July – September, 2015
A variety of weather conditions were seen throughout this spring across Florida. Three months ago, KBDI values over 500 engulfed the southern regions of Florida; going into the start of summer, KBDI values over 500 primarily affect the big bend regions, Gulf coastal areas of FFS Regions 1 and 2, and the Atlantic coastal areas of FFS Region 4. The early part of spring brought moist air into the southeast U.S. as well as the presence of multiple frontal systems which allowed for numerous rainfall events. In the month of May, a prominent high pressure regime over the state caused much drier conditions and allowed the statewide KBDI values to begin creeping back up across the state. The majority of June brought a summer-like weather pattern to the state with the bulk of the rainfall coming from isolated showers and storms in the afternoon hours.
In April, a dominant upper-level low positioned itself over the southwest U.S. which allowed the jet stream to slump south and move over Florida throughout most of the month. These conditions allowed for multiple frontal passages and upper-level forcing which helped to bring many days of rainfall to Florida. The majority of the state experienced average to above-average rainfall in April with the largest surpluses seen in the western panhandle and north-central regions of FFS Region 4. In May, the jet stream was positioned much more northward and a strong ridging pattern sat over the state which promoted hot, sunny conditions along with sinking air in the upper levels. This pattern allowed for a much drier month of May with the majority of the rainfall forming near coastlines and the bulk of the convection occurring in the lower-levels. The majority of the state experienced below-average rainfall in May, with small surpluses seen near the Gulf coast of FFS Regions 2 – 4 and north-central areas of the FL panhandle. In June, the bulk of the jet stream stayed well to the north of Florida; however, troughing patterns were much more frequent which assisted in vertical motion and enhanced the overall coverage of afternoon showers and storms. Moist air and convergence in the lower-levels, as well as upper-level forcing, allowed isolated showers and storms to be more widespread across the state, bringing rainfall to locations that desperately needed it. Despite this fact, the absence of frontal systems deterred any long-term rainfall and as a result, most of the state experienced below-average rainfall with the greatest deficits seen in the southeastern FFS Region 4 area.
In the past three months, the entire state has seen above-average temperatures with the highest anomalies occurring in FFS Regions 1 – 3 around 2 – 4 degrees Fahrenheit above normal. The sea surface temperature (SST) anomalies in the equatorial Pacific strengthened in April thru June which helped to strengthen the El Niño status; however, above-average temperatures and below-average rainfall, were the most consistent conditions seen in Florida in the past three months due to the detachment from the jet stream.
Long-Range Weather Outlook
Looking ahead at the next three months, the Climate Prediction Center (CPC) is forecasting above-average temperatures for the state of Florida with the greatest chances for above-average temperatures occurring over the Florida peninsula. Below average rainfall is being forecasted for the central and western areas of the panhandle, with the best chances for below-average rainfall occurring in the western panhandle region. The rest of the state will possess an equal chance for above-average, normal, and below-average rainfall for the next three months. Now that the upper-levels reflect a summer pattern and the jet stream remains well to the north, the rainfall in Florida for the next three months will likely rely on low-level lifting caused by mesoscale boundaries such as sea breeze fronts in the afternoon and gust fronts caused by storms and showers that have already formed. For optimal rain coverage from these systems, ample moisture will need to be present in the lower-levels along with a good amount of instability from steep lapse rates; both of these conditions have been present recently and look to continue on for at least the next month.
Strong El-Nino conditions are expected to occur for the next three months. The CPC is currently forecasting a greater-than 90% chance that El Niño conditions will continue through the fall of 2015 and about an 85% chance that it will last through the 2015 – 2016 winter. Despite these conditions, a weak ENSO signal is being forecasted for the state of Florida with below-average rainfall in the panhandle and above-average temperatures throughout the state for the next three months. Also, the strong El Niño conditions in the summer will help to strengthen the Atlantic sub-tropical ridge which will in return strengthen the Atlantic trade winds which will help to deter tropical cyclones from forming over the Atlantic. The NOAA is currently forecasting a 70% chance of a below-normal tropical season, and the scientists at Colorado State are forecasting a 15% chance of the state of Florida to experience at least one major (category 3 – 5) hurricane.
Summary and Fire Potential Outlook
Overall, the last three months have supplied about 60% of Florida with below-average precipitation amounts; the greatest deficit of rainfall was seen in the southeastern areas of FFS Region 4. The remaining 40% of the state saw average to above-average rainfall with the greatest surpluses seen over the Tampa areas as well as the northwestern and west-central sectors of FFS Region 4. Currently the CPC is predicting below-average rainfall for the western and central panhandle while the rest of the state is expected to receive an equal chance for above-average, normal, and below-average rainfall for the next three months.
The amount of rainfall that most of the state will see for the next three months will occur from low-level convergence in the boundary layer from sea breezes and outflow boundaries. The coverage of rainfall will rely on the amount of instability and moisture in the boundary layer as well as an absence of ridging in the upper-levels. The CPC is also predicting above-average temperatures for the next three months throughout the state which will help in destabilizing the atmosphere in the afternoon hours by steepening low-level lapse rates.
As the El-Nino conditions strengthen, the CPC is forecasting a greater-than 90% chance of El Niño conditions through the Fall of 2015. Additionally, NOAA is forecasting a below-normal Atlantic hurricane season mostly due to factors caused by the strength of the upcoming El Niño. As the El Niño conditions strengthen in the equatorial Pacific, it will be interesting to see if the ENSO signal in the southeast U.S begins to strengthen which is not being forecasted by the CPC at this time. Below-normal rainfall and above-average temperatures could increase the number of days for optimal prescribed burning conditions; however, it could also raise the chance of any severe wildfire activity.
The next seasonal outlook will be the first week in October 2015.
Drought Outlook (NOAA)