WKU Meteorology

Discussion of Mid-South weather and climate and information about the WKU Meteorology program

WKU Meteorology - Discussion of Mid-South weather and climate and information about the WKU Meteorology program

A warm and dry Thanksgiving will turn into a wet weekend.

  • Thursday:  Sunny with a high near 64° and a low near 35°.
  • Friday:  Sunny with a high near 68° and a low near 42°.
  • Saturday: Clouds increasing throughout the day.  Highs near 65° and lows near 40°.
  • Sunday:  Rain showers likely, with possible snow overnight.  Highs near 45° and lows near 32°.

We couldn’t ask for a better situation to have and be thankful for the weather.  The weather for Thursday and Friday is going to have include temperatures well above average for this time of the year.  Our climatological average for late November shows high temperatures around the mid 50s and lows ranging between 32° F and 35° F.  All of the pleasant weather will move away, however, late Saturday night heading into Sunday morning with the arrival of an approaching cold front.

High pressure at the surface will quickly move out of the area early Saturday, but will dominate the weather over the course of the next two days.  This feature, combined with a ridge aloft will provide for relatively cloudless skies and temperatures that are well above average for this time of the year.  However, a strong jet streak that is currently intensifying a storm in the Gulf of Alaska will propagate to the east just in time to strengthen a developing trough in the northern Plains and cause it to dig southward toward the southeast.  An associated surface low will be centered over Wisconsin and under the front-left quadrant of the jet streak in the eastern portion of this trough, causing the necessary forcing for divergent air in the upper levels and strengthening of the low.  The deepening low and increasing pressure gradient will help to pull in a much colder air mass that will be pinched off by the trough, cause showers and possibly early Monday morning snow and keep rain chances around through the early part of next week.

This trough over the Midwest will pinch off an upper level low and keep persistent rain chances in the forecast through early next week.

Storms that do occur on Saturday and Sunday are not currently expected to reach severe limits as available moisture will be a limiting factor.  Moreover, weak lapse rates and warmer air aloft will inhibit any significant vertical velocities in existing thunderstorms.

Under what could finally be officially labeled as a La Niña, stormy conditions and above average rainfall are likely to continue.  Shown below are graphics from the NOAA Climate Prediction Center detailing typical weather patterns of a La Niña event.

Shown in the bottom picture are the jet stream and rainfall patterns typical of a La Nina.

Model forecasted Niño 3.4 region temperature anomalies indicate a weak to moderate La Niña persisting into the beginning of next year.  This, in combination with the build up of cold air along the Canadian border could mean an active winter for the Midwest and Northeast.

The blue line shows a moderate La Nina sticking around through next spring.

This is certainly something to keep an eye on for the long term.  Areas that were hammered during last year’s winter may see another active winter season given the right storm moving in.

 

Diminishing severe weather threat for Tuesday…

Looking at the 15z run of the RUC model, the severe weather threat for today seems almost non-existent.  An already rain-cooled air mass will not receive any extra convective energy from the sun as cloud cover will be extensive throughout the day.  This will put an end to any significant threat for the Bowling Green area and state of Kentucky.

Clouds will stop any real severe threat today

Despite the absence of a meaningful severe weather threat today, heavy rains are still probable across much of the state.  The latest 500 mb analysis puts the rain-shield sitting across the region continuing its movement toward the northeast.

Storms will continue to swing around the trough axis toward the northeast, placing most of KY under the heavy rains for the remainder of the day

PWAT values are very high and could mean that very high amounts of rain will fall before the night is over.  By tomorrow morning, SREF guidance suggests that close to 1 inch of rain will fall.  Bowling Green has already recorded 0.3 inches at the mesonet station, so the SREF forecast of around 0.85 inches by tomorrow morning is not completely out of the question.

Very high precipitable water amounts will lead to very heavy precipitation today and into tomorrow.

 

The drier air mass is beginning to filter toward the surface by tomorrow morning

Damaging winds may still be possible today and this evening, as bulk shear values still range between 40 and 60 knots.  Some momentum transfer is possible, but winds are not expected to meet severe criteria.

Shear values could indicate some strong winds later tonight.

 

 

 

Severe weather increasingly likely to start the week…

Over the last few days, operational models have been trending more and more towards solutions that show severe weather striking the area on Tuesday.  The Storm Prediction Center has now changed its forecasted “slight risk” area for Tuesday from an area that was contained in the ArkLaTex to one that now extends well into Kentucky.

Severe weather is becoming increasingly likely for Kentucky on Tuesday

Early Tuesday morning, a positively-tilted trough will swing into the area, providing the necessary momentum to push a front that is currently stalled across the state back to the north again as a warm front.  As it does so, most of Kentucky will be in the warm sector of the storm, allowing for a progressively destabilizing atmosphere to take hold in the region.  Mositure will increase ahead of the storm’s trainling cold front, with forecasted dew points in the low 60s.  An elongated area of vorticity and increased shear hint at a developing QLCS across Missouri, Arkansas and Illinois that will likely affect the Warren County area after dark.  With bulk shear values of 40-60 knots, damaging winds ahead of bowing segments of the squall line are the most probable convective mode to impact the region.

The 06z NAM Model forecasted 1000-500 mb shear.

The models are differing as to the timing of the arrival of the system.  The 06z ECMWF and the latest run of the NAM are showing a slowing storm, while the 06z runs of the GFS and NAM show the main line affecting us just after sundown on Tuesday.  Nonetheless, heavy rains and the downward transfer of high momentum winds will make for a stormy evening Tuesday, which could possibly last into Wednesday morning.  The latest forecasted rainfall totals by the HPC show well over an inch of rain falling across the western half of the state, with isolated totals being greater than that.

Heavy rains are going to fall for most of the day Tuesday.

The strong ridge off to our east that has provided for the warm weather and gusty conditions over the last few days will prevent any tornadic activity for our region.  A relatively stable boundary layer will provide a hostile environment for the sustenance of any convective activity across the Ohio Valley.

New look at Texas and Oklahoma drought conditions.

With La Niña conditions expected to stay at current strength or get stronger, the people of Oklahoma and Texas are rightfully concerned about their water conditions.  The map below is an interesting look at how many inches of rain are needed to break the drought.

Notice the two areas that desperately needed the rain this weekend got it.

Comparing this map to a couple of accumulated rainfall maps from Texas and Oklahoma, it’s easy to see that although this rainfall event hasn’t done away with the drought conditions across these areas, a nice dent was put into some of the anomalies.  Look at these maps that show accumulations of  3 + inches across parts of Texas and Oklahoma.

Courtesy of the Texas Mesonet

And in Oklahoma,

Not quite as descriptive as the data from the mesonet, but still gives the idea.

 

I’m sure that the people of Texas and Oklahoma who do not know what a dryline will still be thanking God for them.  Bring it on drylines!

HPC has backed off of totals, but Texas and Oklahoma still wet this weekend.

As I mentioned yesterday, the HPC models tend to overdo forecasted precipitation totals, expecially for long-term periods.  The new 5-day totals still show a significant change to drought conditions in the north Texas/south Oklahoma areas, and also a significant flash-flooding threat.

Notice that the hot-spots are still in the same areas as yesterday, but the forecasted rainfall totals are less than they were.

This is great news for Texas.  A dryline is serving as the main focus of thunderstorm activity today, evident from the map below.

See the tight PWAT gradient west of Midland and Lubbock

As this dryline swings eastward, it will continue to meet up with moisture advecting in off of the Gulf of Mexico.  Some severe weather is possible out of these storms, with strong moisture convergence along the boundary forming (shown in red), upper level divergence in the right exit region of the jet streak and fairly strong low-level helicity values (also shown below).  Isolated tornadoes are possible tonight in the area, but the major concern will be flooding issues.

A cold-front is developing here

The trough over the Four Corners region is helping to bring much needed rain...

This is great news for Texas and Oklahoma.  After this rainfall event has completed, it will be interesting to look at how drought conditions have changed for the area.

Drought relief for Texas?

Over the weekend, the extremely drought stricken areas of Oklahoma and Texas could potentially get some help from mother nature.  Thanks to the blocking pattern that’s currently in place over the eastern United States, any storms that move into the country are forced to go up and over this ridge that’s kept much of our weather pleasant and unseasonably warm.  As long as the blocking pattern stays in place, which only looks to be for about the next 5 days, any forward movement will be halted, placing areas that desperately need some rain under the influence of a cold front.

Currently we can see that almost the entire area contained within Texas and Oklahoma are and have been under an exceptional drought.

Much of the area that looks to be "on fire" may have some of those flames put out.

The models at the NOAA Hydrometeorological Prediction Center, at times over-predicting rainfall totals, still  show two distinct hot-spots for precipitation for the next 5 days.  The 5-day precipitation total published on their website shows a staggering 10.8 inches of rain forecasted for an area near Witchita Falls, TX.

Notice the extremely high numbers over Florida, Oklahoma and Texas

Any totals such as this are way off base given the conditions in the area.  The lack of soil moisture available will inhibit the sustenance of any convection that develops along the boundary.  However, persistent southeasterly flow off of the Gulf of Mexico provides the moisture advection necessary to allow for what will likely be persistent rain showers with some isolated thunderstorms.  Flash-flooding will be a major concern this weekend over the area.

Current surface conditions show the synoptic setup for this weekend.  The main feature causing the HPC models to pick up on the significant rainfall is the high pressure system at the surface, encompassing much of the east.  A tight pressure gradient is developing over Florida , allowing for strong easterly winds over Florida.  Along a stationary boundary located south of Florida, an area of rotation is expected to form in the wake of this tight pressure gradient and the enhanced easterly flow.  This looks to be the source of our next chance of rain in the Bluegrass State, most likely around Tuesday or Wednesday.

Note the position of the frontal boundary from Kansas southward toward New Mexico.

High pressure is still dominating weather in Kentucky through this weekend.

The frontal boundary has not moved much by Sunday morning

The frontal boundary is marked south of Florida.

We’ll have to wait and see how the ridge in the east moves, because that will be one of the steering mechanisms of any rotation and moisture that advects northward from the Caribbean.

Models continue to show potential tropical development around Florida.

The models are continuing to show development of a system along a quasi-stationary boundary located south of Florida.  The windy conditions and rip currents that much of the Sunshine State’s east coast is experiencing will likely sustain their current strength or even worsen in the next couple of days as high pressure centered over Pennsylvania builds.  Where this spin-up tracks will determine if areas in the south experiencing drought conditions will receive any support from mother nature.  Currently, it appears as though this area of vorticity will track up into Georgia and potentially track into eastern Tennessee and Kentucky.  As I mentioned in a previous post, this looks to potentially bring rain to the Bluegrass region and east, and likely increase cloud cover to the west.  This looks to be the next best chance for rain that the state will see, so this is the area to watch heading into the middle of next week.

Next good chance for rain looks to be next Tuesday or Wednesday…

Under the influence of high pressure aloft and at the surface, the weather across the Bluegrass State is beautiful.  For the next few days, the conditions outside aren’t going to change much, if any at all.  Enjoy the nice weather while it lasts, because the models are suggesting the potential for some tropical moisture to infiltrate the area and possibly bring rain to the Bluegrass region and eastern Kentucky.

Gradual height falls heading into the middle of next week show the onset of a trough digging into the area from the Dakotas.  This looks to be a pretty potent trough at this point but with no real moisture advection off of the Gulf of Mexico, any significant rain chances will be dependent on what could be a named tropical system moving in.  Note the area of vorticity near Florida.

The yellow area on the map over central Florida is the important feature here.

The 500 mb height contours show this vorticity maxima following almost a perfectly south-to-north track up toward the Appalachians.

If the trough and this moisture meet up, there could be some heavy rainfall in eastern KY

As this area of rotation moves up the southeast, the majority of moisture associated with it looks to stay in the Bluegrass and eastern Kentucky areas.  Places like Lexington, London, Pikeville, Georgetown and others should at least see clouds increase with the passing system, and possibly some showers.  Until this happens, though, keep enjoying the beautiful weather that we’ve had and will keep having through the weekend and early into next week!

 

Tropical Update and the Madden-Julian Oscillation

Now that we’ve finally passed the peak of the hurricane season and almost the entire eastern half of the country will experience nice weather for a while, it’s a good idea to take a look at the tropics to see what may be brewing there.

As of now, the Atlantic basin has one active tropical system, tropical storm Phillipe.

This storm should transition to post-tropical within a few days.

Phillipe is the only notable area of convection in the Atlantic and Caribbean at the moment.  From the graphic below, a few fronts are evident, and Phillipe is marked.

Notice the large cloud shield moving toward Phillipe. This front is forecast to become quasi-stationary, but will keep Phillipe from impacting land as a tropical system.

The main area that forecasters watch at this time of the year is the Caribbean Sea and the Gulf of Mexico.  These areas are showing little to no activity at the moment, but model forecasts hint at a possible storm within the next 10-14 days.  See the area of vorticity just east of Florida.  The rotation is more pronounced at the 850 mb level.

This is forecast for 300 hours out, so this is something we'll have to wait on and see if it pans out.

With all of that said, it is still important that we’re entering back into La Niña conditions in the Pacific Ocean and that we can expect an active pattern with regards to the Madden-Julian Oscillation.  Under La Niña conditions, vertical wind shear in the areas that hurricanes form within is much less than during an El Niño year.  This is important in allowing the storms to grow.  In addition to that, with a more active Madden-Julian Oscillation, increased latent heat flux provides necessary energy to the atmosphere for the storm to tap into.

So, what is the Madden-Julian Oscillation?  The Madden-Julian Oscillation is a global scale wave occurring within the tropics that promotes areas of enhanced or suppressed rainfall.  It typically lasts between 30 and 60 days, begins in the Indian Ocean and propagates eastward from there.  The phase of the oscillation is characterized by the spatial location of the center of enhanced convection (rainfall).  When we enter the later phases of the cycle, the center of convection reaches closer and closer to the U.S. Pacific Coast.  Relative humidity, westerly winds and other variables increase ahead of the center.  When the conditions are in this phase, tropical cyclones are 4 times more likely to occur (Maloney and Hartmann, 2000).

Below is the GFS forecast of the MJO.  Follow the green and yellow lines to see where the center would be located and read off the magnitude from the plot.

The GFS MJO forecast retrieved 10/04/2011We’ll have to wait and see how the forecast turns out, but in the short-term we could potentially see another storm form within two weeks, this one being more likely to impact the United States.

SOURCES:

Maloney, E.D., and D.L. Hartmann, 2000: Modulation of hurricane activity in the Gulf of Mexico by the Madden-Julian Oscillation.  Science,287,2002-2004.

Wheeler, M.C. and H.H. Hendon, 2004: An all-season real-time multivariate MJO index: Development of an index for monitoring and prediction.  Monthly Weather Review,132,1917-1932.

 

 

Pleasant fall weather to continue for the next several days.

Bowling Green forecast for the next few days:
  • Tuesday: High near 78, overnight low near 46.  Sunny.
  • Wednesday: High near 81, overnight low near 49.  Sunny.
  • Thursday: High near 83, overnight low near 51.  Sunny.

Discussion:

After a brief cool down, the region will see an upper-level ridge build into the area from the west over the next several days.  Coupled with high pressure at the surface, mostly sunny to completely sunny skies can be expected through the week.  Model forecasted temperature guidance has been low for much of the summer.  Considering this, we should have just enough daylight left to bump up forecasted temperatures a degree or two, with the peak in temperatures occurring late this week.

Omega block keeping us dry!

This ridge doesn't begin to weaken until this weekend.

Tomorrow, I’ll provide a tropical update and an explanation of the Madden-Julian Oscillation, a key ingredient for the rest of our hurricane season.