The Difficulties in a Florida Forecast

Each of the last two days, the National Weather Service offices in Peninsular Florida have been caught off guard by the predictable daily pattern.  On each day, the forecast was predicted to be normal: hot and humid with thunderstorms.  On both days, there was a tornado, one near West Palm Beach on Saturday and one near Fort Pierce on Friday.  Naturally, the NWS was surprised and is now adding tornado/funnel cloud potentials to their forecasts, but it demonstrates how no one is immune to letting their guard down against the weather when phenomena go unseen for long periods of time.

 Thunderstorms are part of daily life in Florida.  In fact, Florida leads the nation in thunderstorm days, so correct your friends next time they say they came to Florida for the sun.  This leads to the topic: how many factors go into a Florida thunderstorm forecast?

Here are some of the factors that go into the forecast: the air temperature, the humidity, the 300, 500, 700 and 850-mb patterns, surface highs and lows, precipitable water (PWAT), the time of formation and strength of the seabreeze, local cloud cover, etc.  These factors will be presented in the order they are listed.

Air temperature: obviously it is pretty hot in Florida during the summer.  High temperatures are typically in 88-94°F range, with some days getting up to 98°F but never reaching 100°F.  Without the high temperatures, there are no thermals; the thermals are necessary to cloud formation.  With so much heat at the surface, the thermals can continuously generate.  Morning winds are generally weak, which helps support the thermals.

Humidity: there must some surface moisture to support the formation of clouds.  More details shall come  later.

The 300-mb pattern: this factor is most useful in forecasting the future of surface highs and lows.  From the analysis of the 300-mb map, the forecaster can also see whether upper-level moisture will be moving toward or away from a region.

The 500-mb pattern: conditions at the 500-mb layer, particularly temperature and dewpoint, have long been the most important factor in several forecasting indices.  Surface winds may be conjectured from this layer.

The 700-mb pattern: here is one of the most important factors.  From the winds at this layer, the direction in which thunderstorms will travel may be determined.  This "synoptic flow" most often will determine the trajectory of thunderstorms when the other factors that affect this motion (especially seabreezes) are weak or are cancelled (such as when two storms collide and form a new thunderstorm).  Also, if there is little moisture at this layer, then storm formation greatly diminishes.

The 850-mb pattern: much like the 700-mb pattern, a lack of moisture at the 850-mb layer diminishes storm formation.  Lapse rates above this layer must be low enough to keep the atmosphere unstable to allow for cloud/storm formation.

Surface patterns: the Bermuda High and other highs to the east of Florida greatly favors storm formation, as does a low to the west, because both cause moisture and warmth to push up from the south and into the peninsula.  If highs and lows are positioned so as to drive down winds from the north, then storm formation is diminished. 

PWAT: precipitable water tells the forecaster how much moisture is in the entire atmospheric column.  If this value is low, then storms are not likely to form.  Typical values favoring storm formation are > 2.0".  The distribution matters as well as mentioned in the 700 and 850-mb patterns.

Time of formation and strength of the seabreeze: the timing of the seabreeze is important.  If the seabreeze forms too soon, then the air will be cooled off and the thermals that generate due to daytime heating will be weakened.  If it forms too late, then the east and west coast seabreezes will not be able to collide.  The same is true of weak and strong seabreezes.  On top of all this, the seabreezes can collide and, especially if one or both seabreezes are weak, not generate any storms.  When the east coast seabreeze generates cloud convection, the storms tend to be stationary or move weakly to the west.  When the west coast seabreeze does the same, the storms tend to move to the east.  This is because the synoptic flow is generally from the west moving toward the east.

Local cloud cover: this is the last factor that will be discussed within this post.  If there are clouds already present over an area, then daytime heating is severely diminished, pretty much ending all chances of thunderstorms forming later.