately estimating latent heat release (LHR) (affecting hurricane intensity) and rainfall quantitative prediction. It is preferred that it be flown at or above 20,000 ft. (10) TC-Ocean Interaction Experiment: This is a multi-option, single aircraft experiment acting insupport of upper ocean and air-sea flux measurements made by oceanic floats and drifters. One to three float and drifter arrays will be deployed into one or two mature storms by an AFRC C-130J and provide real-time ocean data. A NOAA P-3 will deploy dropwindsondes and make SFMR and Scanning Radar Altimeter (SRA) measurements within the float and drifter array as the storm passes over it. This work will be coordinated with NASA P-3 deployments of CTD probes.

(11) Hurricane Synoptic Surveillance: This is a multi-option, single or multi-aircraft operational mission that uses dropwindsondes launched from the NOAA G-IV, and the AFRES C-130 to improve landfall predictions of TCs by releasing dropwindsondes in the environment of the TC center. These data will be used by NCEP to prepare objective analyses and official forecasts through their assimilation into operational numerical prediction models. Because the atmosphere is known to be chaotic, very small perturbations to initial conditions in some locations can amplify with time. However, in other locations, perturbations may result in only small differences in subsequent forecasts. Therefore, targeting locations in which the initial conditions have errors that grow most rapidly may lead to the largest possible forecast improvements.  Locating these regions that impact the particular forecast is necessary. When such regions are sampled at regularly spaced intervals the impact is most positive. The optimal targeting and sampling strategies is an ongoing area of research. This addresses IFEX Goal 1. 

For more on the many (other) research oriented activities of the Hurricane Research Division (HRD) - part of the Atlantic Oceanographic and Meteorological Laboratory (AOML) - please follow this link:  http://www.aoml.noaa.gov/hrd/index.html .

(Courtesy NOAA/NWS - July 15, 2008) 

Strong heating over the elevated (Mexican Plateau) desert southwest CONUS causes an area of low pressure to form known as a thermal low.  Since the air pressure is relatively higher over the nearly adjacent ocean areas (Gulf of California and the Tropical Pacific) to the south and west, air flow (from high pressure to low pressure)  begins to bring much more humid air toward the thermal low.  Instability with this lower source (level) of moisture will help in developing thunderstorms in which rain can actually reach the ground (instead of just virga storms) which will additionally add to the boundary layer moisture and help in increasing thunderstorm chances for a prolonged period…at least until the cycle reverses in late summer/early fall (when land temperatures decrease some and the oceanic waters reach their maximum).  Mid and upper level flow around high pressure aloft will also bring mid and upper level moisture into the region from the Gulf of Mexico. 

Of course there can be much variability with the North American Monsoon as to where and how intense the moisture and thunderstorms tend to be.  Where both the thermal and upper level lows/highs set up is of major importance and can mean the difference between all or nearly nothing.  For example, while a good strong upper level ridge over the great plains area will help drive moisture into the southwest CONUS, a weaker ridge or one located further west over New Mexico or northern Mexico will keep the moisture located to the east and over the great plains.  There are also a large number of other variables which can adversly affect the monsoon (see this short paper).  Currently the thermal low is located over N to NWrn Mexico and the upper level ridge is centered over the Rio Grand Valley region of Texas/Mexico. 

According to the the National Weather Service (NWS) out of Tucson, who track the North American Monsoon and its progress, the monsoon “officially” began here in the United States around the 2nd of July, 2008 (when average dewpoints in the Tucson region remain at least 54 deg F. or higher) - see the Monsoon Tracker page.  Past 24 hour rainfall (as of July 15, 2008) is depicted at the top of the page and is very typical for an early season monsoon pattern.  

For a more detailed and fascinating look at the North American Monsoon, see the NWS Tucson’s Monsoon section. 

See also these papers:

Adams, D.K., and A.C. Comrie, 1997: The North American Monsoon. Bull. Amer. Meteor. Soc., 78, 2197-2213.

Douglas, M.W., R.A. Maddox, K Howard, and S. Reyes, 1993: The Mexican monsoon. J. Climate, 6, 1665-1667.

Carleton, et.al., 1990: Mechanisms of Interannual Variability of the Southwest United States Summer Rainfall Maximum. J. Climate, 3, 999-1015.

Higgins, R.W., Mo, K.C. and Yao, Y., 1998: Interannual Variability of the U.S. Summer Regime with Emphasis on the Southwestern Monsoon. J. Climate, 11, 2583-2606.

Higgins, R.W. and Shi, W., 2000: Dominant Factors Responsible for Interannual Variability of the Summer Monsoon in the Southwestern United States. J. Climate, 13, 759-776.

Higgins, R.W. and Shi, W., 2001: Intercomparison of the Principal Modes of Interannual and Intraseasonal Variability of the North American Monsoon System. J. Climate, 14, 403-417.

Castro, C.L., McKee, T. B. and Pielke, R.A., 2001: The Relationship of the North American Monsoon to Tropical and North Pacific Sea Surface Temperatures as Revealed by Observational Analyses. J. Climate, 14, 4449-4473.

Vera, C. et. al., 2006: Toward a Unified View of the American Monsoon Systems. J. Climate, 19, 4977-5000.

Castro, C.L., Pielke, R.A. and Adegoke, J.O., 2007: Investigation of the Summer Climate of the Contiguous United States and Mexico Using the Regional Atmospheric Modeling System (RAMS). Part I: Model Climatology (1950-2002). J. Climate, 20, 3844-3865.

Castro, C.L., Pielke, R.A. and Adegoke, J.O., 2007: Investigation of the Summer Climate of the Contiguous United States and Mexico Using the Regional Atmospheric Modeling System (RAMS). Part II: Model Climate Variability. J. Climate, 20, 3866-3887.