Snow Potential In Jacksonville

A unique forecasting challenge is coming up this week for the potential of winter precipitation on the First Coast.

An unusual long lasting Arctic outbreak of air is holding over Jacksonville all week. A series of freezes is in the forecast.  These charts from Sunday represent the potental for sleet over southern Georgia Friday morning. The top two maps show temperatures and wind at 850 mb which is about 5000 ft. The first map is 1 am with the freezing isotherm on the state line. The second map shows the freezing level moving south by noon over Orlando. This is the new cold air pushing south behind Friday's front. If this were earlier it would be certain to snow in Jacksonville.  But the warmer air in advance of a cold front will cause the boundary layer or air within 3000' of the ground to be warmer earlier in the morning.  The next map is 7 am with the freezing isotherm on the state line.

The bottom two charts are a key tool for determining the type of precipitation in the winter.  Thickness charts like these represent the depth of each layer in the atmosphere.  Colder air makes the air  contract and is less thick.  Warmer air increases thickness.  Snow is a good bet if the last blue line is over your forecast location. This is the 540 thickness line. You can see it is almost over Tampa early in the week on Monday night at 1 am.  The green represents rain.  It is cold and dry in Jacksonville during this time period and temperatures are in the low to mid 20s at the surface. 

But by Thursday night when the rain arrives, the thickness increases over Jacksonville.  This is why the forecast is tricky.  This GFS forecast model run indicates the green over coastal Georgia would be snow.  Sleet is more likely due to possible warmer air just above the ground in northeast Florida.  The accuracy of computer models in pinpointing exact temperatures in the vertical layers of the atmosphere is a challenge that can bust a forecast. 

Super strong cold advection which is the cold air moving south is shown by the parallel black pressure lines (isobars) at right angles perpendicular to the blue thickness dashed lines.  This means cold and windy conditions Friday as the reinforcing new blast of arctic air keeps freezes going into the weekend.

Hail Size Increased In Severe Storm Warnings

When you think of what makes thunderstorms severe, most people assume lightning is a factor.  But it is not part of the criterion for issuing severe warnings.  The National Weather Service (NWS) is responsible for issuing severe thunderstorm warnings when winds reach 58 mph or  higher.  But hail is also part of the factor.  It used to be when hail reached 3/4 of an inch a storm became severe.  But the now the  threashold is 1."

The one inch criterion had been use in hail country of the southern plains and over  the western United States. Hail is common in these locations due to lower freezing levels in the atmosphere and stronger vertical updrafts.  Updrafts result in the formation of hail when water droplets freeze and get carried upward until the they freeze and fall back down.  The cycle repetes until the hail grows too large for the updraft to keep it suspended in the air.

This is a good idea because it reduces the perception of over warning saturation during our busy summer storms season. Warnings are often issued for penny sized or 3/4" hail.  It is rare for 1" golf ball sized hail to occur in our area. Now thunderstom warnings carry more weight and hopefully people will take them seriously when storms hit.

TWO TYPES OF WATERSPOUTS

It is not often we see waterspouts in Jacksonville.  Well today I think we had the most famous waterspout in Jacksonville history. Over 50 of you sent us pictures and the waterspout seemed poised for the sky cameras as it slowly moved north over the river today.  Waterspouts typically are located in the waterspout capital of the world, the Florida Keys.  The Keys provide a perfect environment for an average of 400 waterspouts every year.  Jacksonville's balmy, hot, and humid weather was just like the islands.

Light winds made for favorable waterspout conditions.  Outflow boundaries from previous thunderstorms moves east from the I-75 area and collided with the east coast sea breeze resulting in severe storms with 1" hail and lightning which struck a home on fire near Normandy on the Westside.  As the storms moved north from Putnam and St. Johns county, the perfect placement of the river underneath provided an ideal location for the waterspout.  Fortunately the long length of the river was the reason why the waterspout never moved ashore.  Typically they have a lifespan of only 15 minutes.


There are two types of waterspouts and fortunately we had the weaker non-tornadic waterspout variety.   The other, tornadic waterspouts, begin as true tornadoes over land in association with a thunderstorm, and then move out over the water. They can be large and are capable of considerable destruction. What we had was a fair weather waterspout. They form only over open water. They develop at the surface  and climb up in association with warm water temperatures and high humidity in the lowest several thousand feet of the atmosphere. They are usually small, relatively brief, and less dangerous. The fair weather variety of waterspout is much more common than the tornadic.  Needless to say it was scary for everyone today.  Thanks everyone for all the great photos you emailed.

GETTING HOT THIS WEEK

The hottest month in Jacksonville is July. Temperatures most days in the month are 90 degrees or higher. The end of July begins the start of summer and First Coast locals always sweat out hot afternoons waiting for cooling thunderstorms. In fact, over a 30 year average of July months, 23 days are above 89 degrees.   

Get ready for a return to ninety's all week long into next week. June is often a scorcher.  In 2008, June had 21 days at 90 or above.  That was also the hottest month out of the '08 summer.  The average for 90 degree days in June is 16.  The onset to this upcoming heat wave is due to a lack of forecasted rain and a ridge of high pressure in the Gulf.

The 500 mb ridge in the Gulf results in sinking air over Jacksonville which suppresses thunderstorms.  The blue vectors show NW winds around the right side of the Gulf ridge.  
Looks like the pool or river will be the best places to cool off.

TAKING MY JOB TO THE BEACH

Well some days are better than others at work.  For this interview we got to do something different.  A water interview with professional surfer Karina Petroni sure beats sitting at my computer desk.  Thanks to Josh Hansbrough at Birdman Productions for taking the water shots.