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Tornadoes are one of the high‐impact weather phenomena that can induce life loss and property damage. Here, we investigate the relationship between large‐scale weather regimes and tornado occurrence in boreal spring. Results show that weather regimes strongly modulate the probability of tornado occurrence in the United States due to changes in shear and convective available potential energy and that persisting weather regimes (lasting ≥3 days) contribute to greater than 70% of outbreak days (days with ≥10 tornadoes). A hybrid model based on the weather regime frequency predicted by a numerical model is developed to predict above/below normal weekly tornado activity and has skill better than climatology out to Week 3. The hybrid model can be applied to real‐time forecasting and aide in mitigation of severe weather events. Plain Language Summary: Severe storms that are capable of producing tornadoes, strong winds, and hail may lead to a high number of fatalities and property damage. The relationship between tornadoes and the large‐scale, recurrent weather patterns over the United States is investigated. It is shown that specific weather patterns may alter tornado activity and that persisting weather patterns contribute to greater than 70% of tornado outbreak days. Finally, a hybrid model is created to predict tornado activity, and the skill of the model is better than using a long‐term average out to Week 3, which is an improvement of the current forecasting state. Key Points: The large‐scale weather regimes strongly modulate United States tornado occurrenceA large fraction of tornado outbreak days are associated with a persisting regimeWeekly tornado activity can be predicted using forecast model‐predicted weather regimes [ABSTRACT FROM AUTHOR]

The Pacific-North American pattern (PNA), North Atlantic Oscillation (NAO), and Arctic Oscillation (AO) are three dominant teleconnection patterns known to strongly affect December-February surface weather in the Northern Hemisphere. A partial least squares regression (PLSR) method is adopted in this study to generate wintertime two-week statistical forecasts of these three teleconnection pattern indices for lead times of up to five weeks over the 1980-2013 period. The PLSR approach generates forecasts for the teleconnection pattern indices by maximizing the variance explained by predictor indices determined as linear combinations of predictor fields, which include gridded outgoing longwave radiation (OLR), 300-hPa geopotential height (Z300), and 50-hPa geopotential height (Z50). Overall, the PLSR models yield statistically significant skill at all lead times up to five weeks. In particular, cross-validated correlations between the combined weeks 3-4 PLSR forecasts and verification for the PNA, NAO, and AO indices are 0.34, 0.28, and 0.41, respectively. The PLSR approach also allows the authors to isolate a small number of predictor patterns that help shed light on the sources of prediction skill for each teleconnection pattern. As expected, the results reveal the importance of tropical convection (OLR) for forecast skill in weeks 3-4, but the initial atmospheric flow (Z300) accounts for a substantial fraction of the skill as well. Overall, the results of this study provide promise for improving subseasonal-to-seasonal (S2S) forecasts and the physical understanding of predictability on these time scales. [ABSTRACT FROM AUTHOR]

Characteristics of over 15 000 tropical cyclone (TC) inner cores are evaluated coincidentally using 37- and 85-GHz passive microwave data to quantify the relative prevalence of cold clouds (i.e., deep convection and stratiform clouds) versus predominantly warm clouds (i.e., shallow cumuli and cumulus congestus). Results indicate greater presence of combined liquid and frozen hydrometeors associated with cold clouds within the atmospheric column for TCs undergoing subsequent rapid intensification (RI) or intensification. RI episodes compared to the full intensity change distribution exhibit approximately an order of magnitude increase for inner-core cold cloud frequency relative to warm cloud presence. Incorporation of an objective ring detection algorithm shows the robust presence of rings associated with hydrometeors for 85-GHz polarization corrected temperatures () and 37-GHz vertically polarized brightness temperatures () for differentiating RI with significance levels ≥99.99%, while 37-GHz false color rings of a combined cyan and pink appearance surrounding a region that is not cyan or pink lack statistical significance for discriminating RI against lesser intensification. Rings of depressed and enhanced tied to RI suggest the combined presence of liquid and frozen hydrometeors within the atmospheric column, indicative of cold clouds. The rings also exhibit preferences for those with collocated more widespread ice scattering signatures to be more commonly associated with RI and general intensification. [ABSTRACT FROM AUTHOR]

Differences in the vertical velocity characteristics associated with various cloud populations are evaluated for two simulated cases of tropical cyclone (TC) rapid intensification (RI) under varying wind shear. Within the radius of maximum wind (RMW), preceding RI in the low-shear TC (hurricane Ike of 2008) increased updraught magnitudes for the top 1% of the distribution at 7 km occur, while in the high-shear scenario (hurricane Earl of 2010) RI is led by increased updraught magnitudes of the top 1% of the distribution at 12 km. Three-dimensional analyses of individual updraughts relative to their peak altitude enables direct quantification of processes associated with shallow cumuli, cumulus congestus, deep convection failing to penetrate the tropopause, and convective bursts (CBs). Mean profiles for each convective regime reveal positive contributions in certain variables of each updraught variety towards RI with positive diabatic heating, absolute vorticity, and moisture convergence roles. Within the RMW, CBs are shown to be the primary diabatic heating and vertical mass and vapour flux contributors in both simulations, while Ike includes secondary contributions from deep convection and congestus in addition to noteworthy diabatic heating from apparent stratiform processes that act to spin up the mid-level vortex. Inner-core moisture convergence has divided contributions from shallow cumuli, cumulus congestus, and CBs. CBs act to enhance potential vorticity to the greatest amount and over the deepest vertical layer at low levels. The simulation results overwhelmingly support the aggregate importance of vertically developed deep convection and its associated ice processes to the initiation and maintenance of RI. [ABSTRACT FROM AUTHOR]

Two 1-km Weather Research and Forecasting model simulations of the rapid intensification (RI) periods of hurricanes Ike (2008) and Earl (2010), under low and high wind shear respectively, are performed to evaluate mechanisms linked to the initiation and maintenance of RI. Despite similar simulated intensification time series, the exhibited hydrometeor characteristics differ between the two cases since Ike possesses less asymmetry and less vigorous convection than Earl. An objective method to quantitatively estimate the radius of maximum wind (RMW) with respect to height is introduced for quantifying the primary circulation character of each storm, permitting analysis of where heating is most efficient for driving warm core growth and subsequent intensification. Following eye development in each, diabatic heating remains embedded within the RMW. Earl's RI onset coincides with an absolute maximum in diabatic heating occurring within the RMW, while Ike possesses relatively less diabatic heating inside the RMW throughout and lacks an absolute peak associated with RI. The majority of diabatic heating within the RMW of both cases occurs at subfreezing temperatures, indicative of the importance of clouds associated with ice processes in these RI simulations. Earl's diabatic heating peak is tied to a maximum in convective burst activity residing within the RMW. Earl's preference for more sparse but intense convection is associated with increased sea surface temperatures, surface heat fluxes, moisture convergence, and near-surface equivalent potential temperature relative to the broader, weaker convective character in Ike. [ABSTRACT FROM AUTHOR]