Author Archives: wx

Interesting situation for a couple of hours

Sunday, April 14, 2019  12:30 P.M.

Thunderstorms possible

Sudden heating and increasing dew points for the past few hours have pushed us into an interesting weather pattern for the next 2-3 hours.  A rather diffuse low pressure system has been over us this morning (it brought us the overnight rain).  The low is now moving northeast into northern IN/MI.  That’s why we’ve seen the rapid increase in winds.  But, low level moisture is now dropping!  Also, in the wake of the low, a cold front is now marching from west to east across southern IN and KY.  The front should pass the I-65 corridor between 2 -3 P.M.

Ahead of the front scattered thunderstorms are popping up. Due to the decreasing moisture, no solid line of storms is expected.  But, some isolated storm or two could generate some gusty winds, perhaps (but not likely) even up to 50 mph.  Any thunderstorm threat locally will be over by 3 P.M.  Stronger storms, even a severe storm or two, could pop up over eastern KY later this afternoon.

About those winds…

1).  We are under a Wind Advisory this afternoon.

2).  National Weather Service Definition of a Wind Advisory is sustained winds of 31-39 mph for at least an hour and/or wind gusts of 46 mph to 57 mph.

3).  The current NWS forecast predicts wind gusts up to 40 mph.

4).  The current Hazardous Weather Advisory and Weather Advisory say gusts to 45 mph.

5).  Go figure!  Why issue an Advisory for something they aren’t forecasting?

6).  And, the prediction isn’t even going to happen.  The strongest wind fields are over eastern KY (east of I-75).  Locally, our wind gusts should top out in the mid 30’s.

Stuff

Q is the only letter of the alphabet that does not appear in any state name.

 

Astronomical Spring arrives at 5:58 this evening

Wed., March 20, 2019

Is the Equinox equal?

The Spring (or vernal) Equinox arrives this evening as the sun will be directly above the Equator at 5:58 EDT.  And, even though the word equinox roughly translates into “equal night”, that’s not exactly true.  The common interpretation for the day of the equinox is 12 hours of day and 12 hours of night.   But if you look at the sunrise/sunset table, you’ll see today has 12 hours 8 minutes between sunrise and sunset.  So, day and night are not 12-12 as most people think.

What accounts for the extra 8 minutes?  Our atmosphere! On its roughly 93 million mile journey to Earth, sunlight travels through a vacuum.  For the last 10-20 miles, however, the sunlight encounters a higher density medium (the atmosphere).  The closer it comes to the surface, the higher the density.  When light travels into a region of higher density, it bends.  This bending is properly called refraction.  Or, in the case of the sunlight and Earth, atmospheric refraction.

Courtesy: timeanddate.com

So, when we see the sun sitting on the horizon, it is actually below the horizon.  But, we still “see” the sun because the atmosphere refracts (bends) the sunlight back to our eyes.  From the time we perceive the sun on the horizon until it drops out of our vision, four minutes go by.

So, if the Earth had no atmosphere (we’d be in BIG trouble), sun rise and set would be exactly 12 hours apart.  Because we do have an atmosphere, you must add the extra 4 minutes at sunrise and four more at sunset. Thus, the day of the Equinox has 12 hours 8 minutes between sunrise and sunset

Full moon tonight

The March full moon is most commonly called the Worm Moon (think robins in the early spring).  It has many lessor known names as well…Lenten Moon,  Crow Moon, Crust Moon, Chaste Moon, Sugar Moon and Sap Moon.

Our moon will be “full” at 9:42 P.M.  Moon rise tonight at 7:38 PM EDT.

Wait, one more thing…

Just trying to wrap things up in a neat little package…

Q.  Why does the moon look much larger near the horizon than it does higher in the sky?

Ans:  atmospheric refraction (plus something called the apparent shape of the sky)                         Note:  The moon on the horizon problem is still considered as the Moon Illusion.  People have been trying to explain it for over 2000 years.  Ptolemy was one the first to try.  His suggestion was very similar to the one I gave above.  But, it’s still under debate.

A new twist

Update  5:25 P.M.

The line of storms did strengthen, but not much.  Line should push across Jefferson County without any significant problems.  East of Jefferson County, the line should be stronger, with possibly a severe storm warning or two this evening.

On the positive side, the squall line will actually decrease the strong wind fields  for an hour or two

 

 

Update  4:45 P.M.

Radar IS showing a definite uptick in strength of the approaching thunderstorms.  Between now and 6 P.M., expect a line of strong thunderstorms to move across the metro area.  Expect brief heavy downpours, additional strong gust winds and possibly some hail.

 

 

4:15 P.M. Thurs., March 14, 2019

Line of storms still crawling eastward

As of 4 P.M. the only strong-to-severe thunderstorm left is racing across northern Clark Co. IN.

Elsewhere, the remaining narrow line of storms still lies west of the Louisville area.  No severe activity has been noted in the part of the line headed our way.

The items above have worked about as expected since the weakening trend noticed a couple of hours ago.  However, we are not out of the woods yet.  One of the two short-term models predicts the line to re-intensify as it approaches I-65 and become strong again east of I-65 for a few hours this evening. That’s the HRRR model.  It’s partner, the RAP, does not.

As of this time, local radar does not show any rebuilding… yet.  If that continues and the RAP is correct, we should have minimal effects from the squall line. However, we’ll have to watch out for any intensification of the line as that would increase the potential for possible severe wind gusts – especially along and east of I-65.

WOW

I just noticed that the airport reported 61 mph wind gusts earlier this afternoon.  Bowman Field’s top gust reached 56 mph.  Ft Knox topped out at 53 mph.  Mostly likely, if we do get thunderstorms in the next hour or two, they won’t match those winds!

Timing update

2 P.M. update

Thursday, March 14, 2019

1).  Recent data shows the upper and lower atmosphere are beginning to “uncouple.”  That means the “sweet spot” of atmospheric conditions that has spawned the severe weather to our west for the past few hours is beginning to fade.

2).  The weakening has enabled the line of storms to slow its eastward progression.

3).  New “best estimate” for pushing through Louisville Metro is now 4:30 to 6 P.M.

4).  Until then, surface winds will continue to be very gusty – up to around 40-45 mph.

5).  When thunderstorms arrive, the primary threat will be strong wind gusts – up to 50 mph or so.  Tornado threat has lowered.

New time estimate

1:45 P.M. Thursday, March 14, 2019

1).  Signs  are showing the upper atmosphere and the lower atmosphere are beginning to “uncouple.”  Basically, that means that the “sweet spot” for severe weather production is fading away.

2).  The general weakening will allow the system to slow its eastward progression.

3).  Best time for Louisville now looks to be 4:30 until 6 P.M.

4).  Strong gusty winds (non thunderstorm) will continue.  Thunderstorm winds could be strong (50 mph or so), but tornado threat is lowering.

Tornado Watch

 

1 P.M. update

Supercells still forming over west KY into SW IN.  Several tornadoes have been reported.

Adjust time of arrival in Louisville area to 3 P.M. – 5 P.M. Otherwise, discussion below still looks okay.

March 14, 2019  Noon

Potentially Damaging Storms late this afternoon

One change since yesterday’s post is the addition of additional moisture into the equation.  The very strong wind fields are still in place.  Now with the added moisture, it looks like nature’s two primary severe weather producers – dynamics and thermodynamics – will interact for a few hours this afternoon.  Luckily for us, the worst should be over before reaching the Louisville area.

Super cells are moving along the Ohio River in western KY and southern IL.  Several warnings have already been issued.  This area of severe weather will move rapidly northeast into southwest IN and then central IN over the next few hours.  It will not affect the Louisville area.

With afternoon heating, additional super cells are likely to form.  Models continue to put the primary threat north of the Ohio River.  However, I wouldn’t be surprised to see additional development over western KY early this afternoon.  Any super cell will be capable of producing large hail and tornadoes.

The threat to Louisville area will be between 4 P.M. and 7 P.M.  as a scattered to broken line of thunderstorms moves through.  Primary threat by that time should be strong winds, but depending upon how things evolve over the next few hours, it could get worse (or better).

Updates to follow…

Severe Storm “potential”, take 3

Wed., March 13, 2019  6 P.M.

Thunderstorms likely tomorrow

This is the third time in recent weeks that the Storm Prediction Center(SPC) has put us in an area of potential severe storms.  We are in a “slight risk” region tomorrow.

The first two times, the situation was the same.  Very strong dynamics (wind fields) but little support from the thermodynamics (heat and humidity).  As the old song goes, it takes two to tango.  Both times, the partnership never happened, so the weather didn’t get to dance.

As is common in the early spring, the (almost) same situation exists again.  But, there are some differences.  Strong dynamics (upper winds, strong surface storm) are once again the prominent feature.  But, the dynamics over the midwest/Ohio Valley will be weakening through tomorrow.  Northward flow of heat and humidity is currently strong, but it also is expected to weaken.

So, overall the threat for severe storms over KY and southern IN tomorrow looks pretty small.  The GFS is weak with storm development while the NAM is a little stronger and more in line with the SPC ideas.  However, it keeps storms pretty weak over KY, but does point out a stronger threat over the northern two-thirds of IN tomorrow afternoon.

Here’s my forecast:

Mostly cloudy and breezy tonight…low…58.  70%  chance for showers and/or thunderstorms after 4 A.M. continuing into mid-morning.  (Rain/storms will be most likely along and west of I-65.)

Another area of showers and thunderstorms will likely cross the area between 4 P.M. and 7 P.M.  tomorrow.  Gusty winds around thunderstorms, but severe weather threat is quite low.  Cooler weather arrives Friday.

Stuff

Tomorrow is Pi Day.  3.14 on the calendar.  Use some math, it won’t hurt (much).

 

Stormy evening

Noon, Sat. Feb.23, 2019

Rain, thunder late this afternoon into the evening

Some quick notes…

1).  The forecast for the Ohio River flood in Louisville has lowered about four FEET since yesterday.  Rise above flood stage about midday tomorrow…crest just over 25 feet Tuesday.

2).  Strong storm over the upper midwest will push a cold front through the area this evening.  Showers and thunderstorms will be likely between about 4 P.M. and 10 P.M.  Rain: between .5″ and 1″.

3).  Severe weather is not likely, but we will have strong winds during the night.  Wind gusts are likely above 30 mph over much of the area.  Some gusts could reach into the 40+ mph range near thunderstorms.

4). Strong winds continue overnight and tomorrow – we could see gusts into the 35-45 mph range tomorrow morning.

 

Saturday: wet, maybe stormy

6 P.M. Fri., Feb. 22, 2019

Model differences continue

First, we’ll take a look at rain.  The GFS and NAM have lowered their rain forecast for tomorrow, but the GFS is still about a half-inch heavier.  However, both models are way down from earlier this week.  (If memory serves me, I think the NWS was talking about a 5″-8″ total for the week.  About 2″ fell earlier this week.)  Now, the GFS is around an inch and the NAM is closer to a half-inch!  Current Ohio River forecasts, which include higher rainfall for tomorrow, call for the river to reach flood stage (23′) in Louisville tomorrow late afternoon and crest early next week about 15″ lower than last week’s crest.  If current rain forecasts are correct, that crest forecast will be revised downward.  Good news for the “river rats.”

Over the next 24 hours or so, we’ll see two episodes of rain.  Plenty of heavy rain will fall over southern KY tonight.  Some of the leftovers will push through northern KY/southern IN toward daybreak tomorrow.  That’s the easy part.

Later tomorrow

I hope you’ve heard of the 3 feet of snow at Flagstaff AZ (most in a 100 years) and the 1″ at Phoenix (almost unheard of).  That potent storm is now crossing the southern Rockies and will wreck havoc over the nation’s midsection tomorrow and tomorrow night.  The storm center will stay far to our northwest, but the cold front sweeping eastward south of the storm center is expected to create severe storms late tomorrow.

The highest concentration of severe weather is expected to be over western Tennessee.  The Storm Prediction Center has dropped the northern edge of its “slight risk” area down about 200 miles since this morning.  The western half of KY is in the slight risk area, but IN has been dropped.  But, will severe weather reach as far north as us?

That’s where the models diverge again.  There is no argument about the strong wind fields associated with this storm.  But the GFS and NAM don’t agree that warm, humid air is going to get this far north.  Sometimes the strong dynamics can overcome weak thermodynamics to create strong storms, but usually you’d like go see some help from both sides of the equation.

On that point, the GFS expects the thermodynamics requirement to be met and it’s statistical output prints out a high probability for severe storms around Louisville late tomorrow afternoon or evening.  On the other hand, the NAM holds sufficient moisture south of us and has a much lower severe storm threat for the Louisville area.

This is a gross simplification, but here’s what to watch for tomorrow afternoon.  The NAM is predicting a high tomorrow around 60 – not enough to free much thermodymanics.  Meanwhile, the GFS this morning was saying upper 60’s (latest is 66).  Upper 60’s would significantly rise chance for severe storms.  Still iffy in the mid 60’s, but the lower the “high”, the lower the chance for severe.  So, watch the temperature tomorrow afternoon.  The higher it gets, the higher the storm chance goes.

So, we have the GFS’s erratic behavior lately, the NAM’s accuracy recently, and the SPC backing away from the northward moisture expansion.  As the cold passes through late tomorrow, expect to see a line of heavy showers and some thunderstorms mixed in.  Strong winds will be possible, but “severe” storms are only a small possibility.

NOTE:  By definition, a severe thunderstorm must have at least 58 mph wind gusts.  Many Severe Thunderstorm Warnings are issued for storms that never reach that strength.

Updated forecasts put the Ohio over flood stage by Saturday

6 P.M. Thursday, Feb. 21, 2019

Another heavy dose of rain by Saturday

Models continue to generate 2″-3″ of rain from the expected rain system due to arrive tomorrow night and depart Saturday night.  Adding that information to the river forecasts puts the Ohio River at Louisville over Flood Stage early Saturday morning  AND continuing to rise through early next week.

Meanwhile, most of KY will get some rain tonight.  Only the northern areas should escape the light rain.  Along and north of I-64 should stay dry as a weak disturbance moves east over the Gulf states tonight.  Only the GFS brings rain as far north as Louisville this evening.  And that would be only a very small amount.  The other models keep us dry.  The GFS hasn’t been on its game lately, so I’ll stick with the dry forecast.