000 acus01 kwns 221258 swody1 Storm Prediction Center ac 221257 Day 1 convective outlook National Weather Service Storm Prediction Center Norman OK 0657 am CST Monday Dec 22 2014 Valid 221300z - 231200z ..there is a marginal risk of severe thunderstorms portions lower MS valley...central Gulf Coast... ... Isolated severe storms will be possible today and tonight across parts of the lower Mississippi Valley and Gulf Coast regions. ... In middle-upper levels...broadly cyclonic pattern has taken shape across central Continental U.S....and will be reinforced by several shortwave troughs through remainder of period. Closely spaced perturbations now over eastern Colorado and upper Red River valley region of Texas/OK are expected to merge today over OK...then pivot northeastward across MO/western Illinois tonight. Phasing of combined perturbation with another shortwave trough moving southeastward from northern High Plains will contribute to development of closed 500-mb cyclone tonight over southeastern Nebraska/southwestern Iowa/northwestern MO area. Meanwhile...upstream perturbation and related speed maximum now over southern British Columbia are expected to move rapidly southeastward to nm and West Texas...approaching base of cyclonic-flow field by 12z. These processes will combine to tighten upper-level height gradient considerably east of synoptic-scale trough...leading to expansive and intensifying field of 120-130-kt 250-mb speeds over lower MS and Ohio valleys around 12z. At surface...11z analysis showed lows over northern South Dakota and southwestern OK...connected by frontal zone that in turn extended west-southwestward across northwest Texas and westward/nwwd across E-central/northestern nm. These lows should effectively consolidate/redevelop into single cyclone over MO by 00z...as cyclone aloft begins to take shape over Central Plains. Associated cold front should be located over southern MO...arklatex region and S-central/SW Texas at 00z. Surface cyclone then should deepen and pivot north-northeastward across eastern Iowa...with 12z cold-frontal position roughly along an evv...Jan...gls...cot line. Meanwhile...wavy frontal zone extended northeastward and southwestward from weak frontal-wave low offshore SC...to S-central Florida...then becoming warm front over northern Gulf waters S of Florida Panhandle...MS...al...and la...and offshore Texas coast. This shallow marine/warm front will drift northward through period...moving inland over portions southeastern la...southern MS...southern Alabama and Florida Panhandle. ..lower MS valley...Central Gulf Coast... at least weakly favorable parameter spaces and conditional risk of damaging gusts/tornadoes will exist across this region throughout latter 12-15 hours of day-1 period -- from late afternoon Onward. However...lack of substantial deep-layer lift and weakness/absence of low-level foci indicate very sparse coverage of surface-based thunderstorms...keeping unconditional probabilities marginal at this time. Deep shear will be increasing over this region throughout period...as will low-level moisture and thetae. Moreover...somewhat enlarged/loopy low-level hodographs likely will develop in vicinity of warm front. Subtle large-scale ascent is expected mainly north of I-20 over corridors of weak elevated instability...in relation to right-entrance region of strengthening jet aloft...contributing to elevated thunder potential over middle south and Ohio Valley. However...ascent/destabilization aloft related to middle-upper-level DCVA likely will remain displaced from most favorable low-level thermodynamic fields this period. Moreover...support for sustained/deep convection is lacking given... 1. Shallow slope of warm front... 2. Lack of apparent/additional mesoscale foci to augment low-level lift over Delta/Gulf Coast region...and 3. Likelihood of weak midlevel lapse rates and stable layers aloft per 12z lch radiosonde observation and multiple model-forecast soundings. Surface dew points middle-upper 60s f may offset those poor lapse rates enough to result in 500-1000 j/kg MLCAPE near coast after dark...though sources for low-level forcing appear weak/uncertain at this time. Some potential also exists for small supercells to form within relatively weakly capped and highest-thetae air mass over Gulf late in period...moving eastward toward coast. Present indications are greatest concentration of any such convection will remain offshore through period...though one or two such thunderstorms cannot be ruled out tonight over coastal areas from Florida Panhandle westward across southeastern la. .Edwards/cohen.. 12/22/2014
Posted on: Mon, 22 Dec 2014 13:22:45 +0000
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