INTRODUCTION

This post examines the role of a sea breeze boundary triggering deep moisture convection and the favorable conditions at both the synoptic and mesoscales for their development. To guide us through this, we'll examine a case from June 4, 2006, when the Storm Prediction Center (SPC) issued Mesoscale Discussion (MD) 1064 concerning thunderstorm development along sea breeze boundaries.

Image 1: Mesoscale Discussion #1064. Note the forecaster was primarily concerned with convection along the east coast of Florida. (SOURCE: Storm Prediction Center)

The SPC issues Mesoscale Discussions when conditions are favorable for severe weather development. These provide meteorological explanations for what is currently happening and anticipated to happen at the mesoscale. This particular MD was issued at 1727Z (1:27 PM EDT) on June 4, 2006 concerning the possibility of severe storms developing along the sea breeze boundary on the Florida Peninsula, producing hail and gusty winds.

SEA BREEZE CIRCULATIONS

Before examining the conditions at both the synoptic and mesoscales, we'll review the concepts of the sea breeze circulation in order to relate to the conditions on June 4^th. Sea breeze circulations are thermally-forced, resulting from differential heating of land and water. As the land warms faster than adjacent water, the local pressure drops over land relative to areas over water. The result is cooler air moving onshore to fill the lower pressure. This cool air at the surface forms a boundary with the warmer air over land, resulting in forced lift and potentially convection. The surface air flow is generally perpendicular to the coast, and convection usually occurs just inland. Depending upon the synoptic flow, though, the sea breeze front may penetrate deep inland with a tailwind, or not form at all with a headwind. Convection is usually enhanced when these boundaries intersect another boundary or meet offshore synoptic flow.

Image 2: An idealized cross section of the Florida Peninsula shows the Gulf coast (west) and the Atlantic coast (east). Note the diurnal advance of the colder surface air off both the Gulf and Atlantic that triggers convection along the sea breeze front. (SOURCE: William Smith)

SYNOPTIC SCALE ANALYSIS

In order to understand the conditions at the mesoscale, it is first important to understand synoptic scale conditions present at the time. We'll first review the upper air at 12Z on June 4^th, 2006.

500 MB Analysis

The first impression at 500 mb is the weak wind speeds, with measurements between 15-20 kts over South Florida. While there is a weak trough at 500 mb over the region, vorticity and divergence downstream of the trough is weak in this area. The importance of the weak gradient is that sea breeze circulations form more readily in this environment. The winds are uniformly WNW at 500mb, which impacts the sea breezes on both the Atlantic and Gulf sides of the peninsula.

Image 3: 500 mb Analysis. Note the uniformity of the winds aloft and the rather weak gradient. Conditions like these are favorable for sea breeze formation. (SOURCE: Storm Prediction Center)

Also notice the temperatures at 500 mb associated with this trough are relatively colder than areas in the western Gulf of Mexico at -9°C and -10°C. Falling temperatures at upper levels steepens the environmental lapse rate, which enhances CAPE. The SPC forecaster mentions this feature in the MD:

"VIA PERIPHERY OF EASTERN STATES UPPER TROUGH...TEMPERATURES ALOFT
HAVE COOLED SLIGHTLY ACROSS THE FL PENINSULA SINCE YESTERDAY...WITH
-9.0 C AT TAMPA BAY/-10.0 C AT MIAMI AT 500 MB PER 12Z OBSERVED
RAOBS"

850 MB Analysis

A quick look at 850 MB shows winds are generally W to WNW at 10 knots. Conditions are favorable for a sea breeze with weak winds aloft, but note the apparent lack of shear from 500 to 850 MB.

Image 4: 850 mb Analysis. Again, notice the similar pattern at 850 as at 500. General westerly winds and a weak gradient. (SOURCE: Storm Prediction Center)

Surface Analysis

Focusing your attention on Florida, you'll immediately notice a stationary front draped over the northern half of the peninsula. Consistent with conditions in the UA, there is a weak surface pressure gradient over Florida. Also at the surface, winds at 12Z are generally blowing offshore, with no synoptic-scale motion.

Image 5: Surface Map at 12Z June 4, 2006. The offshore flow suggests the presence of a nocturnal land breeze, with a favorable synoptic environment (weak gradient) in place. (SOURCE: Hydrometeorological Prediction Center)

Notice that at 18Z, little has changed with the surface pressure gradient. However, the surface winds have changed significantly reflecting the sea breeze.

Image 6: Surface Map at 00Z June 5th, 2006. The synoptic scale environment has remained generally consistent and favorable for sea breeze formation. (SOURCE: Hydrometeorological Prediction Center)

MESOSCALE ANALYSIS

Discussion In evaluating the mesoscale, we will look first at two major factors for enhanced convection: CAPE and Vertical Wind Shear. We'll then evaluate the forecast in the MD and discuss how and why the convection occurred.

ML CAPE

We evaluate the potential for strong updrafts by using CAPE, and in particular, we'll use the Mixed Layer CAPE (ML CAPE) in this analysis. The ML CAPE estimates CAPE by assuming a well-mixed layer in the lowest 100 mb. This is important, because on a sunny, warm June day on the Florida Peninsula, we can assume a well-mixed boundary layer through convective eddies.

Image 7: ML CAPE Analysis at 18Z on June 4th, 2006. ML CAPE values soared over Florida, while CIN remained low. The presence of CIN over the water is further indication of the differential heating between the land and adjacent water. (SOURCE: Storm Prediction Center)

At 18Z, you can see that much of the Peninsula had ML CAPE values in excess of 1000 J/kg, approaching 2000 J/kg. Also of note is the uniform lack of CIN over the land, but the presence of weak CIN over the water. If you recall the sea breeze model, cooler air resides over the water, suggesting a weak inversion of cold air at the surface. Thus, the high CAPE values and the relative lack of CIN is a favorable environment for strong updrafts to form.

Vertical Wind Shear

The MD was issued not because thunderstorms may form, but because they may produce hail and microbursts. In order for these storms to have the best chance to produce hail and microbursts, vertical wind shear is necessary to form rotating updrafts.

Image 8: Vertical Wind Shear at 18Z on June 4th, 2006. Very little shear is present over the region, consistent with the uniformity of wind vectors through the layers. However, a small area of wind shear along the east coast is enough to warrant concern for some rotating updrafts. This shear is the result of the sea breeze vector and the synoptic wind vector. (SOURCE: Storm Prediction Center)

The 18Z analysis of Bulk Shear shows relatively little shear over the peninsula, with weak values near 30 kts along the East Coast. If you recall the uniformity of the wind directions at 500 mb and 850 mb, this makes sense. The forecaster is generally concerned about the presence of this shear, albeit weak, combined with the large CAPE values, and a favorable synoptic environment for a sea breeze.

"MODERATE CAPE/MODEST LAPSE RATES IN PRESENCE OF WEAK
VERTICAL SHEAR MAY BE SUFFICIENT FOR ISOLATED/PULSE-TYPE
DOWNBURSTS/HAIL."

Atmospheric Soundings

Examining the sounding at Jacksonville at 00Z on June 5^th (evening of the 4^th), confirms several assumptions. The first is the presence of a sea breeze, which is well-depicted in the wind vectors at the low levels. Uniform westerly's dominate the layers above 850 MB, but easterly's are present at the lowest levels, indicative of a sea breeze circulation. The second is the weak shear present at 30 kts. This appears to be solely the work of the sea breeze. Also note the high ML CAPE over 2500 J/kg.

IMAGE 9: 00Z Sounding at JAX on June 5th, 2006. The sounding shows the large area of CAPE, the presence of the sea breeze below 850 mb and weak vertical wind shear. (SOURCE: Storm Prediction Center)

Storm Evolution

Below is a visible satellite image loop that shows the evolution of the sea breeze boundary, and the subsequent storms.

IMAGE 10: Visible Satellite from 14Z to 23Z on June 4th, 2006. Convection erupts linearly along the west coast of Florida and moves deep inland. Convection along the east coast is visible as well, although it remains more stationary. (SOURCE: Aviation Weather Center)

Note that the sea breeze front pushed well inland from the west coast and the storms developed mainly along the east coast. If you recall the mesoscale discussion, the forecaster predicted the convection would form generally along the east coast.

"GIVEN UNIFORM LOW TO MID LEVEL WESTERLY COMPONENT ACROSS MUCH OF THE PENINSULA...SEA BREEZE SHOULD LARGELY REMAIN NEAR THE EASTERN COAST."

IMAGE 11: Visible Satellite over Florida on June 4th, 2006 at 20Z. Note the sea breeze fronts pusing from west to east across the peninsula. Also visible is a sea breeze front near Jacksonville in NE Florida. Note the relative stable atmosphere behind these fronts. (SOURCE: Aviation Weather Center)

While the sea breeze developed along both coasts, the strong tail wind for the west coast component drove the convection much further inland. Further, the strong head wind for the east coast component enhanced the convergence along that boundary.

Image 12: An idealized cross-section of the Florida Peninsula showing the sea breeze front advances on both coasts. Note with the presence of the synoptic flow from the west, the western component moves deep inland. The eastern component is rather stationary, but with deeper convection. (SOURCE: William Smith)

CONCLUSION

The strong storms that developed along the Florida Peninsula on June 4^th, 2006 were indeed triggered by the sea breeze boundary, but that process was contingent upon a favorable synoptic environment. At the surface, the sea breeze was in full effect. Especially in late spring when water temperatures are still cool and land temperatures are rising, the sea breeze dynamics are in place. The rather weak pressure gradient aloft led to lighter winds and greater uniformity across the region. Further, the presence of the weak trough did introduce colder temperatures aloft, steepening the lapse rate, and thus, enhancing CAPE. Lastly, the weak-moderate synoptic flow west-east over Florida helped drive the western sea breeze component inland and enhanced convergence along the east coast. It also produced weak vertical wind shear as the westerly synoptic flow met the easterly mesoscale flow. The SPC forecaster recognized these features, and issued the MD as a result.