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Tsing-Chang Chen
,
Jenq-Dar Tsay
,
Jun Matsumoto
, and
Jordan Alpert

Abstract

The peak intensity occurrence frequency over the life cycles of parent cold-surge vortices (CSVs) for heavy rainfall/flood (HRF) events is classified into two types depending on their life cycles having two or three peak intensities, denoted as HRF2 or HRF3, respectively. The formation of an HRF2 event from its parent CSV(HRF2) formation is ≤5 days, while the formation of an HRF3 event is ≥6 days. The latter group contributes ~57% of the total number of HRF events. As a result of some model constraints, the formation and development of HRF3 events are not well forecasted by the Global Forecast System (GFS) and regional forecast models. The life cycle and second peak intensity for CSV(HRF3) allow for the introduction of a forecast advisory for HRF3 events. Identification of CSVs and two sufficient requirements for the formation and occurrence of HRF events were developed by previous studies. Nevertheless, two new necessary steps are now included in the proposed forecast advisory. The population ratio for CSV(HRF3) and the regular CSV is only about 15%. The occurrence optimum time t o for the CSV(HRF3) second peak intensity from this vortex formation is about 3 days 6 h. The GFS forecast over t o is utilized to identify CSV(HRF3). Then, the relay of the GFS forecast from the occurrence time of the CSV(HRF3) second peak is used to predict the formation/occurrence of HRF3 events. Six HRF3 events during cold seasons for 2013–16 are used to test the feasibility of this forecast advisory. Results clearly demonstrate this advisory is a success for the forecast of HRF3 events over the entire life cycles of their parent CSV(HRF3)s.

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John O. Roads
,
T. Norman Maisal
, and
Jordan Alpert

Abstract

Precipitation forecasts made by the National Meteorological Center's medium-range forecast (MRF) model are evaluated for the period, 1 March 1987 to 31 March 1989. As shown by Roads and Maisel, the MRF model wet bias was substantially alleviated during this period. As is shown here, the MRF model forecast skill in predicting individual wet and dry events has also increased. We show that there is substantial skill in the model forecasts of precipitation occurrences beyond 2.5 days. These MRF model forecasts have not yet been fully exploited by the forecasting community, in part, because they have not been readily available.

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Tsing-Chang Chen
,
Jenq-Dar Tsay
,
Jun Matsumoto
, and
Jordan Alpert

Abstract

After the onset of the Southeast Asian summer monsoon in mid-May, the South China Sea (SCS) trough is deepened by the intensified monsoon westerlies to facilitate the development of a synoptic cyclonic shear flow. This shear flow forms an environment favorable for the SCS tropical storm (TS)/typhoon (TY) genesis triggered by the surge of this monsoon circulation. This genesis mechanism has not been well documented. Seventeen named SCS TS/TY geneses in May over 1979–2016 occurred under the following environmental conditions/processes: 1) with its maximum located south of 15°N, the intensified monsoon westerlies are extended eastward beyond 120°E, 2) the synoptic SCS cyclonic shear flow is developed by the tropical easterlies fed by a northeast Asian cold surge (or a North Pacific cold-air outbreak) and the intensified monsoon westerlies, and 3) SCS TS/TY genesis is triggered by the surge of monsoon flow. The accuracy of the monthly mean forecasts is limited. However, it is found that SCS TS/TY genesis only occurs after the existence of persistent, strong, monsoon westerlies lasting for at least 5 days. Forecasts from the National Centers for Environmental Prediction Global Forecast System (2004–16) and the Global Ensemble Forecast System (1985–2003) cover these 15 SCS TS/TY geneses. The requirements for SCS TS/TY genesis in May described above are met by the 5-day-mean Southeast Asian summer monsoon circulation. Based on a statistical analysis of 5-day forecasts for these TS/TY geneses, a four-step forecast advisory is introduced. The forecasts for SCS TS/TY genesis can be made 3 days prior to occurrence.

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Tsing-Chang Chen
,
Ming-Cheng Yen
,
Jenq-Dar Tsay
,
Jordan Alpert
, and
Nguyen Thi Tan Thanh

Abstract

The formations of heavy rainfall/flood (HRF) events in Vietnam are studied from diagnostic analyses of 31 events during the period 1979–2009. HRF events develop from the cold surge vortices formed around the Philippines. These vortices’ speed, size, and rainfall, which evolve into HRF events, are enhanced distinguishably from non-HRF vortices, as they reach Vietnam. The HRF cyclone, the North Pacific anticyclone, and the northwestern Pacific explosive cyclone simultaneously reach their maximum intensities when the HRF event occurs. An HRF cyclone attains its maximum intensity by the in-phase constructive interference of three monsoon (30–60, 12–24, and 5 days) modes identified by the spectral analysis of zonal winds. The rainfall center of an HRF event is formed and maintained by the in-phase constructive interference of rainfall and convergence of water vapor flux anomalies, respectively, from three monsoon modes. Forecast times of regional models are dependent and constrained on the scale of the limited domain. For 5-day forecasts, a global or at least a hemispheric model is necessary. Using the salient features described above, a 5-day forecast advisory is introduced to supplement forecasts of HRF events made by the global model. Non-HRF vortices are filtered by threshold values for the deepening rate of explosive cyclones and basic characteristics of the HRF events predicted by the global model. A necessary condition for an HRF event is the in-phase superposition of the three monsoon modes. One-week forecasts for 12 HRF events issued by the NCEP Global Forecast System are tested. Results demonstrate the feasibility of the forecast advisory to predict the occurrence dates of HRF events.

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