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(T. F. “Storm” Walsh III)
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Good day everyone!
The Storm Prediction Center (SPC) has designated the following risk areas regarding Severe Thunderstorms for this morning and afternoon:
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHEAST TEXAS ACROSS CENTRAL AND SOUTHERN LOUISIANA…
Thunderstorms will move across parts of the upper Texas coastal plain and central and southern Louisiana through tomorrow morning. Sporadic, isolated damaging gusts are possible and a brief tornado or two cannot be ruled out.
From the SPC Outlook:
A high-amplitude synoptic pattern is present over the central and eastern U.S., with a progressive ridge from the eastern Gulf to Hudson Bay, and upstream trough arching from southern AB down the High Plains to northern and central MX. By the end of the period, the ridge should extend from Cuba to the Carolinas and northern QC, while the trough curves from northwestern ON across WI and the Ozarks to southeast TX and east-central/south-central MX. A basal shortwave perturbation – now evident in moisture-channel imagery from the South Plains and lower Pecos River area southward across northern MX, will proceed east-northeastward to near a FYV-GGG-GLS-BRO line by 12Z.
At the surface, a weak cold front — now extending from the eastern Dakotas across northwestern KS to northeastern NM — should move to Lake Superior, the Ozarks, and east TX by 12Z. Influence of this boundary on sensible convective threat will be minimal, however, given the presence of a well-defined, strongly baroclinic, convectively generated, outflow/convergence boundary to its east. That boundary — located initially over western AR, northwestern LA and east TX — will proceed slowly eastward across AR, LA and the remainder of southeast TX through the period, as additional convection develops primarily along and behind it.
…Southeast TX/LA outlook area…
Earlier bands of strong to severe thunderstorms over OK and east TX have weakened considerably. Still, farther south, the convective boundary is being reinforced and forced eastward by a line of strong/sporadically severe thunderstorms from the HOU metro south-southwestward across the coastal waters to the BRO area. Overall net motion of the convection across the outlook area should be slow, given the strongly meridional component of flow aloft. However, favorably strong deep/speed shear will aid in occasional organization of segments of the band into small-scale, accelerating LEWP/bow structures capable of locally damaging gusts and embedded bookend/mesovortices. Such structures may develop preferentially where the line of thunderstorms intercepts a prior outflow boundary, as has been observed the past several hours over south-central/southeast TX.
Effective-shear magnitudes of 45-55 kt are present in forecast soundings ahead of the primary convective plume through most of the period, as the basal mid/upper-level shortwave trough and associated height gradient shift eastward. Irregular diurnal heating beneath patches of anvil cirrus will destabilize the foregoing air mass gradually through early afternoon. This, combined with mid-60s to near 70 F surface dew points, should offset weak mid/upper-level lapse rates enough to boost MLCAPE into the 800-1500 J/kg range over most areas.
With time tonight, the convective regime will move farther eastward across eastern LA into a progressively less-favorable/lower-theta-e air mass across northeast LA and MS. As this occurs, overall convective organization and severe potential should lessen further.
Based on the information contained in the outlook, and analysis of the NAM-WRF forecast sounding data from F5 DATA software, the sounding data suggests the best probability for the strongest severe thunderstorms to occur within the outlined area, around approximately 12:00 noon – 3:00 p.m. CST. However, based on recent NWS Doppler Radar loops, and history of the southern extent of heavier thunderstorm activity, I do believe the strongest storms and cells will extend further south over LA. The outline I have drawn is what the model solution suggests, based on various forecast sounding indices. Based on 0-6km shear of 60-70 kts, SBCAPE of 1000 j/kg, and MLCAPE of 500 j/kg over the outline area, a good probability for supercell development does exist. The possibility of a brief isolated tornado activity cannot be ruled out with development of stronger cells, however in my analysis of the forecast sounding data, a lack of helicity, Energy Helicity Index (EHI values), Significant Tornado Parameter (STP values), etc., I would not expect tornadoes to be in the strong range (EF3-EF5).
F5 DATA NAM-WRF BEST SEVERE / TORNADO PROBABILITY (12:00 – 3:00 P.M. CST)
I cannot rule out some fairly strong cells, however based on the above data, combined with weaker, or lack of some other parameters, storms may be limited in their severity. BUT, there’s ALWAYS that one.
Based on weaker mid level lapse rates, and lifted indices of only 0 to -2, the threat for any hail should be minimal.
As evening draws closer, thunderstorms should begin to wane in intensity, given the forecast weakening of Theta-e values as the threat moves into extreme eastern and SERN LA early this evening.
Residents under the risk area should monitor NOAA Weather Radio throughout the day into tonight, and local NWS statements and warnings.
IF A TORNADO WARNING IS ISSUED FOR YOUR AREA…SEEK SHELTER IMMEDIATELY.
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Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS