U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 271957 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0257 PM CDT Monday Apr 27 2015 


Valid 272000z - 281200z 


..there is a slight risk of severe thunderstorms from north-central Texas to the central 
Gulf Coast... 


..there is a marginal risk of severe thunderstorms surrounding the slight risk 
over the general area... 


..there is a marginal risk of severe thunderstorms for portions of central and southern 
Florida... 


... 
Isolated strong to severe storms with hail...locally damaging 
wind...and perhaps a tornado or two may affect parts of 
north-central into eastern Texas and perhaps southwest Louisiana. A 
few strong to possibly severe storms also may occur over central and 
southern Florida. 


The only substantial changes this outlook update include... 
1) the removal of 5 percent tornado probabilities over southeastern la. The 
early day mesoscale convective system has moved into the Gulf and the airmass has become 
less unstable in its wake. Another round of storms is expected to 
move into southeastern la later today/tonight and strong wind profiles will 
support a conditional severe threat for organized storms. 
2) added low severe probabilities for much of the southern half of the Florida 
Peninsula as recent iterations of convection-allowing models show 
the northern Gulf of Mexico mesoscale convective system continuing east-southeastward and affecting the Florida 
Peninsula tonight. 


.Smith.. 04/27/2015 


Previous discussion... /issued 1118 am CDT Monday Apr 27 2015/ 


... 


A middle to upper-level low will edge slowly eastward across the Southern Plains 
in response to an amplifying upstream ridge over the Great Basin 
into northern rockies. A middle-level jet streak observed within the base 
of the upper low per 12z observed soundings will progress generally 
eastward into the central Gulf Coast. 


At the surface...an area of low pressure over north-central Texas will 
develop southeastward into southern la along a cold front advancing southeastward through 
central and southern parts of Texas. Meanwhile...visible satellite and 
surface observations as of middle morning indicate an outflow boundary 
associated with the Bow echo over southeastern la extending westward through the 
Upper Texas coast before curving nwwd along a baz-Sep line where it 
links with the surface low and associated cold front. 


..sern Texas into the central Gulf Coast today into tonight... 


A well-defined Bow echo with a history of fairly widespread damaging 
winds should continue east-southeastward through the remainder of far southeastern la 
late this morning into early afternoon with a risk for damaging 
winds and perhaps a tornado or two over those locations. In the 
wake of this mesoscale convective system...it appears that warm air advection/isentropic ascent atop the 
trailing outflow boundary coupled with deeper-layer forcing for 
ascent related to the above-mentioned middle-level jet streak may 
support the development of thunderstorms this afternoon into tonight from 
southeastern Texas eastward into la. 


Based on the 12z soundings at crp and bro...ample low-level moisture 
surmounted by a well-defined eml will support moderate afternoon 
instability and the potential for organized storm modes including 
supercells given 40-50 knots of deep-layer shear. While large hail 
appears to be the primary hazard...locally damaging winds and 
perhaps a brief tornado or two are possible. 


..N-central into eastern Texas this afternoon/evening... 


As mentioned above...considerable overturning of the air mass has 
occurred over eastern into north-central Texas in the wake of the mesoscale convective system now over 
la. However...cold middle-level temperatures coupled with a warming 
boundary layer should support air mass destabilization by afternoon 
ahead of the surface low and cold front with MLCAPE approaching 
1000-1500 j/kg. Forcing for ascent in advance of the deep-layer 
cyclone is expected to Foster the development of surface-based 
storms by afternoon amidst a kinematic environment featuring 
vertically veering and strengthening winds with height. As 
such...supercells are possible with the risk for large 
hail...damaging winds and a couple of tornadoes spreading southeastward into 
parts of eastern Texas. This severe weather threat should begin to wane by 
middle to late evening as the boundary layer begins to cool and 
stabilize. 


..FL peninsula this afternoon... 


Pronounced westerly deep-layer flow observed by 12z soundings will 
persist today with diurnal storm development likely remaining 
confined to the East Coast sea breeze. While middle-level lapse rates 
will remain poor...daytime heating coupled with a moist boundary 
layer will yield moderate afternoon instability and the potential 
for a few strong to marginally severe storms this afternoon with a 
risk for locally damaging wind gusts. 

Mesoscale Discussion


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sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 272140 
flz000-272245- 


Mesoscale discussion 0484 
National Weather Service Storm Prediction Center Norman OK 
0440 PM CDT Monday Apr 27 2015 


Areas affected...cntrl/srn Florida Peninsula 


Concerning...severe potential...watch unlikely 


Valid 272140z - 272245z 


Probability of watch issuance...20 percent 


Summary...a localized hail/wind threat will exist across eastern 
portions of central/South Florida over the next couple hours. A ww is not 
anticipated. 


Discussion...strong diurnal heating has occurred across central/southern Florida 
with temperatures near 90 f in many locations amidst middle-60s to 
middle-70s surface dewpoints. This has yielded sufficient buoyancy to 
support strong thunderstorm development in spite of warm midlevel 
temperatures. Currently...the strongest thunderstorm is moving across 
Highlands/Okeechobee counties...which formed along a synoptic cold 
front. 50-60 knots of effective shear is supporting marginal supercell 
structures with this storm...which has a history of producing 
one-inch hail. Other thunderstorms are developing farther S between mia and 
pbi along a westward moving sea breeze boundary. Here...thunderstorms will have a 
short residence time over land given Ely motion...but brief hail and 
strong wind gust potential may exist. A brief/weak tornado threat 
could exist if the sea breeze boundary can interact with more 
organized convection farther inland. The isolated nature of these 
threats will preclude ww issuance. 


.Rogers/Thompson.. 04/27/2015 


..please see www.Spc.NOAA.Gov for graphic product... 


Attention...WFO...mfl...mlb...key...tbw... 


Latitude...Lon 25098037 25238081 26248107 27518120 28028088 28128051 
27608021 26557990 25638007 25098037