11PM Day 2 Convective Outlook for Monday, February 16. THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS MONDAY ACROSS PARTS OF CENTRAL AND SOUTHERN CALIFORNIA
SUMMARY
Strong thunderstorms may impact southern California coastal areas, mainly south of Vandenberg AFB into the Los Angeles Basin, and perhaps parts of the Central Valley, on Monday, accompanied by at least some risk for damaging wind gusts and perhaps a couple of tornadoes.
Discussion
General trends concerning the mid/upper flow evolution depicted in prior model runs for this period continue. To the northeast of a blocking mid-level ridge centered over the southern mid-latitude Pacific (between 150-160W longitude), a vigorous short wave perturbation is forecast to undergo further amplification while digging offshore of the British Columbia and Pacific Northwest coast. It appears that this will be accompanied by lower/mid-tropospheric cyclogenesis, and force an initially significant downstream trough inland across California and much of the Great Basin by late Monday night.
As this occurs, deep surface troughing is also forecast to develop farther inland during the latter half of the period, becoming centered near the northern Rockies through the central and southern Great Plains by 12Z Tuesday. Farther downstream, models indicate that warm/dry elevated mixed-layer air will continue advecting northeastward, overspreading much of the central and southern Great Plains and Gulf Basin. At the same time, further lower tropospheric cooling and drying appears probable across much of the Gulf Basin, in the wake of mid-level troughing and an associated rapidly deepening surface cyclone migrating away from the middle and southern Atlantic Seaboard.
It still appears that the inland migrating mid/upper-level troughing will be accompanied by an intense jet (including a 100-110+ kt speed maximum around 500 mb) nosing inland across southern California through the Four Corners. Forcing associated with this feature may be promoting modest cyclogenesis offshore of the California coast by the beginning of the period. The low may deepen a bit further while occluding and migrating inland near or south of the San Francisco Bay vicinity during the day Monday.
Southern California coast
Models continue to suggest that a corridor of low-level moistening, along and ahead of the front trailing the occluding surface low, may gradually contribute to sufficient destabilization to support deepening convective development while spreading inland across coastal areas, mainly south of Vandenberg AFB through the Los Angeles Basin, perhaps beginning as early as late Monday morning. Aided by mid/upper forcing for ascent and strong mid-level cooling,
thermodynamic profiles may become supportive of thunderstorm development, in the presence of strong deep-layer shear. Wind profiles may include 40-50 kt southerly flow around the 850 mb level, with sizable clockwise curved low-level hodographs developing, mainly where the favorable orientation of the Transverse Ranges with respect to the low-level flow contributes to backing of near-surface wind fields. As this occurs, the environment may become conducive to the development of supercells capable of producing tornadoes, in addition to potentially damaging wind gusts.
Central Valley
There is notable spread evident among the various model output, but at least some guidance suggests that the inland migrating low, and perhaps an associated lingering mid-level cyclonic circulation, may contribute to a focused area of lift, weak low-level destabilization and strengthening shear by Monday afternoon. NAM forecast soundings, perhaps more so than other guidance, suggest that the environment might become conducive to the risk for a brief, weak tornado or two.