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.