SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0649 AM CST Mon Nov 09 2020 Valid 091300Z - 101200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF SOUTH FLORIDA AND THE KEYS...AND OVER PORTIONS OF THE CENTRAL/SOUTHERN PLAINS... ...SUMMARY... A tornado or two may occur today over parts of south Florida or the Keys on the back side of Tropical Storm Eta. Marginally severe hail or gusts cannot be ruled out tonight over portions of the south-central Plains. ...Synopsis... In mid/upper levels, a longwave trough will persist over the western CONUS, but with some important synoptic to subsynoptic developments. A currently upstream trough over the Gulf of AK and southern AK will dig southeastward to BC and the Pacific Northwest by the end of the period. As that occurs, the broad, marginally closed cyclone now over the Great Basin will eject east-northeastward across the central Rockies to the adjoining High Plains. By 12Z tomorrow, the associated 500-mb low should be located over the northwestern KS/ southwestern NE area, with trough south-southwestward over southeastern NM. Elsewhere, a broad mid/upper cyclone over the southeastern Gulf, southern FL, the northwestern Caribbean, and central/western Cuba, has enveloped the circulation of T.S. Eta, which should become the dominant midlevel vorticity max for the broader cyclone. At the surface, 11Z analysis showed a cold front across portions of northeastern/central MN to a weak low in the ONL/YKN area, then southwestward to another low near LHX. The Arctic frontal branch extended from there northwestward over northwestern CO to the northern Rockies, while a Pacific front extended from the southern low south-southwestward over south-central NM. A dryline was drawn from the southern low southward close to the eastern NM/TX line, then across the Big Bend area of TX. By 00Z, the combined cold front should extend from the western Lake Superior/MN Arrowhead region to western IA, central KS, through a low over the southwestern KS/southeastern CO area, then south-southwestward over far west TX. The front will overtake the dryline generally from north-south this evening and overnight. By 12Z, the front should reach eastern Lower MI, southwestern WI, to a weak low over northwestern MO, then across eastern KS to west-central TX and northern Coahuila. ...South FL/Keys... T.S. Eta is forecast by NHC to take a broadly cyclonically curving and generally southwestward track through the period, around the broader cyclone noted above. The T.S. should shift toward a position over the southeastern Gulf that will effectively render it a meandering, cutoff low amidst weak steering flow on days 2-3. See latest NHC advisories for tropical watch/warning info and specific track/intensity guidance. Forecast wind radii and existing/progged convective structure indicate that Eta will remain asymmetric this period with convective bands well east and northeast of center, across the outlook area. Weak lapse rates and marginal instability -- as sampled by available central/southern FL RAOBs since 06Z -- are expected to continue, but with typically rich near-surface moisture for a tropical cyclone, and sufficient SRH (especially for any rightward/deviant motions of sustained, deeper cells) to support a threat for a brief tornado or two. Outer/peripheral convection associated with Eta may continue to affect parts of this area well into tonight. However, as the system pulls away westward, low-level flow, boundary-layer shear, and hodograph size each will decline gradually with time over land, likewise ramping down the already marginal supercellular tornado potential. ...Central/southern Plains... Isolated to scattered thunderstorms are expected to develop this evening and especially overnight across western parts of the outlook area, near the front and dryline, with greater convective coverage likely over the more-strongly frontally forced KS portion. Activity should spread eastward and northeastward across the remainder of the area overnight, with the most intense cells offering a threat for marginally severe hail and or gusts. Strengthening deep-layer shear and lift with time will be the most favorable factors, while instability remains modest. Time series of forecast soundings suggest steep low/middle-level lapse rates above a nocturnally stabilizing airmass near the surface, with increasing moisture in and just atop the boundary layer. This will support 500-800 J/kg MUCAPE, locally/briefly reaching near 1000 J/kg, amidst 40-50 kt effective-shear vectors. The most favorable juxtaposition of CAPE-shear parameter space and greatest coverage probabilities appears to be over parts of central/south-central KS this evening. Large-scale ascent should decrease southward across OK into greater moisture, as will frontal forcing, while forecast soundings show a warm nose or "CAPE robber" around 500 mb. This leads to questions over coverage, but with the potential for isolated strong-severe convection as far south as west-central OK. ..Edwards/Kerr.. 11/09/2020
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