John P. Monteverdi

Shear Parameter Thresholds for Forecasting Tornadic Thunderstorms in Northern and Central California

Abstract A study of 39 nontornadic and 30 tornadic thunderstorms (composing 25 tornado “events,” as defined in the text) that occurred in northern and central California during the period 1990–94 shows that stratification of the stronger tornadic events (associated with F1 or greater tornadoes) on the basis of 0–1- and 0–6-km positive and bulk shear magnitudes is justified statistically. Shear values for the weaker F0 events could not be distinguished statistically from the “background” values calculated for the nontornadic (null) thunderstorm events observed during the period. Shear magnitudes calculated for the F1/F2 events suggest that these tornadoes had developed in an environment supportive of supercell convection. Hindcasting the tornado events based upon shear thresholds produced a high probability of detection (POD) and low false alarm ratio (FAR), particularly for the stronger (F1/F2) events. Although the current sample size is limited and the conclusions drawn from it should be considered preli...

The Cool-Season Tornadoes of California and Southern Australia

Abstract Examples of cool-season tornadic thunderstorms in California and southern Australia are examined. Almost one-half of the reported Australian tornadoes and the majority of those in California occur in the cool season. It is shown that in both areas the typical synoptic pattern shows an active midlatitude trough just upstream, with a strong jet streak aloft. In both areas the tornadic thunderstorms occur with weak to moderate levels of thermodynamic instability in the lower troposphere but with extremely high values of low-level positive and bulk shear. Statistical tests on null cases (nontornadic thunderstorms) in the Central Valley of California indicate that large values of 0–1-km shear provide a discriminator for more damaging (F1–F3) tornadoes, whereas bulk measures of buoyancy, such as CAPE, do not. Australian case studies and tornado proximity soundings show similar characteristics. A “cool-season tornadic thunderstorm potential” diagnostic for Australian conditions, based on regional NWP an...

An Analysis of a Mesocyclone–Induced Tornado Occurrence in Northern California

Abstract One documented F2 tornado and several other unconfirmed tornadoes were reported in Californias Sacramento Valley on 24 September 1986. The synoptic pattern which occurred that day was one long-recognized by California operational meteorologists as being associated with severe weather in the state. The present study documents this event and shows that the parent thunderstorm had supercellular characteristics and that the tornado was mesocyclone-induced. As is the case elsewhere when severe thunderstorms are observed, the mesoscale environment established a focus for late morning and early afternoon deep convection. A quasi-stationary leeside trough acted in concert with local channeling effects to promote the advection of relatively moist air to the northern portions of the Valley. This channeled flow contributed to low-level shear which was much stronger than that evident in the Oakland hodograph and one which was comparable to that found with supercell and mesocyclone development elsewhere in t...

First WSR-88D documentation of an anticyclonic supercell with anticyclonic tornadoes: The Sunnyvale-Los Altos, California, Tornadoes of 4 May 1998

Abstract On 4 May 1998, a pair of tornadoes occurred in the San Francisco Bay Area in the cities of Sunnyvale (F2 on the Fujita scale) and Los Altos (F1). The parent thunderstorm was anticyclonically rotating and produced tornadoes that were documented photographically to be anticyclonic as well, making for an extremely rare event. The tornadic thunderstorm was one of several “pulse type” thunderstorms that developed on outflow boundaries on the left flank of an earlier-occurring thunderstorm east of San Jose. Satellite imagery showed that the tomadic storm moved northwestward along a sea-breeze boundary and ahead of the outflow boundary associated with the prior thunderstorms. The shear environment into which the storm propagated was characterized by a straight hodograph with some cyclonic curvature, and by shear and buoyancy profiles that were favorable for anticyclonically rotating updrafts. Mesoanticyclones were detected in the Monterey (KMUX) radar data in association with each tornado by the Nationa...

Convective and Rotational Parameters Associated with Three Tornado Episodes in Northern and Central California

Abstract An overview of the synoptic and subsynoptic controls on three tornado episodes (seven tornadoes) in northern and central California during December 1992 is presented and compared to the “prototype” documented for the 24 September 1986 mesocyclone-induced F2 event in the Sacramento Valley. Convective and rotational parameters calculated interactively on the Skew T/Hodograph Analysis and Research Programs supported anecdotal evidence that the tornadoes considered in this study were mesocyclone induced. The study indicates that careful subsynoptic analyses and evaluation of buoyancy and shear parameters can establish a mesoscale focus for supercellular development in California “cold sector,” low buoyancy-moderate shear thunderstorm environments.

The Single Air Mass Disturbance and Precipitation Characteristics at San Francisco

Abstract Precipitation in California is generally associated with frontal systems. The authors observations suggest that although the three major frontal cyclone categories are responsible for the bulk of the annual precipitation in the San Francisco Bay Area, a fairly large proportion can be ascribed to non-frontal, single air mass disturbances related only to positive vorticity advection aloft. Daily precipitation at San Francisco International Airport for the period 1966–75 was compared with the 1200 GMT surface and 500 mb charts. Each 24 h precipitation amount was associated with either a frontal or a, non-frontal disturbance. The analysis showed that between 20% and 50% of the total precipitation at San Francisco during the 10-year period was non-frontal in nature. In addition, the temporal variability of frontal precipitation was much higher than that of non-frontal precipitation. The results suggest that the “wetness” or “dryness” of a given season in California is related to frontal characteristi...

Terrain Influences on Synoptic Storm Structure and Mesoscale Precipitation Distribution during IPEX IOP3

Abstract The influence of topography on the evolution of a winter storm over the western United States and distribution of precipitation over northern Utah are examined using data collected during the third intensive observing period (IOP3) of the Intermountain Precipitation Experiment (IPEX). The analysis is based on high-density surface observations collected by the MesoWest cooperative networks, special radiosonde observations, wind profiler observations, Next-Generation Weather Radar (NEXRAD) data, and conventional data. A complex storm evolution was observed, beginning with frontal distortion and low-level frontolysis as a surface occluded front approached the Sierra Nevada. As the low-level occluded front weakened, the associated upper-level trough moved over the Sierra Nevada and overtook a lee trough. The upper-level trough, which was forward sloping and featured more dramatic moisture than temperature gradients, then moved across Nevada with a weak surface reflection as a pressure trough. Over no...

A Supercell Thunderstorm with Hook Echo in the San Joaquin Valley, California

Abstract This study documents a damaging supercell thunderstorm that occurred in Californias San Joaquin Valley on 5 March 1994. The storm formed in a “cold sector” environment similar to that documented for several other recent Sacramento Valley severe thunderstorm events. Analyses of hourly subsynoptic surface and radar data suggested that two thunderstorms with divergent paths developed from an initial echo that had formed just east of the San Francisco Bay region. The southern storm became severe as it ingested warmer, moister boundary layer air in the south-central San Joaquin Valley. A well-developed hook echo with a 63-dBZ core was observed by a privately owned 5-cm radar as the storm passed through the Fresno area. Buoyancy parameters and hodograph characteristics were obtained both for estimated conditions for Fresno [on the basis of a modified morning Oakland (OAK) sounding] and for the actual storm environment (on the basis of a radiosonde launched from Lemoore Naval Air Station at about the t...

Recent rainfall trends on the margins of the subtropical deserts: A comparison of selected northern hemisphere regions

Abstract An analysis of recent (1898–1972) seasonal rainfall data from California (winter) and Mexico (summer) provides no evidence of an inverse relationship such as has been reported by Winstanley for climatically analogous areas of Eurasia and Africa. However, California and Old World winter rain trends have been remarkably similar during this time period. Also, summer rain in northwestern Mexico (Mazatlan) correlates highly with winter rainfall in southern California (San Diego) when allowance is made for a 7-year lag. Winstanley suggested that the expansion and contraction of the circumpolar vortex is a primary control of rainfall trends on the margins of the subtropical deserts. The alternative proposed here is that, at least on this time scale, variations in the longitudinal position of troughs and ridges in the upper air westerlies are more important.

An Analysis of the 7 July 2004 Rockwell Pass, California, Tornado: Highest-Elevation Tornado Documented in the United States

AbstractThis manuscript documents the tornado in the Rockwell Pass area of Sequoia National Park, California, that occurred on 7 July 2004. Since the elevation of the tornado’s ground circulation was approximately 3705 m (~12 156 ft) MSL, this is the highest-elevation tornado documented in the United States. The investigation of the storm’s convective mode was performed mostly inferentially on the basis of an analysis of the radar imagery from Edwards Air Force Base (which was in clear-air mode on this day), objectively produced soundings and/or CAPE estimates from two mesoscale models, an objectively produced proximity sounding and hodograph, and analyses of satellite imagery. The nearest Weather Surveillance Radar-1988 Doppler (WSR-88D) in Hanford, California, could not be used to observe this storm because of terrain blockage by the Sierra Nevada, and the nearest sounding sites were too distant and in a different meteorological environment on this day. The near-storm environment may have been favorable...