WKU Meteorology

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!

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.

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.

Now that we’re in the month of October, everyone has begun to relax, enjoy football season and get ready for the changing seasons.  It is important to remember, however, that a “second severe weather season” begins for much of the south around this time of the year.  The fall “season”, though not typically producing the prolific outbreaks that we’d witness during the spring time, can still cause a lot of devastation.  It is important to be aware of the weather during this time of the year; with decreasing daylight, there is a greater chance of a nighttime tornado.

Below is a map showing just how affected our area can be.

Many thanks also goes to the WKU Department of Geography and Geology for providing the GIS software used to create this graphic.

As you can see, many strong and long-lived tornadoes have hit the Bluegrass State over the last half-century.  South-central and western Kentucky have been much more active in regards to tornadoes during this time.  As recently as last year, Warren County witnessed an EF1 tornado touch down.  Moreover, several EF3 and one EF4 tornadoes have touched down in western Kentucky, ranging in location from south of Paducah toward Hopkinsville and northward toward Owensboro.

So the logical question would then be, why severe weather this late in the year?  We’re all aware that with the changing seasons, solar input decreases with the decreasing duration of daylight.  But, what does that have to do with anything?  This magnifies an increasing temperature gradient across much of the United States, allowing for upper-level winds (most importantly the jet stream winds) to increase in speed.  The increase in the upper-level winds helps to create stronger frontal systems to impact the country.  During this transitional season, the last bit of summer hangs on long enough to interact with one of these frontal systems, allowing for a warm and moist air mass to be in place just in time for the arrival of one of these frontal systems.  Ultimately, all of the ingredients can be in place for strong to severe storms.

Another reason to pay attention to the weather here during the “second severe weather season” is the forecasted return of La Niña.  During La Niña conditions, more significant outbreaks are seen, longer track tornadoes are seen, and more violent tornadoes are seen (Knowles and Pielke, 2005).  The La Niña influence on the positioning of the jet stream places the Ohio Valley, and certainly Kentucky, in an area of focus for fall tornadoes.

SOURCES:

Knowles, J.B., and R.A. Pielke Sr., 2005: The Southern Oscillation and its effect on tornadic activity in the United States. Atmospheric Science Paper No. 755, Colorado State University, Fort Collins, CO 80523, 15 pp. (Originally prepared in 1993, published as a Atmospheric Science Paper in March 2005).

Durkee, J.D., and G. Conner:  Fact Sheet – El Nino/La Niata and Kentucky Tornadoes. Kentucky Climate Center Fact Sheets. Accessed 10/2/2011.  http://kyclim.wku.edu/factsheets/ninokentuckytornadoeses.html

As the upper level low departs and as a drastic change in air mass occurs overnight, a slight gradient wind event is occurring.  The strongest pressure gradient is farther north; however, current sustained winds measured at Mesonet sites around Southern Kentucky and throughout Kentucky are reading pretty breezy at around 15 mph with the highest wind gusts approaching 30 mph.  The interesting part about the readings is the Warren County Mesonet Site seems to be getting the highest wind readings whereas surrounding Mesonet Sites are reading wind speeds lower.  Gusts have been strongest at the Warren County Mesonet Site as well.

As the weekend approaches, a drastic cool down will occur, with highs expected to be in the low 60s on Saturday.  Saturday Night, with mostly clear and nearly calm conditions expected, the first frost of the season is forecast.  This would be right on time for Bowling Green based on the average date for the first frost of the season, which is October 2.

After Saturday, a ridge will build into much of the CONUS, and more seasonable temperatures will be in place.

The cut-off system has been picked up by the jet-stream, and a jet streak will help dig the a trough over Central Canada down into The Mid South bringing high temperatures down into the 60s for the weekend with Saturday being the coolest day in a long time, where the high temperature will struggle to make it too far above 60°F as a very strong northwest flow will be in place continuing to dig the trough into Georgia.  The high amplitude pattern will also cut off slightly over the Eastern States.  As it moves off to the northeast temperatures will rebound during the weekend and into next week, and a broad trough will move in over much of the CONUS.

The first full week of Fall 2011 looks mostly quiet with the temperature fluctuations along with the cut-off low being the main stories.

Yesterday, the weather over The Mid South was dominated by rather prolific dry slotting in the cut-off low pushing the most active weather to the Atlantic States.  Today, the main activity is in the Deep South as drier more stable air is in place once again over the Mid South.

Skies should remain mostly clear today with a few clouds moving through.  The latest GOES Sounding (11GMT) suggests a high of around 73°F, but winds out of the south should provide enough Warm Air Advection to reach a a high of 75°F.  Models suggest a small shortwave in the cut-off low should move through here tomorrow increasing the chance of rain, and Thursday should be the warmest day of the week.

As we head closer to the weekend, models suggest the cut-off low should be picked up by the jet stream and a high amplitude pattern will ensue with a trough in place over the Mid South by the weekend pushing high temperatures down to the low to mid 60s.

Highlights

-First day of Fall starts out cool and wet

-Cut-off 500 hPa low retrogrades westward

-Wet weekend in store

A potent upper level low is digging and progressing east-southeast at the moment over the midwest.  We have seen a nice drop in temperatures over the past few hours as the cold front that is currently draped over west-central KY has passed through.  Post-frontal rain showers have formed ahead of the upper level trough over eastern Missouri and will progress east-southeast towards the area.  Rain showers could start overnight but looks to begin after morning commute but may have to deal with patchy fog that may form under clear skies tonight.  Rain should exit the area by midday or early afternoon.  Also, the upper level low will continue to spin and become cut-off from the main flow overnight tonight bringing around a shortwave on the backside of the upper low that will bring the showers into the area tomorrow morning.  Skies should stay partly cloudy rest of the day today with a high around 71 and an overnight low around 50.  Clouds will start to roll in during the early morning hours.  The two figures below show the evolution of the cut-off low from this morning to midnight tonight, respectively.

GFS and NAM both show a halt in the progression of the 500 hPA cut-off low over northern Illinois. NAM actually has the cut-off low retrograding westward over northeastern Missouri, whereas GFS has is staying over northern Illinois.  Regardless, we will be impacted with showers throughout the weekend due to shortwaves transversing around the cut-off low. Saturday evening through Monday morning the Ohio River Valley could see on the higher end of one inch of precipitation in a few areas. However, expect most areas to be around a half inch during this period.  Highs will not fluctuate much and will tend to stay around the 70 degree mark with lows in the low 50s.  Does look like at the beginning of the work week we will start to see a drying, warming trend.

Friday: Partly Cloudy, with high 71 and low 50. North winds around 6 mph.

Saturday: 40% chance showers , mainly after mid-morning. High 71 and low 53. Wind becoming south at 5 mph.

Sunday: 50% chance showers with high 73 and low 53.  South winds at 5 mph.

Highlights

-Rain to start work week

– Temperatures will gradually decrease throughout the week from upper 70s to low 70s

– Atmosphere revamps for another round of possibly strong T’storms Wednesday evening

A neutrally tilted shortwave trough axis extending from Minnesota to Missouri is ejecting eastward towards our region.  As it does so, moisture is building into the area off southerly winds ahead of this system.  Showers will impact much of the Ohio River Valley  overnight tonight and continue throughout the day ending late Monday.  The cold front associated with this first system is rather weak, but still expect to receive a half  to three-quarters of an inch of precipitation.  The 500 hPa map below shows the shortwave that will impact our weather at the start of the week.

Clouds will diminish overnight Monday giving way to partly cloudy skies on Tuesday with a high of 79 and an overnight low of 63.  However, winds will be persistent from the south around 8-12 knots bringing in moisture from the Gulf. That said, clouds will be increasing during the afternoon on Tuesday as showers from the next system will reach our area early Wednesday morning.  There is instability for the next system to tap into bringing chances for scattered strong thunderstorms.  However, since there is a lack in deep moisture, it will keep severe storms to a minimum (Skew-T below). There is sufficient enough shear to structure a squall line that will encompass much of the Ohio River valley. Wednesday evening.  If a strong storm does form in the area, the storm will be limited to small hail and damaging straight line winds.  A 500 hPa map below shows the system that will impact the region Wednesday into Thursday.

The front will move slowly east of us on Thursday with chances diminishing on Thursday as the day goes on.  Friday through Sunday looks to be great with highs only reaching into the lower 70s and lows in the lowers 50s under sunny skies.

Monday: Showers/T’storms likely throughout the day into the overnight hours.  High around 75 and low around 64 and south winds persistent at 8-12 knots.  Expect half inch of precipitation.

Tuesday:  Partly cloudy skies with a high around 79 and a low around 64.  Slight chance of showers late Tuesday night.

Wednesday: 50% chance of Showers/T’strorms later in the afternoon with a high around 78 and a low around 60.  Some strongs could be severe with small hail and damaging winds.

After a welcome dose of rainfall and cooler temperatures this past week, a ridge of high pressure will build into the Bowling Green region this weekend. This means that picture-perfect weather will be the rule. Highs are expected to be in the upper 60s once again today, with temperatures warming into the mid 70s under mostly sunny skies on Saturday and into the low 80s on Sunday. Overnight lows will be on the cool side this weekend, with lows in the low 50s expected tonight and upper 50s tomorrow night.

Our next shot at rainfall will begin late Sunday, with off and on chances of precipitation early next week as a stationary front meanders around the area. Until then, get outside and enjoy the nice weather!