Wills' Geek Weather

2 P.M. update

Sat. may 30, 2009
So far, the expected line of thunderstorms has not developed.  It looks like a line might be trying to form from west of Indy southwestward to the Wabash River north if Evansville.  If so, ideas discussed in previous post should be pushed back into the 4-6 P.M. time frame.  Otherwise, we may miss the rain entirely.  (Current cluster of showers/t-storms west and southwest of Louisville will dip southeast of Metro Louisville and should not bring any rain to us.)

Severe Storm threat this afternoon

11 A.M. Sat., May 30, 2009

A line of thunderstorms is expected to form northwest of Louisville early this afternoon, then race southeast over southern In/central KY.  Currently, timing looks like storms moving through Louisville area between 2-4 P.M.  But, it's hard to get a good handle on timing now since the line hasn't formed yet.
    We don't have a whole lot of heat and humidity to work with, but the upper level wind fields are fairly strong, so it may be that nature has "just enough" of everything it needs to send a fast-moving line of gusty storms across the Ohio Valley this afternoon.  Storm Prediction Center has us in a "slight risk" for severe storms category with straight-line winds as the primary threat.
    What to Expect:  Quick-hitting line of thunderstorms to cross the Louisville Metro during the mid-afternoon.  Brief heavy rains likely.  Strong, and possibly severe, winds as the line moves through.  (Wind gusts to 50 mph or higher in a few areas.)  From rain onset to end should be 30-45 minutes.

Update

1:15 P.M. Friday, may 8, 2009

Although the MCS/derecho has decreased in size a lot over the past three hours, it is still maintaining a strong/severe line of thunderstorms as it comes east.  And, it is taking a slightly more northerly path than previously.  Thus, the threat for severe wind gusts may now reach as far north as I-64 between 3-5 P.M.  Nevertheless, the PRIMARY area of concern remains over central/southern KY where more instability is available.  This is still a very potent system with the potential for widespread severe wind damage south of the Louisville area.  However, the system does continue to slowly weaken/shrink as it races east.  Let's hope that trend continues.

Severe storms?

Friday morning, may 8, 2009
A very serious severe thunderstorm situation appears headed toward southwestern KY.  Large, powerful derecho has been creating widespread damaging winds (80 mph plus), and a few tornadoes, over southern Missouri.  System will probably weaken very slowly as it moves across extreme southern IL into southwestern KY by early afternoon.  Widespread wind damage should continue with this system. 

While this system is bad news for western and southern KY, it is good news for the WAVE 3 viewing area.  Derecho system will pass south of us - dropping into central TN by early evening IF it can last that long.  That path is important.  North of the system's path, there will be plenty of rain and thundershowers, but any severe weather is unlikely.  The potentially dangerous weather is along and south of the path.

Some information of derechos from the Storm Prediction Center...

What is a derecho?
A derecho (pronounced similar to "deh-REY-cho" in English is a widespread and long lived windstorm that is associated with a band of rapidly moving showers or thunderstorms.

Because derecho is a Spanish word the plural term is "derechos".

What is the origin of the term "derecho"?
The word "derecho" was coined by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa, in a paper published in the American Meteorological Journal in 1888.. Dr. Hinrichs chose this terminology for thunderstorm induced straight-line winds as an analog to the word tornado. Derecho is a Spanish word which can be defined as "direct" or "straight ahead" while tornado is thought by some, including Dr. Hinrichs, to have been derived from the Spanish word "tornar" which means "to turn".

DERECHO PRODUCING STORMS

What kind of storms cause derechos?
Derechos are associated with a band of showers or thunderstorms that are often "curved" in shape. These bowed out storms are called "bow echoes". A derecho can be associated with a single bow echo or multiple bow echoes. The bow echoes may vary in scale and may die out and redevelop during the course of derecho evolution. Further, derecho winds can be enhanced on a smaller scale by embedded supercells within the derecho producing storm system.

Tornado Watch

1 P.M. Friday, April 10, 2009
Tornado Watch has been extended northward to cover just about all the WAVE 3 viewing area.  Not to worry.  Little has changed.  Primary upper system will move over KY this afternoon keeping primary threat from winds/tornadoes from the southern third of KY down into TN.  After our hailers around the area this morning, the rest of the afternoon should be quieter here - some showers/t-storms, but severe storm chances look very slim.

deja vu?

Midday Thursday, April 9, 2009

We've had a long series of upper troughs digging into the western U.S. and then curving around toward the eastern U.S.  Most recent one should be here tomorrow with a rainy day.  Earlier, it looked like we'd see a lot of rain (1-2") in Louisville and higher totals over southern Indiana.  Plus, the models were taking the weakening surface low north of the Ohio River.  That would open the door to severe weather.

The 12Z runs this morning have made a big shift in keeping the upper low and the surface low south of (or right over) the Louisville area.  If that verifies, it really changes things for us:  1). rain starts later (midnight or after) and ends up being about an inch less than previous expectations (good news for flood-prone southern Indiana), and 2). severe thunderstorm threat drops south (out of the Louisville area) although southern KY and TN could see strong to severe storms late tonight and/or late tomorrow.

Severe storm threat diminishing.

5 P.M. Sunday April 5, 2009

Low pressure cutting across southern Indiana and a very strong upper level wind field, seemingly ideal conditions for severe storms, so far have combined for a lot of rain, but not the outbreak of severe storms expected earlier.  Moisture has been the problem as the instability of the atmosphere has remained very weak.  So far, the upper air dynamics have been unable to overcome the lack of strong instability.  And, at least as far as our area is concerned, it looks like the system is running out of time.  Numerous showers and thunderstorms cover southern Indiana and western KY.  As of 5 P.M., the strongest thunderstorm nearby was over southern Breckenridge County and moving eastnortheast about 40 mph.  Up until 7 P.M., the initial line of thunderstorms will move through the Louisville area and most of KY west of I-65.  If anything strong or severe is going to develop, it'll be with this line.  But, that remains doubtful as instability remains very low and will be dropping.

So, in a nutshell, the threat for severe thunderstorms is very small and diminishing quickly.

Tornado Watch

2 P.M. Sunday, April 5, 2009

Tornado Watch until 7 P.M. for western half of KY and SW quarter of IN. 

Certainly looks like the moisture is going to get here and the classic severe pattern remains in place.  So far, nothing is going on, but look for that to change soon.  We'll probably start getting severe reports by 3 P.M. out of western KY and southern IL.

Severe storms possible by late afternoon.

11 A.M. Sunday, April 5, 2009

Conditions continue to look more and more like we'll be under the gun for severe storms later today (probably in Louisville area between 5 and 8 P.M.).  Strong surface low will cross southern IL/IN this afternoon and evening.  That puts all the "dynamics" in place for severe storms over southern IN and KY.  Only missing ingredient currently is moisture and that seems to be racing northward along the Mississippi Valley and spreading eastward.  In fact, thunderstorm initiation has occurred earlier than anticipated, so it looks as though enough moisture will arrive in time to be in play as the cold front moves across southern IN and KY late this afternoon and evening. 
   Assuming moisture does arrive, the scenario should play out about like this - several supercells should pop up over western KY and southwest IN early this afternoon.  Would not be surprised if they produced some tornadoes.  With time (and before it approaches the I-65 corridor), the system should evolve into more of a linear thunderstorm complex.  Primary threat would then shift to hail and (especially) high winds. 
   However, there is a chance that some additional discrete supercells could still form out ahead of the primary thunderstorm line.  If so, watch out! They could spin up some tornadoes.
     Trying to read the Storm Prediction Center's mind...I'd imagine we'll go under a Tornado Watch this afternoon.  However, for most of us the primary threat will not be from tornadoes.  Rather, straight line wind gusts of 50-70 mph could be fairly common.

Rainy night

7 P.M. Thursday, April 2, 2009

SPC hates to give up, but it looks like any threat for severe weather is fading fast.  So, we'll probably be needlessly under a Tornado Watch for several more hours.  But, a large area of moderate to heavy rain/thunderstorms will move over the area for the next few hours.
Shakespeare got this one right centuries ago - "Much Ado About Nothing."