U.S. Severe Weather Forecast
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000 acus01 kwns 111953 swody1 Storm Prediction Center ac 111951 Day 1 convective outlook National Weather Service Storm Prediction Center Norman OK 0151 PM CST Wednesday Dec 11 2013 Valid 112000z - 121200z ..no thunderstorm areas forecast... ..Lee of lower Great Lakes... time series of forecast soundings in these areas suggest lake-effect convective snow bands are in environment of near-maximum buoyancy for entire period at this time...with no lightning detections yet observed. While MUCAPE up to about 75 j/kg may persist through remainder afternoon...coldness of surface-midlevel thermodynamic profiles and resultant/implied lack of precipitation-particle phase diversity suggest lightning risk...while nonzero...is minimal. As such...if any thunder occurs it should be very brief/isolated...and general thunderstorm probability now appears to be less than 10%. .Edwards.. 12/11/2013 Previous discussion... /issued 1027 am CST Wednesday Dec 11 2013/ ... Some eastward progression has been noted since yesterday...with height rises over the Pacific northwest/northern rockies...but a large-scale trough still encompasses much of the Continental U.S. East of The Rockies. A separate lower-latitude closed low just off the northwest Baja California coast will begin a transition to a more open wave as an upstream shortwave trough approaches the Pacific northwest late. Moistening in advance of the northern Baja California low will likely remain insufficient for deep convection across Southeast Arizona/SW nm until the day 2 period. ..the Lee of lakes Ontario and Erie today... Cyclonic flow will persist over the Great Lakes today...with lake effect snow bands most pronounced in the Lee of lakes Erie and Ontario. Temperature profiles will cool some aloft from the 12z observations and convective band depth should increase...especially over Lake Ontario. However...thermodynamic profiles will remain quite cold through the convective cloud depth /colder than -6 to -8 c from the local upward/ and likely limit mixed phase...which will limit the potential for lightning production. By tonight... low-level flow will veer to northwesterly and weaken with trough passage... bringing an end to the long-fetch trajectories over Lake Ontario. ..S Florida/Keys through tonight... Moistening has occurred since yesterday...with precipitable water values now ranging from 1.75-2 inches. Weak buoyancy is expected over S Florida today with daytime heating...and this buoyancy will be maintained through tonight within the northern periphery of a moisture plume emanating from the northwest Caribbean Sea. Still...there is no clear focus for storm development...and a few convective showers appear more probable than thunderstorms...given the rather modest lapse rates aloft.
000 acus11 kwns 111332 sels mesoscale discussion Storm Prediction Center mesoscale discussion 111332 nyz000-111730- Mesoscale discussion 2076 National Weather Service Storm Prediction Center Norman OK 0732 am CST Wednesday Dec 11 2013 Areas affected...east end of Lake Ontario to the Tug Hill plateau of New York Concerning...heavy snow Valid 111332z - 111730z Summary...the earlier disruption to the overnight Solitary Lake effect snow band may only have been temporary...as a new solitary band potentially becomes re-established this morning. If this continues to occur...snowfall rates of 2-3 inches per hour may redevelop through the morning with the most intense parts of the band. Discussion...during the past couple of hours...buf vwp data indicate subtle backing of the flow just above the surface. This could be linked to isallobaric forcing associated with a middle-level perturbation crossing the lower Great Lakes region that is elongated over the southern periphery of the deeper polar vortex -- per moisture channel imagery. The minor wind shift may explain the earlier disruption to the antecedent solitary snow band that affected the local area through much of the night. However...with mass fields at the mesoscale-Beta scale reestablishing a quasi-steady state given the elongation of the middle-level perturbation...it is entirely possible that another solitary snow band will continue reforming during the next couple of hours in a similar location as the overnight band...and persist through the day. The 12z buf radiosonde observation suggests deep westerly/unidirectional flow through the troposphere that would favor the development of such a band via maximization of fetch length atop relatively warmer lake waters. This should continue to support the maintenance of an above-lake convective boundary layer. Mosaic radar imagery already indicates regenerating convection along the long axis of the lake that could continue re-consolidating into a band this morning. The moderately deep dendritic growth zone centered around 850 mb as sampled by the 12z buf radiosonde observation and the shallow -- though intense -- convective nature of the precipitation could Foster snowfall rates of 2-3 inches per hour...especially if a solitary band redevelops. The highest snowfall rates would occur around west-facing slopes surrounding the Tug Hill plateau...and in association with any isolated thunderstorms /see day-1 convective outlook for additional details/. .Cohen.. 12/11/2013 ..please see www.Spc.NOAA.Gov for graphic product... Attention...WFO...bgm...buf... Latitude...Lon 43667641 43787568 43827527 43637513 43477538 43437573 43437619 43467654 43667641