I hope this helps clear up anything with troughs and ridges...these are some AFD's from my area in eastern NC, written by the wonderful staff at the Newport/Morehead City, NC NWS Forecast Office!
AREA FORECAST DISCUSSION...DELAYED
NATIONAL WEATHER SERVICE NEWPORT/MOREHEAD CITY NC
520 AM EDT FRI AUG 18 2006
.SHORT TERM /TODAY THROUGH SUNDAY/...NE-SW ORIENTED STATIONARY FRONT
JUST OFF THE COAST WILL SLOWLY WEAKEN AND DISSIPATE AS IT DRIFTS
INLAND TODAY THROUGH SATURDAY. SURFACE WIND FLOW WILL BACK FROM EAST
NORTHEAST TODAY TO SOUTHEAST ON SATURDAY...THEN SOUTHWEST ON SUNDAY.
Same AFD, the 520 AM one, further down, split so I dont have to copy the entire Short Term Forecast:
I KEPT IN SLIGHT CHANCE OVER THE COASTAL WATERS
ONLY FOR THE USUAL NIGHTTIME CONVECTION AND KEPT THE REMAINDER OF
THE AREA DRY. CHANCE POPS ON SUNDAY IN PREVIOUS ZONE PACKAGE STILL
LOOKS GOOD AS A COLD FRONT WILL BE HEADED OUR WAY FROM THE
NORTHWEST.
Notice them mentioning a boundary moving back inland today and dissipating overland...then notice also the mention of another front moving in Sunday...now inbetween this time the ridge builds in slightly, and then gets kicked back out.
From Long Term, still the 520 AM discussion:
.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...EXTENDED PERIOD WILL
BECOME A BIT MORE ACTIVE. THE BROAD UPPER RIDGE NOW COVERING MUCH OF
THE SOUTHERN HALF OF THE NATION WILL RETROGRADE TO CA/NV/AZ BY THE
END OF THE WEEK AND A BROAD UPPER TROF DEVELOPS OVER THE EASTERN
HALF OF THE COUNTRY. PREVIOUSLY MENTIONED COLD FRONT IS FORECAST TO
STALL OUT ACROSS THE AREA FOR MOST OF THE WEEK.
Note that the ridge is forecasted to move out as it is replaced by a broad upper trough.
Que Evil Music:
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEWPORT/MOREHEAD CITY NC
955 AM EDT FRI AUG 18 2006
.SHORT TERM /REST OF TODAY/...LOOKING AT OBS OVERNIGHT THE WEAK
BOUNDARY OFF THE COAST ACTUALLY PUSHED INLAND FOR A BRIEF
PERIOD...PRODUCING SOME SCT SHRA NEAR THE COAST. THE BOUNDARY IS
AGAIN OFFSHORE...
What is this? Just a few hours later...the boundary pushes onshore, as previously forecasted, but then instead of dying, it decides to push back offshore, not like forecasted...oh my...just a few hours and the forecast is already kind of wrong...
Evil Music X2:
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEWPORT/MOREHEAD CITY NC
333 PM EDT FRI AUG 18 2006
.SHORT TERM /TONIGHT/...THE BOUNDARY PUSHED BACK INLAND
TODAY...WHICH TRIGGERED SOME SCT SHRA.
Wait...time out...lets review, front is forecasted to move inland and die...front actually moves inland, then back out to sea, front is still forecasted to move inland and die, now front is finally inland, (again) and should die...ok, moving on...now we move to the 333 PM (same discussion as above) Long Term about that front that is suppose to move in Sunday:
Still forecasted to move just west of here on sunday...however during the week they do not want to forecast where exactly the lows will be as the front may set up just a little bit further east or west and throw forecasts off, so they leave in chance of precip until they have more information...BY SUN THE 500MB RIDGE WILL RETROGRADE TO THE W AND ALLOW A TROF
TO DEVELOP OVER THE EAST COAST. A BOUNDARY WILL ADVANCE TOWARDS
THE AREA SUN...BUT SHOULD REMAIN JUST W OF THE AREA MORE IN RAH
AREA. THE BOUNDARY WILL SINK SE ON MON THROUGH WED...INCREASING
RAIN CHANCES THROUGH THE WEEK WITH POSSIBLE WEAK LOWS RIDING ALONG
IT BRINGING HEAVY RAINFALL. WILL NOT TRY AND PIN POINT LOWS OR
FRONTAL POSITIONS AT THIS TIME...AND WILL CONTINUE CHANCE WORDING
THROUGH THE WEEK.
Now that I have showed that fronts are tricky to forecast, you maybe asking..."How does that affect whether or not tropical cyclones hit the East Coast or the Gulf of Mexico????" I will get to that in a second...first my NWS sources: http://iwin.nws.noaa.gov/iwin/nc/discussion.html <==is where I retrieved the discussions, however they do update and will not be there tomorrow because the newer ones will take their place...
Now...these troughs moving in and breaking down the ridge, then washing out (dieing) and allowing the ridge to build back in can affect tropical cyclones paths by allowing them to feel a weakness in the ridge (where the front has broken down the ridge) and then they start moving northward, which can be good if the weakness stays there, however if the front dies out and allows the ridge to build back in, then the ridge shoves the tropical cyclone back to the west and towards land. Think of the ridge as a big bully, pushing and shoving the storm westward...now these series of troughs and ridges are constantly moving, reforming, dieing, getting stronger, everyday something with them changes, including intensity. This creates some doubt and error in the longer range forecasts, ie. 3-7 days out, however inside the 3 day forecast they still cause some errors, thus that is why you should focus on the cone of uncertainty, instead of that small, squiggly black line. Another reason you focus on the cone is because systems are not a point, but that is the beginning of another mile long post for another subject/day. Some examples include Hurricane Isabel:
Here is the source: http://www.nhc.noaa.gov/archive/2003/di ... .023.shtml?THE BIG QUESTION CONTINUES TO BE WHAT WILL HAPPEN BEYOND THE 5-DAY
FORECAST PERIOD. IT IS STILL IMPOSSIBLE TO STATE WITH ANY
CONFIDENCE WHETHER A SPECIFIC AREA ALONG THE U.S. COAST WILL BE
IMPACTED BY ISABEL. THIS WILL LIKELY DEPEND ON THE RELATIVE
STRENGTH AND POSITIONING OF A MID-TROPOSPHERIC RIDGE NEAR THE EAST
COAST AND A MID-LATITUDE TROUGH TO THE WEST OR NORTHWEST AROUND THE
MIDDLE OF NEXT WEEK. UNFORTUNATELY...WE HAVE LITTLE SKILL IN
PREDICTING THE EVOLUTION OF STEERING FEATURES AT THESE LONG RANGES.
This is 7 days before Isabel hit Eastern NC and then moved on up thru the North East. The different positions and strenth of the trough and ridge was in the air, no one really could say "It will hit here" just due to us not knowing enough information about these ridges and troughs, plus the fact we really did not know where it would finally settle/set up...
Here is another trough affecting a hurricane, this one was forecasted to hit the east coast for a short time, however it didnt, last years Hurricane Irene.
Source: http://www.nhc.noaa.gov/archive/2005/di ... .025.shtml?EXCLUDING
THE UKMET MODEL...THE REST OF THE NHC MODEL GUIDANCE IS TIGHTLY
CLUSTERED ABOUT THE PREVIOUS FORECAST TRACK THROUGH 72 HOURS WITH
LESS THAN A 60 NMI SPREAD. AFTER THAT...HOWEVER...THE MODELS
DIVERGE SIGNIFICANTLY BASED ON HOW THEY HANDLE THE DEVELOPMENT OF A
WEAKNESS IN THE SUBTROPICAL RIDGE OFF THE SOUTHEASTERN U.S. BY DAY
4. THE DYNAMICAL MODELS...ESPECIALLY THE CANADIAN...ARE MUCH LESS
ROBUST WITH DEVELOPING A SIGNIFICANT WEAKNESS IN THE RIDGE.
As you can see in this example, there were significant differences, just 4 days out, in the models to where exactly the trough would set up and break down the ridge, if at all. Luckly for us, the trough set up, broke down the ridge, and kicked Hurricane Irene out of here.
In short, we simply can not forecast where the trough will park itself, if at all, in the longer ranges, and that sigificantly effects where exactly the hurricanes will go for an East Coast threat. If anyone tells you they can forecast exactly where the ridge will set up in a month then they are lying to you and are most likely either drunk and/or high, or just want to make some quick dollars. However, there are experts who have been studying these massive beasts for more than 20 years and, looking back at the way things are set up now compared to a similar set up year, can tell you which areas are more likely to be hit, such as the east coast is more favored in "x" pattern, but those predictions take lots of time to compile and also are to be taken lightly, as they can easily be wrong.
